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explain the electrolytic refining of lithium from its ore

electrolytic refining - meaning, electrorefining of copper, metals

electrolytic refining - meaning, electrorefining of copper, metals

Electrolytic refining is a process of refining a metal (mainly copper) by the process of electrolysis. As far as the mechanism of the process is concerned, during electrolysis, a large chunk or slab of impure metal is used as the anode with a thin strip of pure metal at the cathode. In this setup, an electrolyte (metal salt aqueous solution) depending on the metal is often used.

The clean or pure metal is formed at the cathode when the electrical current of a sufficient voltage is applied by dissolving impure metal at the anode. Electrolytic refining is also sometimes referred to as Electrorefining.

The below-given table outlines the methods used to refine five metals. It is necessary to choose the electrolyte and other conditions so that both anodic dissolution and metal deposition proceed with high efficiency while none of the impurity metals can move from the anode to the cathode. Clearly, there must be no passivation of the anode and the aim is to produce a deposit at the cathode of good quality, sometimes extremely crystalline. Additives are applied to the electrolyte when necessary to impose the right behaviour on both electrodes.

Here we will take an example of electrolytic copper refining to understand the process more clearly. Copper is usually mined from its coal known as blister copper. It is about 98 to 99 per cent pure. However, the electro-refining process can easily make it 99.95% pure which makes it a good product to be used in electrical components.

A block of impure copper is taken as an anode or positive electrode. Copper sulfate which is acidified with sulphuric acid is used as a graphite-coated electrolyte along with pure copper tubes, as a cathode or negative electrode. In this phase of electrolysis copper sulfate divides into a positive ion of copper (Cu++) and a negative ion of sulfate (SO4). The positive copper ion (Cu++) or cations travel towards the negative electrode made of pure copper where it absorbs the electrons from the cathode. Cu atom is deposited on the cathodes graphite layer.

The cathode is coated with graphite in the process of electrolytic metal processing or merely electro grinding so that the concentrated material can be easily removed. This is one of the most growing electrolysis procedures.

However, anodes metallic impurities are also mixed with SO4, forming metallic sulfate in the electrolyte solution and dissolving. Impurities such as silver and gold that are not produced by a solution of sulphuric acid-copper sulfate settle down as the anode sludge or dust.

In modern times, the main use of copper is in the form of an electrical conductor. Copper has a very high per unit volume electrical conductivity. It can be easily drawn through strings, whether single or multi filament, which can be conveniently and regularly twisted without undue toughness of operation. Copper wire is readily tinned, has superior soldering properties and is resistant to corrosion at points of contact.

In the process of electrolytic gold refining, hydrochloric acid is used as an electrolyte. A thin sheet of gold is made the cathode and a gold alloy is made the anode. When ion transfer takes place a pure gold with higher purity rate is transferred to the cathode. This method is also called as the cycle of Wohlwill.

Similarly, crude silver is made the anode and more refined silver is made the cathode. The electrolytic process is the same as discussed above. The only exception or difference is that nitric acid bath is used where the silver anodes get dissolved. Thus, pure silver of about 99.9 per cent is obtained.

Throughout metallurgy, grinding and refining plays a crucial role. Every metal obtained from its ore is typically unclean in nature. Refining is a way to remove impurities and produce high purity metals. Impurities are extracted by different methods from raw metal depending on material properties and impurity. Some different techniques apart from electrolytic refining or electrolysis that are used in crude metal purification include;

Electrolytic refining is a much more common process than electro winning, and such plants exist on a scale ranging from 1000 to 100,000 tons per year throughout the world. These are typically part of a larger project to extract raw metals from both waste and main ores and retrieve them.

The system must, therefore, be designed to handle a metal feed of variable quality and result in a concentrate of all the materials present in a shape that can be further processed. Electrorefining also offers a very high metal purity.

Using a molten salt or non-aqueous electrolyte, electrorefining methods are used and are indeed the focus of further research. This is due to the possibilities they offer to increase current densities and refining through lower oxidation states that are not stable in water (e.g. copper refining via Cu+ would nearly halve the energy requirement).

commercial lithium production and mining of lithium

commercial lithium production and mining of lithium

Most lithium is commercially produced from either the extraction of lithium-containing salts from underground brine reservoirs or the mining of lithium-containing rock, such as spodumene. Lithium production from clay sources is expected to become commercially viable, though perhaps not until 2022.

Lithium is a metal commonly used in batteries like the rechargeable ones found in laptops, cellphones, and electric cars as well as in ceramics and glass. It is the lightest metal on Earth and is soft enough to be cut with a knife when in its elemental form.

Much of the lithium produced today is extracted from brine reservoirs called salars that are located in high-elevation areas of Bolivia, Argentina, and Chile. In order to extract lithium from brines, the salt-rich waters must first be pumped to the surface into a series of large evaporation ponds where solar evaporation occurs over a number of months.

Potassium is often first harvested from early ponds, while later ponds have increasingly high concentrations of lithium. Economical lithium-source brines normally contain anywhere from a few hundred parts per million (ppm) of lithium to upwards of 7,000 ppm.

When the lithium chloride in the evaporation ponds reaches an optimum concentration, the solution is pumped to a recovery plant where extraction and filtering remove any unwanted boron or magnesium. It is then treated with sodium carbonate (soda ash), thereby precipitating lithium carbonate. The lithium carbonate is then filtered and dried. Excess residual brines are pumped back into the salar.

Lithium carbonate is a stable white powder that is a key intermediary in the lithium market because it can be converted into specific industrial salts and chemicalsor processed into pure lithium metal.

In contrast to salarbrine sources, extraction of lithium from spodumene, lepidolite, petalite, amblygonite, and eucryptite requires a wide range of processes. Because of the amount of energy consumption and materials required, lithium production from mining is a much more costly process than brine extraction, even though these minerals have a higher lithium content than the saltwater.

Of the five minerals, spodumene is the most commonly used for lithium production. After it is mined, spodumene is heated to 2012 degrees Fahrenheit and then cooled to 149 degrees. It's then crushed and roasted again, this time with concentrated sulfuric acid. Ultimately, sodium carbonate, or soda ash, is added, and the resulting lithium carbonate is crystallized, heated, filtered, and dried.

Several companies are exploring the extraction of lithium from clay in Nevada, including American Lithium and Noram Ventures. The companies are testing different production methods, including sulfuric acid leaching.

Converting lithium into metal is done in an electrolytic cell using lithium chloride. The lithium chloride is mixed with potassium chloride in a ratio of 55% to 45% in order to produce a molten eutectic electrolyte. Potassium chloride is added to increase the conductivity of the lithium while lowering the fusion temperature.

When fused and electrolyzed at about 840 degrees Farhenheit, chlorine gas is liberated while molten lithium rises to the surface, collecting in cast-iron enclosures. The pure lithium produced is wrapped in paraffin wax to prevent oxidization. The conversion ratio of lithium carbonate to lithium metal is about 5.3 to 1.

The top five countries for lithium production in 2018 were Australia, Chile, China, Argentina, and Zimbabwe. Australia produced 51,000 metric tons of lithium that year, the latest for which figures are available. Total global production, excluding the U.S., amounted to 70,000 metric tons.

electrolytic refining

electrolytic refining

The list of pure non-ferrous metals so widely used in modern industry includes many that are produced by electrolytic means, such as copper, zinc, nickel, aluminum,magnesium, lead, sodium, cadmium, calcium and many others. In the case of some metals, ordinary fire or chemical methods of production are important, but in others, 100 per cent of the metal used is produced by electrolysis. The commercial production of electrolytic metals had its origin a century ago when James Elkington, an English electroplater, invented a process for refining copper electrolytically; later, about 1890, aluminum was first produced on a commercial scale by electrolysis, followed by lead in 1905, nickel in 1910, and zinc about 1915.

There are a number of reasons why metals produced by electrolysis have played such an important part as they have in modern industry. In the first place, metals produced in this way are usually exceptionally pure; the purities shown in the following table are obtainable in commercial practice.

The peculiar properties of high-purity metals include: extraordinary high resistance to corrosion, high malleability, high electrical conductivity, and others of a similar nature. High-purity lead gives exceptional service in chemical plant construction, the zinc die-casting industry depends upon high-grade zinc metal, high conductivity copper is essential in the electrical industry, pure nickel finds a multitude of uses due to its special physical properties, re-refined aluminum is extremely soft and malleable, has a brilliant mirror surface which will not dull or corrode, and is very difficulty soluble in acids or alkalies. Such properties are most readily and economically developed by producing the metals electrolytically.

A second reason for the employment of electrolytic methods is that, often, they provide the most economical method of separating a valuable metal from the gangue, slag, or other metals with which it is combined. Also, in the case of metals standing high in the electromotive series, such as sodium and aluminum, electrolysis provides the only practical means for breaking down the oxidized compounds and preparing the metal in reduced, metallic form.

All electrolytic operations depend upon two basic factors: the first is the volume or quantity factor which is related to amperes; the second is the energy or pressure factor which is related to volts.

The quantity of metal deposited from an electrolyte is in accordance with the laws enunciated by Faraday, which state in effect: (1) that the quantity is proportional to the number of ampere-hours, and (2) that a given number of ampere-hours will deposit an equivalent amount of any metal, the equivalent being determined by dividing the atomic weight by the valence. Experiment and calculation prove the validity of Faradays laws and show that, if a current of 1000 amperes passes through a cell for one hour (or one ampere for 1000 hourshowever one wishes to figure it), the following weights of metal will be deposited if the currentand cell efficiencyis 100 per cent:

From the table it is evident that the same amount of current that will deposit 8.5 pounds of lead will deposit only 6 ounces of beryllium or 12 ounces of aluminum. The current efficiency of an electrolytic cell can be calculated by comparing the amount of metal actually deposited by a current passing through the cell for a given time with the amount which should be deposited,according to Faradays law, by the same current passing for the same length of time.

The voltage required to operate a cell is a different matter entirely, as it is dependent on several factors such as electrolyte resistance, and contact resistances, as well as the decomposition voltage. The latter factor cannot be simply expressed, but may be said to represent, in electrical units, the heat units which must be supplied to decompose a compound such as copper sulphate or aluminum oxide in such a way that the energy content of the component elements is restored to the level existing before the compound was originally formed. For each compound and electrolyte, a definite potential or pressure must be provided before any current (amperes) will flowthis is the decomposition voltageand, once it is reached, the current continues to flow at the constant pressure of the decomposition voltage, the metal being deposited according to Faradays law. The voltages usually required to cause current to flow through a cell are low in aqueous electrolytes and only slightly higher in molten electrolytes, despite the fact that the resistance of molten electrolytes is sufficient to generate the heat required to keep the electrolyte fluid. The conditions of electrolysis vary so greatly for different metals and different electrolytes that no set relations can be given, though it may be noted that cell voltages vary from about 0.25 v. for electrolytic copper refining with soluble anodes to about 5 v. for aluminum reduction with carbon electrodes.

it is obvious that the two factors are multiplied. It is possible to calculate I accurately, but as E is dependent on a number of factors, W can be determined only by trial. As metal is deposited by direct current, the power requirement of an electrolytic plant must also include line-losses, rectifier losses and transformer losses. The average power or energy requirements for some metals is as follows:

The field of electrolytic production of metals can be conveniently separated in two main divisions which may be termed, briefly, electro-refining and electro-winning. The operations are similar in that both employ direct current, which passes from anodes to cathodes in a suitably arranged cell containing an electrolyte, and in that in both types of operation, pure metal is deposited on the cathode. The essential difference between the two processes is that in electro-refining an impure alloy anode is used which is dissolved by the action of the current, thus replenishing in the electrolyte an amount of metal approximately equivalent to the amount deposited at the cathode: in electro-winning, however, the anode is made of some material such as lead or carbon which is insoluble in the electrolyte, making it necessary to replenish the metal content of the electrolyte by the addition of a metal compound which is soluble in the electrolyte. Though the amount of refined metal deposited per ampere-hour is the same in both processes, electro-winning requires more energy per pound of metal deposited than does electro-refining, and cell voltages are therefore much higher for electro-winning operations, with resulting higher power consumption (note second half of table above). In electro-refining, when the cell is operating properly, no gases are evolved at either anode or cathode: in electro-winning, a gas is always evolved at the anode when the cell is operating. (Commonly, oxygen or chlorine is thus produced.)

Usually the object of electrolytic refining is to separate one metal in pure form from an alloy containing a high percentage of the desired metal, copper for instance, and a number of other metals dissolved in the copper or intimately mixed with it. The impurities can be classified in two groups: (a) those metals which can be more easily oxidized than copper, e.g., iron, nickel, and (b) those metals less easily oxidized than copper e.g., gold, silver.

In the actual copper-refining operation, the impure alloy of copper, nickel, gold, etc., is cast in a thin, flat plate constituting the anode: the cathode is usually a sheet of pure electrolytic copper. Anodes and cathodes (30 to 40 of each) are immersed in a cell containing copper sulphate and sulphuric acid which serves as an electrolyte. The action of the electric current can be easily visualized of it is recalled that the action at the anode is always oxidizing and at the cathode reducing. The passage of the current tends to oxidize iron, nickel, and copper at the anode. No gold or silver will oxidize at the anode as long as there is metallic copper present. The metal oxides are soluble in the sulphuric acid of the electrolyte and, as sulphates, are free to move to the cathode. At the cathode the copper sulphate, being more easily reduced than other sulphates present, is broken down to copper metal and sulphuric acid. The sulphuric acid returns to the anode to dissolve more copper, thus completing the cycle. None of the other sulphates in the electrolyte will be decomposed at the cathode as long as there is copper sulphate for the current to work on. In this way, the copper is separated by preferential solution from gold and silver which remain at the anode in metallic form so finely divided that they form a slime; it is separated from iron and nickel at the cathode by preferential decomposition of the sulphate.

The energy relationships in a cell of this type are characteristic of those in all cells where soluble anodes are used. When a compound such as copper sulphate is formed from copper, oxygen, and SO3, a certain amount of energy is released or made available in the form of heat or equivalent electrical units. Conversely, when copper sulphate is decomposed to form copper, oxygen, and SO3 exactly the reverse is truean equal amount of energy (heat or electrical) is absorbed. In the cell with soluble anodes, both reactions are proceeding at the same time, so that energy required for decomposition is derived from the energy resulting from combination, the final result being that no energy is absorbed or dissipated. Hence, the only new electrical energy required to operate the soluble anode cell is represented by the voltage necessary to overcome polarization, cell resistance, etc. This voltage is relatively small in the case of copper sulphatesulphuric acid electrolytes.

Other impure metals are refined in exactly the same way, though it is necessary to adjust the electrolytes and other operating conditions to suit each individual metal, the essential requirement being that the electrolyte must be composed of solution of a soluble salt of the metal to be refined.

A number of electrolytic lead plants are in operation, separating lead from copper, antimony, bismuth, gold, silver, etc. In this operation the anode is only 98 per cent lead, with antimony, copper, gold, silver, etc., present as impurities. A solution of lead fluosilicate and hydro-fluosilicic acid forms the electrolytesulphuric acid would not do, as lead sulphate is insoluble. The action in the cell is, with obvious modifications, identical with that in the copper refining cell, with the exception that any tin present in the anode is dissolved with the lead and deposited on the cathode.

Nickel is refined electrolytically from metallic nickel or nickel sulphide anodes containing as impurities iron, copper, gold, silver, platinum, palladium etc. The electrolyte contains, principally, nickel sulphate in a nearly neutral solution. Extraordinary precautions must be taken to ensure that the electrolyte at the cathode is free of copper, iron, and other impurities. Each cathode is suspended in an individual porous- walled compartment to which pure electrolyte is fed at a predetermined rate. The impure electrolyte leaving the anodes is removed from the cell and is purified by removal of iron, copper, etc., before being returned to the cathode compartments for precipitation of pure nickel on the cathode. Otherwise, the basis of operations resembles that in copper refining.

The anodes used in refining base metals nickel, copper, leadby electrolysis usually contain gold, silver, platinum, palladium, etc., as well as selenium and tellurium in some amount. In every case, these metals, being less easily oxidized than the principal metal constituting the anode, are not dissolved by the electrolytic action, and remain at the anode in finely divided form. As the principal metalcopper, for instanceis largely dissolved, the weight of anode slime is small in relation to the weight of the original anode, and the previous metals (and selenium and tellurium) are concentrated in the slime, along with antimony, arsenic, lead, bismuth, etc.

The methods employed for separating and recovering the metals present in the slimes are determined by the composition of the slime in each individual case. The slime may be roasted, treated chemically, melted and re-electrolyzed, oxidized, leached, etc., with the ultimate object of recovering antimony, bismuth, selenium, tellurium andarsenic as by-products and producing an alloy composed principally of precious metals usually gold and silver, though platinum metals are often present in important amounts. The precious-metal alloys are then treated chemically and by electrolytic refining methods to separate and recover the individual metals.

The object of electro-winning operations is to decompose an oxidized compound of a metal and deposit a pure metal on the cathode, not only separating the metal from gangue, but actually reducing it to metallic form for the first time. (Normally, in electro-refining, the metal appears in metallic form in the anode, the necessary reduction having been affected by smelting or some similar operation.) Electro-winning thus provides a method of producing metal from an ore without smelting.

The operation of an electro-winning process requires that the metal to be recovered be in some form in which it can be dissolved in a suitable electrolyte, while the anodes must be composed of some material insoluble in the electrolyte. The cathodes may have any suitable form, but consist usually of the metal it is sought to recover.

The action in the cell is relatively easily explained, and can perhaps best be illustrated by theproduction of electrolytic zinc. In this operation, a neutral solution of zinc sulphate is fed to the cell which contains a series of lead anodes and zinc-covered cathodes. The electric current leaving the anode would tend to oxidize the lead, but as lead oxide is not soluble in sulphuric acid, the surface of the anode becomes inert and water from the electrolyte is decomposed to liberate oxygen gas and hydrogen ions. At the cathode, the reducing action of the current decomposes zinc sulphate, depositing metallic zinc on the cathode and leaving sulphate ions in solution. Thus neutral zinc sulphate solution entering the cell is converted to metallic zinc, oxygen and sulphuric acid (hydrogen ions and sulphate ions). The sulphuric acid so generated is overflowed from the cell and is used to dissolve zinc oxide made by roasting zinc sulphide concentrates.

The acid dissolves zinc oxide in preference to iron oxide, but some other oxides also dissolve in the acid, and the solution must be purified before being returned to the cell as neutral zinc sulphate, where the cycle is repeated.

The essence of the operation is that zinc oxide is chemically dissolved, enters the cell where oxygen and zinc are produced and acid regenerated to dissolve more oxide. Basically, nothing more or less has happened than that the cell provides a mechanism for the electric current to supply the energy necessary to decompose ZnO to its elementsZn and O. This is the fundamental difference between electro-winning and electro-refining, and it is this energy requirement that necessitates higher voltages in the electro-winning operation. As electricity is purchased on the basis of volts and amperes, and as the ampere requirement is constant (Faradays Law), the higher voltage means higher power cost. For this reason, electro-winning operations are almost invariably located in low-cost power areas.

Some of the worlds copper is produced by leaching and electrolysis. The basis is exactly the same as described for zinceven to the use of a sulphatesulphuric acid electrolyte. Nickel is not produced by this means, due to the insolubility of nickel oxide. However, the process could easily be employed if nickel carbonate, for example, were available as a raw material. Cadmium is recovered from zinc plant by-products by an electro-winning process resembling the zinc process.

Aluminum, Magnesium: Aluminum and magnesium are both produced by electro-winning methods100 per cent of the aluminum and most of the magnesium is made in this way. Neither of these metals can be reduced from compounds in an aqueous solution, so molten salt electrolytes are employed. Basically, the operations are the same as those already described, but the techniques are quite different.

In the case of aluminum, the anode is composed of carbon, the cathode is a bath of molten aluminum at the bottom of the cell, and the electrolyte consists of molten sodium-aluminum fluorides in which is dissolved pure Al2O3. The pure Al2O3 is prepared by chemical means and is added to the cell at regular intervals. The electric current decomposes the Al2O3,depositing pure aluminum on the cathode, while the oxygen released at the anode burns the carbon to form CO2 which escapes from the cell. The anode is thus insoluble in the electrolyte, but is constantly being consumed and must be continuously renewed. There are no by-products from the cell.

The production of magnesium is rather different. The most satisfactory electrolyte is molten magnesium chloride mixed with an alkayli-chloride, usually sodium chloride. A graphite anode is used and a steel cathode the latter serving merely to conduct the current away from the cell. Magnesium metal is released from the chloride at the surface of the steel cathode and, as the metal has a lower specific gravity than the chloride, it rises to the surface of the molten electrolyte, there to be removed by ladling. The chlorine generated at the anode is conducted through pipelines to a furnace where it is reacted with carbon and MgO to produce anhydrous MgCl2, which in turn is charged to an electrolytic cell, thus completing the cycle. Where the Mg is produced from seawater, the chlorine is reduced to hydrochloric acid which is used for conversion of Mg(OH)2 to MgCl2.

In electro-winning processes, only those metals soluble in the leach solution are recovered by electrolysis. If an ore contains precious metals, it is most unlikely that they will be recovered, as, for instance, the dilute sulphuric acid used for leaching copper and zinc ores and calcines do not dissolve gold or silver. The application of electro-winning methods is, therefore, usually limited to ores which do not contain precious metals in sufficient amount to justify their recovery, to light-metal ores which usually contain no precious metals, or to relatively high-grade materials such as concentrate, mattes, and by-products in which the precious metal content is so high that the residue from leaching can be further processed by smelting methods to recover the gold and silver present.

lithium production processes - sciencedirect

lithium production processes - sciencedirect

Technologies used for producing lithium chemicals and lithium metal from mineral sources, salt lake, salar brines, saline water, etc., are reviewed in this chapter. Processes treating lithium-bearing hard rocks normally involve first thermal treatment of these rocks at high temperature, followed by water leaching to release lithium values into solution. When salt lake or salar brines are used to recover lithium, solar evaporation is commonly used to concentrate lithium and precipitate salts of major elements such as K, Na, Mg, Ca, etc. Leach liquors or concentrated brines are then further treated using precipitation, ion exchange, etc., to remove residual contaminants. Carbonation using soda ash or carbon dioxide is preferred to precipitate lithium carbonate as the final product whereas lithium hydroxide is frequently recovered via electrodialysis and crystallization. These products usually are of battery grade (99.5% purity) and could be further processed to produce high purity compounds (>99.9%) by redissolution, ion exchange, and reprecipitation steps. Salar brines are currently used as dominant feedstock for the production of lithium compounds around the world principally due to low operation cost and high reserves as compared to those from mineral sources. A brief evaluation of the economics of processing brines and spodumene ores from several commercial projects is also provided in this chapter.

electrolytic cell - definition, diagram, working, applications, faqs

electrolytic cell - definition, diagram, working, applications, faqs

An electrolytic cell can be defined as an electrochemical device that uses electrical energy to facilitate a non-spontaneous redox reaction. Electrolytic cells are electrochemical cells that can be used for the electrolysis of certain compounds. For example, water can be subjected to electrolysis (with the help of an electrolytic cell) to form gaseous oxygen and gaseous hydrogen. This is done by using the flow of electrons (into the reaction environment) to overcome the activation energy barrier of the non-spontaneous redox reaction.

The electrolyte provides the medium for the exchange of electrons between the cathode and the anode. Commonly used electrolytes in electrolytic cells include water (containing dissolved ions) and molten sodium chloride.

Here, two inert electrodes are dipped into molten sodium chloride (which contains dissociated Na+ cations and Cl anions). When an electric current is passed into the circuit, the cathode becomes rich in electrons and develops a negative charge. The positively charged sodium cations are now attracted towards the negatively charged cathode. This results in the formation of metallic sodium at the cathode.

Simultaneously, the chlorine atoms are attracted to the positively charged cathode. This results in the formation of chlorine gas (Cl2) at the anode (which is accompanied by the liberation of 2 electrons, finishing the circuit). The associated chemical equations and the overall cell reaction are provided below.

Frequently Asked Questions FAQsWhat are the key differences between electrolytic cells and Galvanic cells? The cell reactions of electrolytic cells are non-spontaneous whereas the cell reactions of Galvanic cells are spontaneous. Galvanic cells generate electrical energy from chemical reactions whereas electrolytic cells generate non-spontaneous redox reactions from an input of electrical energy. What are the three primary components of electrolytic cells? The three main components of electrolytic cells include the cathode, the anode, and the electrolyte. In electrolytic cells (as is the case in most electrochemical cells), oxidation occurs at the anode and reduction occurs at the cathode. What kinds of charges are held by the electrodes of electrolytic cells? In electrolytic cells, the cathode is negatively charged and the anode is positively charged. The positively charged ions flow towards the cathode whereas the negatively charged ions flow towards the anode. What are the uses of electrolytic cells? Electrolytic cells can be used to produce oxygen gas and hydrogen gas from water by subjecting it to electrolysis. These devices can also be used to obtain chlorine gas and metallic sodium from aqueous solutions of sodium chloride (common salt). Another important application of electrolytic cells is in electroplating. How do electrolytic cells work? When an external electric current flows into the cathode of the electrolytic cell, the resulting negative charge attracts the dissociated positive ions present in the electrolyte. This results in the deposition of the positively charged ions onto the cathode. At the same time, the negatively charged ions flow towards the anode, which is positively charged.

The cell reactions of electrolytic cells are non-spontaneous whereas the cell reactions of Galvanic cells are spontaneous. Galvanic cells generate electrical energy from chemical reactions whereas electrolytic cells generate non-spontaneous redox reactions from an input of electrical energy.

The three main components of electrolytic cells include the cathode, the anode, and the electrolyte. In electrolytic cells (as is the case in most electrochemical cells), oxidation occurs at the anode and reduction occurs at the cathode.

In electrolytic cells, the cathode is negatively charged and the anode is positively charged. The positively charged ions flow towards the cathode whereas the negatively charged ions flow towards the anode.

Electrolytic cells can be used to produce oxygen gas and hydrogen gas from water by subjecting it to electrolysis. These devices can also be used to obtain chlorine gas and metallic sodium from aqueous solutions of sodium chloride (common salt). Another important application of electrolytic cells is in electroplating.

When an external electric current flows into the cathode of the electrolytic cell, the resulting negative charge attracts the dissociated positive ions present in the electrolyte. This results in the deposition of the positively charged ions onto the cathode. At the same time, the negatively charged ions flow towards the anode, which is positively charged.

electrolytic refining: silver - gold - copper

electrolytic refining: silver - gold - copper

The refinery takes the bullion purchased by the receiving department, and carrying more than 200 parts of precious metals in 1,000, or, in mint parlance, over 200 fine, and separates and refines the various metals contained therein, using electrolytic processes exclusively.

The residue from the silver-cells, together with crude gold bullion, is treated in cells having a chloride electrolyte. These produce fine gold and leave a residue containing silver chloride. The latter is reduced to the metallic state with zinc and is then treated in the silver-cells.

The various waste solutions and the wash-waters, after being freed from the bulk of their precious metals, still contain copper and other metals. These are removed by scrap-iron, and are then treated in the copper-cells, having a sulphate electrolyte. These cells produce pure copper, and collect a residue containing lead, some gold and silver, and all the metals of the platinum group that were in the bullion. This residue is relatively small, and is melted into bars and stored until sufficient accumulates to warrant treating it for platinum, etc.

The refinery occupies three large and three small rooms. The large ones are, a melting-room, 30 by 34 ft.; a cell-room, 39 by 46 ft.; and a washroom, 30 by 33 ft. The small rooms are used as foremans office, laboratory, and generator-room, respectively.

A. The Anodes are made up of crude silver-bullion, together with gold-bullion that is too low in gold to be easily made up into gold anodes. The endeavor is to make a mixture, such that the anodes will run about 600 thousandths in silver, 300 thousandths in gold, and the remaining 100 thousandths in base-metals. The metal is melted in No. 100 graphite crucibles, in Rockwell melting-furnaces of the open-top mint type, heated with crude oil. A drawing of these furnaces is given in Fig. 2. The furnaces are used for melting both the crude metals for the anodes, and the fine gold- and silver-products of the refinery that are to be cast into bars. Fig. 3 is a view of the melting-room. In the background are the furnaces; in the foreground, to the left, is a truck-load of anodes; in the center a truck loaded with gold bars (dark), and behind it a truck loaded with silver bars (white).

The anodes are cast in open cast-iron molds, and are of the dimensions given in Fig. 4. They are suspended from the conductors by C-shaped hooks of gold, which pass through the hole at the top of the anodes and over bars which form the conductors for the current. The anodes are immersed for their full depth in the electrolyte.

B. The Cathodes are made of sheets of silver, 1000 fine, 0.051 in. thick (No. 16 B. & S. gauge) and 4 in. wide. They are immersed for 8.5 in. in the electrolyte, and are bent over at the top so as to hang from the conductors.

The crystallized silver that collects on the cathodes is loose and is removed daily. To facilitate this stripping, the cathode sheets are treated with a dope, consisting of silver nitrate, copper nitrate, and hydrochloric acid, all mixed together, and painted on with a rag. The sheets are then dried in the dry-room. One dose of this dope lasts two or three months; then the deposits begin to stick, and the plates are re-treated.

C. The Electrolyte consists of water with 3 per cent, of silver, as silver nitrate, from 1.5 to 2.5 of free nitric acid, and a little glue. The latter is dissolved and poured in as a thick liquid. The effect of the glue is to toughen the deposit of silver on the cathode.

The electrolyte dissolves and retains the copper and other soluble base metals. These do no harm until the solution becomes so strong that the purity of the silver deposited on the cathodes is affected, when it has to be changed.

D. The Cells are of brown earthenware and their dimensions are shown in Fig. 5. Experience has shown that they are too shallow for advantageous work. There is only a small space between the bottom of the cell and the lower end of the anodes, and the slimes that collect in this space soon cause short-circuits which stop the action of the cell. A new set of cells, 18 in. deep inside, instead of 12 in., is about to be installed. These deeper cells will allow longer cathodes to be used, and, since the cores that have to be re-treated will be of the same size, there will be a reduction in the percentage of metal to be re-treated.

The cells are placed end to end in a double row on two long benches, 12 on one bench and 6 on the other. This allows all the cells to be easily inspected and attended to, from one side or the other of the benches. These cells are the dark ones on the second and third benches in Fig. 6.

The anodes and cathodes are hung in alternate rows from maple strips, 2 1/8- in. apart from center to center, which extend across the cells. Along the top of each is laid a gold strip, bent into the form of an inverted trough. These gold strips are connected by screws alternately to the positive and negative busbars, and form the conductors. There are 19 of these across each cell, 10 supporting four cathodes each and 9 supporting four anodes each. The bus-bars are of copper and extend along the main wooden frame that covers the top of the entire bench of cells. All woodwork and the copper bars are coated with biturine solution, an asphaltic paint that comes from Australia, to protect them from the action of the acids.

The solution in the cells is kept in motion by two glass propellers in each cell. This prevents the heavier solutions from settling to the bottom, and makes the deposition uniform over the whole cathode.

Each propeller, 2 in. across, is made in one piece with a glass rod, which runs up vertically between the electrodes, and is driven by a cord running in a grooved pulley at its top. The vertical glass rods, as well as the line-shaft, are carried by a wooden frame above the cells, as shown in Fig. 6.

E. The Current is a direct one of 15 volts, and passes through the 18 cells in series, as shown in Fig. 7. The amount of current is such as to give a density of 8.3 amperes per square foot of cathode-surface. There are 40 cathodes per cell and each has a normal immersion of 8.5 in. The end rows of cathodes have only one effective surface, so the total cathode-surface per cell is:

Fig. 8 is a view of the generator-room and shows the machines and the switch-board. The generators are driven by current obtained from a public power-line and furnish direct current of the required potential for the different operations.

F. Centrifugal Machines are used to separate the moisture from the different products of the refining process, and to wash them free from soluble matter. There are two of these machines. No. 1 belongs primarily to the silver process, and is used exclusively for silver or products charged with nitric compounds. No material containing chlorides is ever placed in it. Centrifugal No. 2 is similar to No 1, but is reserved for the gold process and for solutions carrying chlorides.

The rotors of the centrifugals are of earthenware and provided with ducts for the escape of the liquids. When in use, the rotor is lined with one thickness of 7-oz. duck, and in this bag is placed the material to be treated. A different filter-bag is kept for each different kind of material that is washed.

Briefly, the anodes are dissolved; pure silver collects on the cathodes; copper and other metals forming soluble nitrates go into the bath, and gold and other insoluble metals are left as a sponge on the anodes.

As the dissolving action progresses, the anodes are taken out at intervals and the sponge of insoluble metals is shaken off into an earthenware jar, by knocking them against its sides. This spongy material is crude or black gold with about 10 per cent, of silver and 1 per cent, of base metals. After washing in centrifugal machine No. 2, it is melted into anodes for the gold process.

When the anodes are eaten down so that they barely hold together (which takes about 48 hr), they are removed, all the loose spongy material is knocked off, and the hard cores that remain are treated in the horizontal cells, to be described later. New anodes are then hung in their places.

The electrolyte is tested at intervals to determine its strength in silver, and if this test shows that the bath is too low in silver, its strength is brought up by adding strong silver nitrate solution.

The test for silver is made by gradually adding a standardized solution of ammonium thiocyanate, NH4SCN, to a sample of the bath, a little ferric sulphate solution having been previously added as an indicator. When all the silver has been precipitated, the ferric salt gives a red color. This is Volhards method, and is given in detail by Sutton.

When the bath contains about 8 per cent, of copper it has to be changed, since the silver deposited on the cathodes begins to be contaminated with the copper. This spent electrolyte is treated in the scrap-copper tank to recover the silver, and then passes on to the scrap-iron tank, where the other metals contained in it are caught, as will be described under the head of Copper-Refining.

The pure silver collects in a crystalline condition on the cathodes, which are lifted out daily and cleaned over large porcelain jars. At first, the deposit is loose and fern-like, and most of it can be removed by knocking the cathodes against the sides of the jars. Gradually a firmer deposit collects that will not knock off, and this has to be removed with a scraper, when it comes away in sheets and leaves the cathode entirely clean. This pure silver is washed in centrifugal machine No. 1 until free from acid and soluble salts, and then is whirled until dry enough for melting, when it is made into fine bars.

A second product of this process consists of the slime that accumulates in the bottom of the cells. This contains black gold that has dropped from the anodes, as they dissolved, and also crystalline silver that failed to stick to the cathodes. This slime is transferred to the horizontal cells for re-treatment.

The operation in the horizontal silver-cells is the same in principle as in the vertical, but the mechanical details are different. There are two independent sets of the horizontal cells, each having three cells in series. These show at the right-hand end of the first and second benches in Fig. 6. The anodes consist of the cores of the silver anodes from the vertical cells, the slime from the bottom of the vertical silver-cells, and the silver reduced from the silver chloride slime from the gold-cells. These materials are contained in a wooden basket or tray. The current is led into this mass by a candle, made of equal parts of gold and silver, the lower end of which is buried in the material. The cathodes consist of graphite plates on the bottom of the cells. The crystalline metallic silver is deposited on these cathodes, and is removed at intervals with a long-handled dipper of hard rubber. The electrolyte is the same as that of the vertical silver-cells. The current, about 50 amperes, passes through the three cells in series. This gives a current- density of 14.3 amperes per square foot of cathode surface, and requires a potential of 5 volts per cell, or a total of 15 volts.

The baskets are made of maple, and all the joints are dovetailed, so that there is no metal in their construction. The bottoms are made with slats, and the baskets are painted all over with biturine solution. They are considerably smaller than the cells, so that the deposited silver can be scraped and gathered from the cathodes through the space between a basket and the side of its cell.

The material to be treated is retained on five layers of 7-oz. duck placed in each basket, and the edges are brought up on all sides above the top of the basket. This cloth shows as a white frill around the tops of these cells in Figs. 6 and 14. The baskets are suspended in the electrolyte by cleats resting on the tops of the cells.

The material left in the basket, after all the silver has been dissolved, is crude or black gold, and is transferred to centrifugal machine No. 1 and washed. It is then dried in the dry-room, melted, and used with other metal to make gold anodes for the gold process.

The spent electrolyte from both the vertical and the horizontal cells contains silver nitrate and the soluble nitrates of the base metals that were in the original bullion. These solutions and the nitric wash-waters from the centrifugal machine are passed over scrap-copper suspended in wooden tanks, which precipitates the silver and leaves the base nitrates in solution. These tanks are in the washroom, as shown in Fig. 9.

The precipitated silver is washed and dried in centrifugal machine No. 1, and then is melted and cast into bars. These are added to melts of low-grade gold and made into silver anodes for the vertical silver-cells. At times, this precipitated silver has been dissolved in nitric acid to make silver nitrate for the electrolyte, but it is often impure, and a better electrolyte is obtained by dissolving pure silver; hence the practice is not common.

A. The Anodes, of the same size as the silver ones shown in Fig. 4, are made from high grade gold-bullion and crude gold- products from both the gold and the silver refining processes. They carry about 90 per cent, of gold, and it is desirable that the silver-content be limited to about 7 per cent., since a greater amount interferes with the operations. Copper is less objectionable than silver. The metal for the anodes is melted in the furnaces shown in Figs. 2 and 3. The anodes, hung by C-shaped hooks of pure gold from the conductors running across the top of the cells, are immersed 7.5 in. in the electrolyte.

B. The Cathodes, strips of pure gold 4 in. wide by 0.012 in. thick (No. 28 B. & S. gauge), weigh about 4.5 oz. They are bent over at the top, so that they can be hooked over the conductors crossing the top of the cells. They are immersed to a depth of 6 in., and are allowed to remain in the cells until they weigh about 160 oz., when they are removed and used as the anodes for the second set of cells. By this re-deposition the fineness of the final product is raised to about 999.7.

The gold is deposited on the cathodes so tightly that stripping is impracticable, and when the final cathodes have been formed, the deposit with its original cathode sheet is all melted down together. Hence, the original strips have to be made of pure gold in order to maintain the quality of the product.

C. The Electrolyte is a trichloride solution, carrying in the first set of cells 70 g. of gold per liter, and from 10 to 12 per cent, of free hydrochloric acid, and in the second set, only 60 g. of gold per liter, but with the same amount of acid.

During the operation, the electrolyte decomposes and drops particles of metallic gold, which collect in the slimes. This lowers the strength of the solution in gold, and when it gets below 4 per cent, of gold, the deposit on the cathode is soft and tends to crumble. To prevent this, the bath is tested daily to determine its strength in gold, and if found to be low, is restored to the desired standard by the addition of strong solution.

The test of the electrolyte for gold is made with ferrous ammonium sulphate. A solution of this salt is made up of such strength that 1 cc. of it will precipitate 27.5 g. of gold. Then, to a liter of electrolyte is added 3.5 cc. of Fe (NH4)2 (SO4)2 solution, which is capable of precipitatiug 96.25 g. of gold more than the bath is likely to contain. The excess of the ferrous salt is then determined by titrating with potassium permanganate, using a solution such that 1 cc. of K2Mn2O8 will oxidize 1 cc. of Fe (NH4)2(SO4)2. On dropping the permanganate into the solution, its purple color is destroyed as long as any of the ferrous salt remains, but when the latter is completely oxidized, an additional drop will retain its color, indicating the end of the reaction.

After a week, the electrolyte becomes spent and takes on a dirty dark-green color, due to the accumulation of copper-salts in the solution. When it reaches this condition, the gold-deposit on the cathodes is soft, and the electrolyte has to be changed.

The gold chloride for the electrolyte is made by dissolving gold-bullion in hydrochloric acid by the aid of an electric current. Anodes of gold 990 fine are hung in strong hydrochloric acid, in five cells slightly larger than those used for the gold-refining process, and the cathodes, also of gold, are hung in porous cups filled with strong hydrochloric acid. On passing a current of 500 amperes at 25 volts through the cells, the anodes are dissolved, giving a solution of gold trichloride in the cells; but, owing to the porous cups, there is no gold deposited on the cathodes. Since hydrochloric acid fumes are

liberated in the process, it is performed under a glass-inclosed hood connected to a flue, shown in the right background in Fig. 6. The gold chloride solution obtained from these cells has a strength of from 375 to 500 g. of gold per liter.

D. The Cells are of white royal Berlin porcelain, and have the dimensions shown in Fig. 11. The electrolyte, like that in the silver-cells, already described, is kept in motion by one glass propeller in the center of each cell, revolved by a vertical glass rod.

The space between adjacent cells is covered with a porcelain strip about 1 by 3 in. in cross-section, clamped to the rim of the cells, and having a series of notches to receive the porcelain bars which support the conductors across the tops of the cells from which the electrodes are hung.

There are three rows of anodes and four rows of cathodes in each cell. The rows of anodes alternate with the rows of cathodes, and are 2 5/8 in. from center to center. There are two cathodes on each row, making eight cathodes per cell, and there are three anodes on each of two rows, but only two on the center row, making eight anodes per cell. The center anode is omitted to give room for the circulating propeller. The drive for the propellers is similar to that for the silver-cells. The arrangement of these parts is shown in Fig. 6, where the gold-cells (white) occupy the left foreground.

To the copper bus-bars, which are bolted to the top of the porcelain strips between the cells, are screwed the ends of the conductors that extend across the cells. These conductors are gold strips bent into an inverted trough shape, and fit the top of the porcelain cross-bars. The electrodes hang from these conductors.

E. The Current, a direct one of 15 volts potential, passes through the 14 cells of each set in series, as shown in Fig. 12, requiring nearly 1 volt per cell. The total amount of current is 180 amperes. There are eight cathodes in each cell in parallel, each having an immersed area of 4 x 6 in. = 24 sq. in. Four of the cathodes have both sides available for the reception of deposits and four have only one side available, thus making 12 cathode-surfaces of 24 sq. in. each, or a total of 2 sq. ft. The current being 180 amperes, the current-density is 90 amperes per square foot of cathode-surface.

F. Centrifugal Machine No. 2 is identical with No. 1, described under the silver process; but this one is used exclusively for gold-products and material charged with chloride waters, which would precipitate silver chloride if it came in contact with solutions of silver-salts. A different filter-bag is used for each kind of material. This machine is located in the wash-room (Fig. 9).

G. The Drying-Room is of brick, has an iron door, is heated with steam and is built into one corner of the cell-room. It is about 5 by 6 ft. It shows in the central background of Fig. 6. It is used to dry fine gold cathodes, and other gold-products, before charging them into the melting-pots.

H. Vats and Tubs.The vats used for the precipitation of the gold from the spent electrolyte are made of brown earthenware and stand on platform-trucks, for convenience in moving them about. They are 2 by 4 ft. in area, and 2 ft. deep.

The tub used for the reduction of the silver chloride to metallic silver, by means of zinc and sulphuric acid, is made of wood, and lined with lead. It is 2 by 4 ft., and 2 ft. deep, and mounted on a truck, similar to the earthenware ones.

Briefly, the anodes are dissolved in the electrolyte, and refined gold is deposited on the cathodes. All the metals in the anodes, including those of the platinum group, go into solution, except the silver and some lead. The last two form chlorides and drop to the bottom of the cells as the anodes dissolve. About 10 per cent, of the anodes is left as undissolved tops, and has to be remelted.

It is desirable that the anodes should not carry more than about 7 per cent, of silver. When more than this amount is present, the coating of silver chloride that forms on the anodes is thick enough to retard the dissolving action. When the anodes contain less than about 7 per cent, of silver, they can be treated in a single set of cells, and the gold-deposit on the cathodes will be considerably over 999 fine. But when more than 7 per cent, is present, so much silver chloride is formed at the anodes that, in dropping off, some of it is caught by the circulating currents, and carried mechanically to the cathodes, where it clings to the rough surface of the gold-deposit and lowers its fineness to less than 999. When handling such anodes high in silver, it has been found advisable to deposit the gold on the cathodes of one set of cells, and then transfer these cathodes, after washing them, to a second set of cells, where they are used as anodes and the gold is redeposited almost pure.

The gold anodes are made exclusively from the gold from the silver-cells, which assays about 875 thousandths gold, from 100 to 125 thousandths silver, and a small amount of base metals. This gives, in the first cells, cathodes about 998.7 fine, which, on being re-treated in the second set of cells, produce gold about 999.7 fine. It has generally been considered necessary to boil the crude gold from the silver-cells with concentrated sulphuric acid before casting it into anodes for the gold-cells, in order to reduce the silver to less that 7 per cent. The desire to do away with this acid treatment, and still produce a high grade of gold-deposit, led to the experiment of redepositing the first gold cathodes.

The same amount of current at the same voltage is used in both sets of cells. The electrolyte in the first set carrier 70 g., that of the second set 60 g. of gold per liter. With the exception of this difference in the strength of the electrolyte, the operation in both sets of cells is identical.

The gold cathodes from the second set of cells are carefully washed in a porcelain filter, dried in the dry-room, melted and cast into fine bars about 1,000 oz. in weight, which may be sold as mint bars, or alloyed with copper and made into coins.

The copper in the anodes goes into solution in the electrolyte ; and as long as the proper amount of gold is maintained in the solution, it does no harm until the amount reaches about 4 per cent., when the gold begins to deposit soft and fall from the cathode. Then the electrolyte has to be changed.

The metals of the platinum group also dissolve in the electrolyte; and while they occur in such small quantities in the bullion that they can hardly be detected, the quantity accumulated in the solution by the dissolving of many anodes is quite appreciable, and is recovered as described later, under Copper-Refining.

The silver in the anodes forms at the anodes insoluble silver chloride, a part of which, in the first set of cells, is removed at intervals by taking out the anodes and brushing and jarring off the silver chloride into an earthenware jar. Most of the silver chloride, however, drops to the bottom of the cells.

The slime in the bottom of the cells also contains metallic gold, which comes from the decomposition of the electrolyte, and does not deposit on the cathodes. This decomposition of the electrolyte seems to be due to the displacement of its gold by the copper dissolved from the anodes. In the first set of cells, with anodes containing 10 per cent, of silver, the slimes are about 600 thousandths gold and 800 thousandths silver, and in the second set, with anodes almost free from silver, they are 960 thousandths gold and only 40 thousandths silver.

The slimes from the bottom of the cells, and the silver chloride that has been removed from the anodes, are washed free from soluble chlorides in centrifugal machine No. 2, using hot water in order to carry off the lead chloride, and are treated in a lead-lined tub with granulated zinc, which precipitates the silver in a metallic condition, the zinc becoming zinc chloride. The granulated zinc is stirred into the mass of silver chloride and a little sulphuric acid is added to start the reaction. At first, the wet slime is a gelatinous mass characteristic of silver chloride, but as the reaction progresses it becomes more and more gritty. The mixture is tested towards the end of the process for the presence of silver chloride, and when there is no longer any present, sufficient sulphuric acid is added to dissolve any zinc that remains.

The test for silver chloride is made by treating a sample of the slime with ammonium hydrate, and then adding a few drops of hydrochloric acid to the clear solution. If there should be any silver chloride present, it would be dissolved by the ammonia, and would re-precipitate on adding the hydrochloric acid.

The granular silver with its gold-content, after being washed in centrifugal machine No. 2, to remove all soluble salts, is transferred to the anode-basket of the horizontal cells of the silver process for the recovery of the silver; and the gold is afterwards obtained from the basket-residue.

The wash-waters from the slimes and from the gold cathodes, together with the spent electrolyte from both sets of cells, are placed in earthenware vats, and a concentrated solution of ferrous sulphate is added to the liquid. This precipitates the gold, which is allowed to settle by long standing. The liquor, which still contains platinum-, copper-, and iron-salts, is decanted, and sent to the scrap-iron tank for further treatment, as described later under the head of Copper-Refining. The gold that remains after decantation is washed and dried in centrifugal machine No. 2, melted with low-grade bullion and cast into anodes, in which form it re-enters the process and is re-treated.

This process is used at the San Francisco Mint to work up the copper occurring as base metal in the bullion, and to recover the copper used to precipitate the silver from the various wash-waters. It is similar to the commercial process of copper-refining; but it is of special interest here, because the metals of the platinum group, taken into solution in the previous operations, have now accumulated in sufficient quantities to be recovered. Fig. 13 gives a diagram of the process.

The wash-waters and spent electrolyte from all parts of the refinery, from which the gold and silver have been recovered, are sent to the scrap-iron tank, and there deposit their copper, lead, and any precious metals, including those of the platinum group, that have escaped from the previous operations. This tank is in the wash-room (see Fig. 9).

The sludge of cement-copper from this tank is washed and drained in wooden tubs with filter bottoms, whence it is transferred to other filter-tubs and allowed to air-dry, and then is melted down and cast into anodes for refining.

The copper anodes contain lead derived from the silver-bullion, metals of the platinum, group derived from the gold-bullion, and small amounts of gold and silver. They are 5 by 14 in. by 3/8 in. thick, and are immersed 13 in. in the electrolyte.

The cathodes are started on sheets of lead 3.75 by 15 in., and when both sides have been coated with a copper-deposit of sufficient strength, the copper is stripped off the lead and returned to the cells. This does away with the repeated melting and rolling of sheet-copper cathodes, similar to those of the precious metals used in the gold and silver processes. The cathodes are immersed 11 in. in the electrolyte and receive deposits on both sides. When completed, these cathodes are washed free of the electrolyte, dried, and added to melts of coin-metal, without previous melting into bars.

The electrolyte is copper sulphate and contains 3 per cent, of copper as sulphate, and from 3 to 4 per cent, of free sulphuric acid. The cells are placed in a series of steps, so that the electrolyte flows through them by gravity. A steam-ejector lifts the electrolyte from the sump at the lower end and returns it to the head-tank, from which it again flows through the cells. These tanks are shown against the wall in Fig. 14.

The gold, silver, and metals of the platinum group are insoluble in the sulphate electrolyte, and drop to the bottom of the cells as slimes when the anodes are dissolved. These slimes are collected, washed with dilute sulphuric acid, dried, and melted into bars. These bars are stored until sufficient have accumulated, when they are treated for the separation of the various precious metals, especially those of the platinum group, that they contain.

The Treasury Department maintains five refineries for the treatment of the gold- and silver-bullion deposited at the various mints and assay offices. The original installation in each case was the nitric acid process of refining. This was succeeded some 30 years ago by the sulphuric acid process, which in turn is now being displaced by the electrolytic process.

The electrolytic process was installed in the Philadelphia Mint, and in the San Francisco Mint. It will be used in the New York Assay Office upon the completion of the new building; and the refinery of the New Orleans Mint, where the amount of work is comparatively small, will then be the only government refinery using the sulphuric acid process.

The mints and assay offices accept bullion carrying more than 200 thousandths precious metals. The refining-charges run from 1 cent an ounce on good silver-bullion, up to 8 cents an ounce on bullion carrying 800 thousandths base. The charges on ordinary gold-bullion average 4 cents per ounce. On account of these high charges on very base bullion, most of it is sent to private refineries, where the facilities for handling this grade of material are better, and the refining-charges are consequently less than at the mints.

In the silver process at the San Francisco Mint, the initial treatment of the bullion is in vertical cells. These are a modification, devised in the Philadelphia Mint, of the Moebius cells. The scraps from the vertical cells are re-treated in the horizontal cells, which are a modification of the Thom cells. Both types of cells have their advantages and disadvantages.

For refineries where the silver-bullion is the product of cupel-furnaces, and carries less than from 50 to 60 thousandths gold, and not more than from 10 to 20 thousandths base metal, there is no question as to the superiority of the horizontal process.

In mint-work the case is different. The bullion carries from 100 to 150 thousandths base and from 300 to 400 thousandths gold; the base requires an excess of acid to put it in solution, and the large amount of gold necessitates current for parting, in addition to that needed to dissolve the silver. The presence of the excess acid and of the heavy currents tends to destroy the filter-cloths quickly.

The gold process used at all the mints is the invention of Dr. Emil Wohlwill, of Hamburg, Germany, and was the outcome of experiments to separate platinum from gold. It was introduced by him into several refineries in Europe, and was first installed in this country in the Philadelphia Mint; but, so far as I know, no private refinery in this country is using it.

The electrolytic process of gold-refining possesses three advantages that are important in mint-work. First, it produces purer gold than the old processes. The elimination of the last trace of silver from the gold removes the brittleness from the ingots used for coinage, so that they roll and press much better than alloys of the same fineness in gold, but made of slightly impure gold. Second, the process permits the saving of all the platinum metals without serious inconvenience. Third, the operations do not give off, as did former processes, great quantities of acid fumes, such as used to cause frequent complaints from the people living in the vicinity of the mints, which were all located in cities.

The electrolytic process of gold-refining has three disadvantages as compared with the sulphuric acid process. First, it is more expensive. Second, more care and intelligence are required to conduct it. Third, the losses are liable to be greater on account of having gold in solution in the electrolyte.

In mint-work, the advantages more than offset the disadvantages ; but in commercial work, the advantages mentioned are of less importance, and the large amount of precious metal invested in the process, with the resulting loss of interest, would be almost prohibitory of its use. This feature in not so important to the government, as the metal so tied up may be considered as part of the gold reserve, and is accounted for at the time of annual settlements.

lead refining by electrolysis

lead refining by electrolysis

A solution of lead-fluosilicate, containing an excess of fluosilicic acid, has been found to work very satisfactorily as an electrolyte for refining lead. It conducts the current well, is easily handled and stored, non-volatile and stable under electrolysis, may be made to contain a considerable amount of dissolved lead, and is easily prepared from inexpensive materials. It possesses, however, in common with other lead electrolytes, the defect of yielding a deposit of lead lacking in solidity, which grows in crystalline branches toward the anodes, causing short circuits. But if a reducing-action (practically accomplished by the addition of gelatine or glue) be given to the solution, a perfectly solid and dense deposit is obtained, having very nearly the same structure as electrolytically-deposited copper, and a specific gravity of about 11.36that of cast-lead.

Lead-fluosilicate may be crystallized in very soluble, brilliant crystals, resembling those of lead-nitrate and containing four molecules of water of crystallization, with the formula PbSiF6,4H2O. This salt dissolves at 15 C. in 28 per cent, of its weight of water, making a sirupy solution of 2.38 sp. gr. Heated to 60 C., it melts in its water of crystallization. A neutral solution of lead-fluosilicate is partially decomposed on heating, with the formation of a basic insoluble salt and free fluosilicic acid, which keeps the rest of the salt in solution. This decomposition ends when the solution contains, perhaps, 2 per cent, of free acid; and the solution may then be evaporated without further decomposition. The solutions desired for refining are not liable to this decomposition, since they contain much more than 2 per cent, of free acid. The electrical conductivity depends mainly on the acidity of the solution.

My first experiments were carried out without the addition of gelatine to the fluosilicate solution. The lead-deposit consisted of more or less separate crystals that grew toward the anode, and, finally, caused short-circuits., The cathodes, which were sheet-iron plates, lead-plated and paraffined, had to be removed periodically from the tanks and passed through rolls, to pack down the lead. When gelatine has been added in small quantities, the density of the lead is greater than can be produced by rolling the crystalline deposit, unless great pressure is used.

The Canadian Smelting-Works, Trail, British Columbia, have installed a refinery, making use of this process. There are 28 refining-tanks, each 86 in. long, 30 in. wide and 42 in. deep, and each receiving 22 anodes of lead-bullion with an area of 26 by 33 in. exposed to the electrolyte on each side, and 23 cathodes of sheet-lead, about 1/16 in. thick, prepared by deposition on lead-plated and paraffined-iron cathodes. The cathodes are suspended from 0.5 by 1 in. copper-bars, resting crosswise on the sides of the tanks. The experiment has been thoroughly tried, of using iron-sheets to receive a deposit thicker than 1/16 in.; that is, suitable for direct melting without the necessity of increasing its weight by further deposition as an independent cathode; but the iron-sheets are expensive, and are slowly pitted by the action of the acid-solution; and the lead-deposits thus obtained are much less smooth and pure than those on lead-sheets.

The smoothness and the purity of the deposited lead are proportional. Most of the impurity seems to be introduced mechanically through the attachment of floating particles of slime to irregularities on the cathodes. The effect of roughness is cumulative : it is often observed that particles of slime attract an undue amount of current, resulting in the lumps seen on the cathodes. Samples taken at the same time showed from 1 to 2.5 oz. silver per ton in rough pieces from the iron cathodes, 0.25 oz. as an average for the lead-sheet cathodes, and only 0.04 oz. in samples selected for their smoothness. The variation in the amount of silver (which is determined frequently) in the samples of refined-lead is attributed not to the greater or less turbidity of the electrolyte at different times, but to the employment of new men in the refinery, who require some experience before they remove cathodes without detaching some slime from the neighboring anodes.

Each tank is capable of yielding, with a current of 4,000 amperes, 750 lb. of refined-lead per day. The voltage required to pass this current was higher than expected, as explained below ; and for this reason, and also because the losses of solution were very heavy until proper apparatus was put in to wash thoroughly the large volume of slime produced (resulting in a weakened electrolyte), the current used has probably averaged about 3,000 amperes. The short-circuits were also troublesome, though this difficulty has been greatly reduced by frequent inspection and careful placing of the electrodes. At one time, the solution in use had the following composition in grammes per 100 c.c.: Pb, 6.07; Sb, 0.0192; Fe, 0.2490; SiF6, 6.93, and As, a trace. The current passing was 2,800 amperes, with an average of about 0.44 volts per tank, including bus-bars and contacts. It is not known what was the loss of efficiency on that date, due to short-circuits; and it is, therefore, impossible to say what resistance this electrolyte constituted.

Hydrofluoric acid of 35 per cent., used as a starting-material for the preparation of the electrolyte, is run by gravity through a series of tanks for conversion into lead-fluosilicate. In the top tank is a layer of quartz 2 ft. thick, in passing through which the hydrofluoric acid dissolves silica, forming fluosilicic acid. White-lead (lead-carbonate) in the required quantity is added in the next tank, where it dissolves readily and completely with effervescence. All sulphuric acid and any hydrofluoric acid that may not have reacted with silica settle out in combination with lead as lead-sulphate and lead-fluoride. Lead- fluosilicate is one of the most soluble of salts; so there is never any danger of its crystallizing out at any degree of concentration possible under this method. The lead-solution is then filtered and run by gravity into the refining-tanks.

The electrical resistance in the tanks was found to be greater than had been calculated for the same solution, plus an allowance for loss of voltage in the contacts and conductors. This is partly, at least, due to the resistance to free motion of the electrolyte, in the neighborhood of the anode, offered by a layer of slime which may be anything up to 0.5 in. thick. During electrolysis, the SiF6 ions travel toward the anodes, and there combine with lead. The lead and hydrogen travel in the opposite direction and out of the slime; but there are comparatively few lead ions present, so that the solution in the neighborhood of the anodes must increase in concentration and tend to become neutral. This greater concentration causes an E. M. F. of polarization to act against the E. M. F. of the dynamo. This amounted to about 0.02 v. for each tank. The greater effect comes from the greater resistance of the neutral solution with which the slime is saturated. There is, consequently, an advantage in working with rather thin anodes, when the bullion is impure enough to leave slime sticking to the plates. A compensating advantage is found in the increased ease of removing the slime with the anodes, and wiping it off the scrap in special tanks, instead of emptying the tanks and cleaning out, as is done in copper-refineries.

It is very necessary to have adequate apparatus for washing solution out of the slime. The filter first used consisted of a supported filtering-cloth with suction underneath. It was very difficult to get this to do satisfactory work by reason of the large amount of fluosilicate to be washed out with only a limited amount of water. At the present time the slime is first stirred up with the ordinary electrolyte several times, and allowed to settle, before starting to wash with water at all. The Trail plant produces daily 8 or 10 cu. ft. of anode residue, of which over 90 per cent, by volume is solution. The evaporation from the total tank-surface of something like 400 sq. ft. is only about 15 cu. ft. daily; so that only a limited amount of wash-water is to be usednamely, enough to replace the evaporated water, plus the volume of the slime taken out.

The tanks are made of 2-in. cedar, bolted together and thoroughly painted with rubber-paint. Any leaks are caught underneath on sloping-boards. Solution is circulated from one tank to another by gravity, and is pumped from the lowest to the highest by means of a wooden pump. The 22 anodes in each tank together weigh about 3 tons, and dissolve in from 8 to 10 days, two sets of cathodes usually being used with each set of anodes. While 300-lb. cathodes can be made, the short-circuiting gets so troublesome with the spacing used that the loss of capacity is more disadvantageous than the extra work of putting in and taking out more plates. The lead-sheets used for cathodes are made by depositing about 1/16- in. metal on paraffined steel-sheets in four of the tanks, which are different from the others only in being a little deeper.

The anodes may contain any or all of the elements, gold, silver, copper, tin, antimony, arsenic, bismuth, cadmium, zinc, iron, nickel, cobalt and sulphur. It would be expected that gold, silver, copper, antimony, arsenic and bismuth, being more electronegative than lead, would remain in the slime in the metallic state, with, perhaps, tin, while iron, zinc, nickel and cobalt would dissolve. It appears that tin stands in the same relation to lead that nickel does to iron, that is, they have about the same electromotive forces of solution, with the consequence that they can behave as one metal and dissolve and deposit together. Iron, contrary to expectation, dissolves only slightly, while the slime will carry about 1 per cent, of it. It appears from this that the iron exists in the lead in the form of matte. Arsenic, antimony, bismuth, and copper have electromotive forces of solution more than 0.3 volt below that of lead. As there is no chance that any particle of one of these impurities will have an electric potential of 0.3 volt above that of the lead with which it is in metallic contact, there is no chance that they will be dissolved by the action of the current. The same is even more certainly true of silver and gold. The behavior of bismuth is interesting and satisfactory. It is as completely removed by this process of refining as antimony is. No other process of refining lead will remove this objectionable impurity so completely. Tin has been found in the refined-lead to the extent of 0.02 per cent. This we had no difficulty in removing from the lead by poling before casting. There is always a certain amount of dross formed in melting down the cathodes; and the lead-oxide of this reacts with the tin in the lead at a comparatively low temperature.

The extra amount of dross formed in poling is small, and amounts to less than 1 per cent, of the lead. The dross carries more antimony and arsenic than the lead, as well as all the tin. The total amount of dross formed is about 4 per cent. Table I. shows its composition.

The electrolyte takes up no impurities, except, possibly, a small part of the iron and zinc. Estimating that the anodes contain 0.01 per cent, of zinc and soluble iron, and that there are 150 cu. ft. of the solution in the refinery for every ton of lead turned out daily, in one year the 150 cu. ft. will have taken up 72 lb. of iron and zinc, or about 1 per cent. These impurities can accumulate to a much greater extent than this before their presence will become objectionable. It is possible to purify the electrolyte in several ways. For example, the lead can be removed by precipitation with sulphuric acid, and the fluosilicic acid precipitated with salt as sodium fluosilicate. By distillation with sulphuric acid the fluosilicic acid could be recovered, this process, theoretically, requiring but one-third as much sulphuric acid as the decomposition of fluorspar, in which the fluorine was originally contained.

For the treatment of slime, the only method in general use consists in suspending the slime in a solution capable of dissolving the impurities and supplying, by a jet of steam and air forced into the solution, the air necessary for its reaction with, and solution of, such an inactive metal as copper. After the impurities have been mostly dissolved, the slime is filtered off, dried and melted, under such fluxes as soda, to a dore bullion.

The amount of power required is calculated thus: Five amperes in 24 hours make 1 lb. of lead per tank. One ton of lead equals 10,000 ampere-days, and at 0.35 volts per tank, 3,500 watt-days, or 4.7 E.H.P.-days. Allowing 10 per cent, loss of efficiency in the tanks (we always get less lead than the current which is passing would indicate), and of 8 per cent, loss in the generator increases this to about 5.6 H.P.-days, and a further allowance for the electric-lights and other applications gives from 7 to 8 H.P.-days as about the amount per ton of lead. At $80 per year, this item of cost is something like 65 cents per ton of lead. So this is an electrochemical process not especially favored by water-power.

The cost of labor is not greater than in the zinc-desilverization process. A comparison between this process and the Parkes process, on the assumption that the costs for labor, interest and general expenses are about equal, shows that about $1 worth of zinc and a considerable amount of coal and coke have been done away with, at the expense of power, equal to about 175 H.P.-hours., of the average value of perhaps 65 cents, and a small amount of coal for melting the lead in the electrolytic method.

Tables V., VI., VII. and VIII. give the results obtained experimentally in the laboratory on lots of a few pounds up to a few hundred pounds. The results in Tables VI. and VII. were given me by the companies for which the experiments were made.

The success thus attained in the electrolysis of lead, generally accepted hitherto as impracticable, may give some encouragement to the employment of similar methods in the treatment of some of the other metals, especially as it is shown to be possible to apply simple means to obviate the chief trouble, spongy deposits.

how does lithium mining actually work and will we have enough?

how does lithium mining actually work and will we have enough?

Lithium is one of the most important metals of the 21st century. It makes possible the rechargeable battery technology found in mobile phones, laptops, and electric vehicles. But where does lithium actually come from and what goes into mining for lithium?

There are two main sources of lithium: mines and brine water. Most of the worlds lithium (87 percent) comes from the latter source. Among brine water sources, briny lakes (known as salars) offer the highest concentration of lithium (1,000 to 3,000 parts per million). The salars with the highest lithium concentrations are located in Bolivia, Argentina, and Chile.

Lithium obtained from salars is recovered in the form of lithium carbonate, the raw material used in lithium ion batteries. The production process is fairly straightforward and requires only natural evaporation, which leaves behind not only lithium, but also magnesium, calcium, sodium, and potassium.

The lithium content of ocean water is far lower, hovering around 0.17 parts per million. However, about 20 percent of the lithium in seawater can be recovered using a combination of membranes, filters, and ion-exchange resins.

Brine mining is normally a lengthy process that takes anywhere from eight months to three years. But scientists are working to develop technology that can extract lithium and other valuable materials including gold, zinc, copper, and silica from the brine water used by geothermal power plants. Collecting lithium from geothermal brine could make the whole lithium production process far faster and cheaper than the natural evaporation processes that are normally used.

In 2010, for instance, Simbol Materials received a $3 million grant from the U.S. Department of Energy to develop just such a technology. Simbol adapted technology originally developed by Lawrence Livermore National Laboratory to create a series of filters and adsorption materials that can capture lithium and other materials in the brine water pumped out of the ground by geothermal plants to generate energy. After removing the lithium and other materials, the water is sent back to the geothermal plant for re-injection below ground.

New technology has also opened up the possibility of recovering oilfield brine, the briny water that bubbles up when oil wells are being drilled. Last year, for instance, MGX Minerals developed a method for extracting over 83 percent of the lithium in oilfield brine.

The remaining 13 percent of the worlds lithium is found in more traditional mines. Lithium concentrations in hard rock (pegmatites) are higher than those found in brine, but the mining process has a higher cost and a larger environmental footprint. Still, hard-rock lithium mining can be competitive, at least in mines that are already in operation.

Over 145 minerals contain lithium, but only five (spodumene, lepidolite, petalite, amblygonite, and eucryptite) are utilized in lithium extraction. Of those five, spodumene provides the largest proportion of all mineral-derived lithium. In 2011, spodumene yielded 12,500 tons of lithium, while the other sources provided only about 1,500 tons.

After spodumene is mined, its heated to 1100C, then cooled to 65C and ground up, mixed, and roasted with concentrated sulfuric acid. The sulfuric acid kicks off a reaction in which lithium sulfate replaces hydrogen. The slurry is then filtered and a number of additional compounds are added. After its pH level is adjusted, the mixture is concentrated through evaporation. Finally, soda ash is added to create lithium carbonate.

The smallest currently available source of lithium is that contained in recycled electronics. While lithium recycling is not yet capable of yielding lithium pure enough for reuse in batteries, it can be used in glass and ceramics, the second-largest lithium-consuming industry after the lithium ion battery industry. But lithium recycling remains a niche market, and only one recycling facility for lithium ion batteries exists in the U.S.

In the future, hectorite clay deposits could provide a significant source of lithium. Procuring the clay itself would be easy, but it would need to be leached or roasted in order to extract the lithium. Clay has not yet been exploited as a source of lithium, but analysts say that for American manufacturing purposes, mining lithium-rich clay in Nevada could be almost as cost-effective as importing it from Chile.

Whether the lithium mining industry turns toward clay, oilfield brine, or some other recovery method, its clear that the rapid increase in lithium consumption over the past few decades requires an equal expansion of lithium sources and production sites. In 2009, the lithium ion battery industry accounted for 21 percent of all annual lithium consumption. Today, that figure has almost doubled, and battery production especially batteries for the manufacture of electric vehicles will continue to gobble up a progressively larger share of lithium. A single electric vehicle requires as much lithium as 10,000 mobile phones, and global electric vehicle sales will essentially double in 2021, then double again by 2025. In other words, expanding access to lithium must remain a priority for the EV and electronics industry.

A more diverse lithium supply could also help break up the oligopoly that currently controls the trade. Today, just four companies (Chiles SQM, U.S.-based FMC Corp and Albemarle Corp, and Australias Talison) produce 85 percent of all lithium.

Finally, expanding lithium mining operations could insulate lithium prices against potential shocks. For instance, if Chile (which holds over half of all the worlds known lithium reserves) was destabilized, prices would spike dramatically, as they did in China a few years ago when an Australian spodumene shortage led to a 300 percent price increase. Price increases like these could threaten the entire lithium ion battery industry and the sustainable energy future along with it.

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