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miller process of refining gold ppt

miller gold refining process by chlorination

miller gold refining process by chlorination

The term refining has been very often applied to the removal of base metals from the noble ones, or, in other words, separating the oxidizable from the non-oxidizable ones. When used in this sense it is generally applied to the term bullion, which is an alloy carrying gold and silver, the bullion being pure when it contains gold and silver only. A special term, parting, is borrowed from assaying, and means the separation of gold from silver. In the foregoing processes it is hard to. draw the line where one begins and the other ends, but the term refining is used generally here to indicate the separation or partial separation of whatever metals are present with gold.

In order to purify gold and remove the silver from it by any process analogous to the cementation with nitre or salt in a reasonable time, the alloy must be in such a fine state of division that the silver will be removed almost instantly, or diffusion must be assisted by actually keeping the alloy in amolten state. Nitric acid, the active ingredient in the first cementation process described, would not be effective in this case, since even if nitrate of silver did form, the temperature is so high that it would be decomposed into oxide of silver, and finally metallic silver and oxides of nitrogen. In fact, nitric acid itself would not be stable at this temperature.

The second active agent was chlorine, or hydrochloricacid, and as the former is known to be much more active thanthe latter, a process of refining by this means was devised byMr. Lewis Thompson in 1838. The alloy was melted in avessel and a stream of chlorine allowed to pass over it. Thesilver was rapidly converted into chloride, and the gold wasthus rendered pure. Later experimentalists found that there was no volatilisation of gold in this process, and that the addition of a chloride of the alkalies or alkaline earths prevented the volatilisation of the chloride of silver formed. It is somewhat difficult to believe that no gold volatilises during such an operation, for if pure gold is heated in a current of chlorine, yellow feathery scales of the chloride will sublime on the upper part of the tube some distance from the flame. This volatilisation is not appreciable until the silver has been removed.

Nearly 30 years after Thompsons discovery, Francis Bowyer Miller, an assayer in the Sydney Mint, discovered and perfected what is known now as Millers process. He found that by passing chlorine gas into the molten alloy that practically the whole of the chlorine first passed in united with the silver, which in this way could be almost perfectly removed from the bullion as chloride. By passing chlorine over molten alloy the silver would be slowly removed, but most of the chlorine would escape unaltered; the chloride of silver coat also would prevent contact after a short time.

It was assumed by Miller that chloride of gold would not form at the temperature of molten gold, since the compound is decomposed at a dull red heat, but although this assumption was incorrect, the operation of passing chlorine through the molten gold does not allow of volatilisation of the gold while silver is present. The covering of borax and chloride of silver also appears to minimise, if not prevent, this loss.

The process was so successful that it has been adopted at all Australian Mints. Before Millers invention, the gold coins made at the Sydney Mint were alloyed with silver, since the cost of removing this and replacing it by copper was more than the value of the contained silver.

The following particulars of the operation at the Melbourne branch of the Royal Mint are partly taken from an account supplied to T. Kirk Rose by Mr. Francis R. Power, and partly from inspection of the process, and particulars kindly supplied through Mr. Wardell, the Deputy Master.

Bullion is received at the Royal Mint in many forms. It comes as alluvial gold, admixed with impurities, as retorted gold, and as smelted gold, more or less admixed with other metals. All classes of bullion are melted with the object of obtaining a sample, the assay of which will accurately represent the bar. Ordinary samples are melted with a flux consisting of:

If the gold is very impure more nitre is added to this flux, a sample is taken to check the subsequent toughening, which is done either with chlorine or nitre until sufficient of the base metals is extracted to enable samples representative of the mass to be taken. Since after weighing and assaying any particular bar loses its individuality, all that is aimed at in the preliminary partial refining is to eliminate any metals or elements which prevent the bullion from being homogeneous, no gold and no silver is removed by this operation.

The furnaces used for melting the gold are shown in section in Fig. 1. They are cylindrical instead of the usual square sectioned ones, since this form is more economical in fuel, and are more readily cleaned from adhering clinker. They are 12 inches in diameter, and 21 inches deep. Five fire bars, 1 inches square, and 18 inches long, rest in a cast-iron frame, D, 12 inches by 2 inches, and are supported by a bar at the back. These bars ere six inches above the floor. The ash pit, F, is a cast-iron box, below the level of the floor, and the draught passes through a grating at floor level covered with an adjustable damper plate, M. The escaping gases pass through the flue L, which leads to a series of condensing chambers, thence to the stack, which is 80 feet high. The furnace itself is built of arched fire bricks, B, 9 inches by 4 inches, tapering from 2 to 2 inches. These are set in an iron cylinder, A, 21 inches in diameter, and 3/8 inch thick. The cylinder rests on an iron plate, C, 5/8 inch thick, and 22 inches in diameter, with a 12 inch hole in the centre. This plate is supported on brickwork. The vertical cylinder is surrounded concrete rammed in, N. When this is set the facing plate is moved out 2 or 3 inches,thus providing an air jacket between the concrete and the external plate of the furnace, keeping the latter cooler. The upper surface of the furnace is also protected by a cast-iron plate. The furnace cover consists of three fireclay tiles, two being 20 inches by 6 inches, the third being smallerall are bound with iron bands. The middle one is perforated with a inch hole, through which the chlorine delivery pipe passes.

The pot in which the gold is melted prior to passing chlorine through is made of clay of fine texture, similar to French clay. These are 10 inches high, 5 inches in diameter, 3/8 of an inch thick at the top, and gradually increasing to 1 inch at the bottom. These pots are fitted loosely into a guard pot for safety; the guard pot is a plumbago crucible, 8 inches high, 6 inches internal diameter, 5/8 inch thick at the top, and of an inch thick at the bottom. This stands in a cylindrical fire brick 5 inches in diameter, and 2 inches high. Fire-clay lids, dished to catch any gold projected by too rapid a current of gas, and having a slit in them to allow of sliding them over the pot without shifting the chlorine pipe are provided.

The, pipe stem is 24 inches long, tapering from 3/8 to an inch at the end inserted into the gold, and is wedge-shaped to facilitate the escape of the chlorine when resting on the bottom of the pot. The bore is 1/8 inch in diameter. The thin edge of the pipe stem is attached to the branch delivery pipe by a piece of inch rubber, about 2 inches long, which connects with an ebonite junction, G, 3 inches in length, with a bore of 1-10 inch, turned with a ring round the middle, which acts as a rest for the 8oz. weight, H, used as a sinker for the pipe stem. One end of the ebonite junction is inch in diameter, the other inch, the latter being connected by a stout rubber tube 3 or 4 inches long, to a 14 inch lead pipe, an inch in diameter, which is connected by a rubber junction to a glass stopcock, I, from the spigot of which a lb. lead weight, J, is suspended to prevent the pressure of gas from blowing it out.

The vessel is made of boiler plates riveted together, and lined with sheet lead. The lead is corrugated below so as to give greater strength, but is smooth inside, steam is admitted through a pressure regulator through a trunnion on one side, and fills the space between the lead and iron, thereby providing a steam jacket, the pressure, and therefore the temperature of which is controlled by the regulator; any condensed steam is drained off by means of a pet cock provided; steam escapes through a pipe leading through the other trunnion.

One comer of the generator is attached by means of a connecting rod to a crank which gives it an oscillating motion, thereby preventing the chemicals from packing or becoming baked on the hot lead surface. Holes about 6 inches in diameter left in the flat cover of the generator. These serve for the introduction of the manganese dioxide and acid, and also for washing out the salts. After the charge is exhausted, leadfaced, flat iron covers are removed: these are readily fastened in position by the pressure of a screw bolt at their centre. The screw passes through a horizontal bar, the ends of which slip under projecting lugs riveted on to the generator. The acid is supplied through a siphon pipe near the centre of the flat surface of the generator; the longer limb being sufficient to prevent the acid being thrown out by the internal pressure of the gas.

The chlorine escape pipe passes through the cover, and is connected with a fixed leaden pipe at a point opposite one of the trunnions, so that the oscillation of the generator will only give a minimum movement at this point.

The two generators are used alternately, except when it is required to refine a large amount of gold; in such a case the two generators are used together. When the charge is exhausted in one generator it is connected to the second, and the chlorinegas still remaining is displaced by gradually filling the vessel with water. One of the cover plates is then removed, and the charge is emptied into an underground drain.

The gas passes through a pair of earthenware jars provided with two necks, in which many of the acids or salts mechanically carried over are retained. From these the gas delivery pipe leads to a distributing vessel with two necks, and partly filled with manganese chloride solution. A pressure gauge of 1 inch glass tube 15 feet high is luted to the bottom of this vessel, and fixed to the wall by brackets. Since one inch of gold will balance a column of about 19 inches of water, the liquid in this tube must be from 10 to 11 feet in height to force chlorine through the 7 inches of gold in the pots. The pressure exerted is about 5lb. per square inch. A four-way tube of lead or pottery is pressed through the second neck of the vessel, and each arm is connected by thick rubber to glass stopcocks to which inch lead pipes are joined, these pipes leading to sets of four and five furnaces.

When the flow of chlorine through the gold is stopped the chlorine escapes through a safety pipe. This is provided by having a two-necked earthenware vessel containing such a quantity of water that when the pressure of the gas exceeds the working pressure required, the end of a glass tube, passing to the bottom of the vessel, and connected above the neck with a 4 inch lead pipe ten feet high, becomes unsealed, and the gas escapes through the water in large bubbles, passing thence through a glass pipe inclined at an angle at the top of the lead pipe into the air. When sufficient gas has escaped to reduce the pressure to the working limit the pipe is automatically sealed.

The guard with the clay liner is placed in the furnace, and 2 or 3 ounces of fused borax added. It is heated until dull red. The ingots are then added, the weight of these being about 700 oz. Fuelis added, and the dampers opened. As soon as the gold ismelted, the lid is put on and the pipe stem, carefully annealed and heated to bright redness, is pushed to the bottom of the pot, chlorine at the same time being gently turned on to avoid the plugging of the tube through the gold solidifying in it.

The supply of chlorine is adjusted so as to avoid projection of globules. This can be told by feeling the pulsations of the rubberwhen the gold contains much silver or base metals the absorption of the chlorine takes place rapidly but quietly, very little motion of the molten metal being apparent, but near the end of the operation the gas must be admitted in a fine stream only. The chlorine is not dried before it passes into the gold, but the small amount of water vapor present does not affect the operation. It was formerly the practice to dry the gas with strong sulphuric acid, but this has been abandoned as unnecessary.

The order in which the metals chloridise has never been determined. Iron appears to come off separately, and is attacked at the commencement, while silver and copper remain and come off practically together.

When the bullion contains much base metal the consumption of chlorine is greater, and the time occupied longer, but not in proportion to the amount contained, since a more rapid stream of chlorine can be safely admitted than when the base metals and silver are nearly all removed.

The usual time allowed is four hours for a pot containing about 700oz. of bullion. If the bullion is nearly fine a much shorter time would suffice; for instance, 2 per cent, of silver and 0.5 per cent, of base would take about 1 hours; 3.5 of silver and 1.5 of base, two hours.

When the operation is nearing completion the flame issuing from the holes or slit in the lid becomes small, and alters in appearance; it becomes very luminous, has a brown edge, and if a white, rough, cold surface is plunged into it will become coated with a yellowish-brown tinge. It consists mainly of ferric, chloride, with traces of silver chloride and gold. As soon as this stain appears the current of gas is reduced, andallowed to pass in for another 15 minutes, when the clay pipe is withdrawn, and the clay pot lifted out of the guard pot. The pot is allowed to stand under a hood to carry off the fumes until the gold has solidified. The liquid chloride and fluxes are then poured into a mould provided with a hood. The pot is then broken, and the cone of gold dropped into the guard pot, and cast into two flat ingots, 12 by 4 by 1 inches. These bars, while still hot, are dropped in dilute sulphuric acid, and then water, and are still hot enough to become dry.

A modification of the process has been introduced. After the chlorine has been passed into the bullion for some time the chloride of silver which forms occupies twice the space that the silver did, and rises in the pot. When much silver was present the chloride was ladled out from time to time, to prevent it overflowing, and poured into a mould on top of the furnace. This practice has now been adopted for all the bullion. The chloride is baled out in such a way that any drip falls back again into the crucible. When the chloride film becomes thin some gold is also picked upthe last pourings are, therefore, put into a separate mould, when the gold present solidifies, the chloride is poured off, and the gold returned to the crucible. The last portion of silver remains on top of the gold, bone ash is added to thicken it, after which the refined gold is stirred and poured, the pot being returned at once to the furnace to be used for a fresh lot of bullion.

The chlorides contain 5 to 10 per cent, of gold in feathery particles. Generally speaking the gold is not present as chloride when the chlorine is passing, but chloride of gold has been found in the silver chloride when the process has been experimentally carried too far. The chlorides are supersaturated with chlorine and this is copiously evolved on cooling. To recover this gold 7 per cent, of their weight of bicarbonate of soda is added cautiously and without stirring, which produces a shower of globules of reduced silver, and these, falling through the chlorides, carry down nearly all the gold; another lot is needed to carry the balance down. The pot is lifted out, the button allowed to settle and solidify, when the chlorides are poured into a mould 12 by 10 by 2 inches. The silvery button obtained contains from 40 to 60 per cent,of gold. Thegold in bars and in the buttons contain 99.85 per cent, of the gold issued for refining, the balance being practically in the pot. The maximum quantity left in the silver is 1 part in 10,000, but it is usually from half to one-third of this quantity.

The cakes of impure chloride of silver are sewn up in coarse flannel bags and boiled with water in a wooden vat for four or five hours. The salt present in the cake helps in the solution of the cuprous chloride present. The cakes are placed ultimately with wrought plates 1/8 inch thick, in a cast iron tank lined with similar plates. The plates are prevented from touching the bags by means of laths of wood, otherwise copper would be reduced in the bags and would be difficult to separate from the silver. The reduction is slow on starting, unless either some liquid is left from some previous operation or some chloride of iron is added. The bath must be heated by a jet of steam and kept boiling from two to four days. When the operation is complete no hard lumps can be felt in the bag. The silver is reduced in this way while the copper is not. When reduction is complete the silver is washed and smelted without fluxes, the average grade of the metal so produced being about 980. The total loss of silver amounts to about 1.6 per cent.

gold chlorination process by miller

gold chlorination process by miller

Millers Gold Chlorination process was introduced by F.B. Miller. The refining process employs chlorine gas, which passed into molten gold covered with a layer of borax and silica, and reacts with most of metals present in the molten charge. Platinum group metals do not react. Basically, gold is slightly attacked in the first moments and all the chlorides formed rise to the surface and are removed with the layer of borax and silica until the viscosity of the layer is sufficient to remove them. During the process some chlorides are volatilized and pass out of the furnace. Once is complete the initial reaction, there is a fast generation of non-volatile compound called Miller Salt that must be removed from the surface of the molten charge. It is important to avoid the formation of gold chlorides in order to avoid losses. One solution is to the problem is to cover the molten charge with mixture composed by borax and silica.When the metallic gold is as fine as 99%, gold chloride is produced in the molten charge in high levels and starts to appear in high proportion in the fumes that must contact the slag. Essentially, the addition of chlorine gas is reduced step by step. The indication of the end of the process is given by a peculiar colour on a cold pipe held in the issuing fume. At this moment, the addition of chlorine is stopped and the final slag is removed and the gold is poured in a cast. The final purity of gold is 99.5 to 99.8% and the minor elements are silver and small quantities of copper.

It has been noted that the slag may content 1-2% of gold as small beads and tiny particles of gold produced by the reaction of chlorine gas and the molten charge. Usually, the gold is recovered by adding sodium carbonate to the molten charge. The best point of addition is over the slag, but the final decision depends on the experience of the operator. During the process, part of the silver is reduced due to the reaction between silver chloride and sodium carbonate, and the metallic product goes to the bottom. The product is retreated again and the silver trapped into the slag is treated separately. In order to optimize the process, some variations were introduced initially, but their results were not satisfactory. One of these changes was to add air after chlorine in order to oxidized part of the base metals.

With E.B. Millers Gold Chlorination Process of refining impure gold with chlorine gas (patented in Britain in 1867) https://docs.google.com/viewer?url=patentimages.storage.googleapis.com/pdfs/US2997508.pdf

refining gold jewelry scraps - ganoksin jewelry making community

refining gold jewelry scraps - ganoksin jewelry making community

When questions arise about manufacturing quality gold jewelry, manufacturers are eager to talk shop with their peers and industry experts. They want to know if they are using the right alloy for a specific application, casting at the appropriate times and temperatures, and annealing properly when work hardening a piece.

Rarely, though, does the conversation turn to refining,an area of jewelry manufacturing that poses more questions than answers in many manufacturers minds. Refining is a practice that must be done precisely and methodically to ensure the full recovery of gold, as well as an end product that is free of impurities, which can lead to quality problems when the metal is reused in production. (See Cracking Up.)

But refining doesnt have to be a mystery to manufacturers. There are several methods commonly used to recover metal. Some operations are suitable for use by manufacturers and jewelers who wish to refine in-house, while others are designed for commercial refiners who handle large lots. The following are the most common methods used in the jewelry and gold refining industries. Ganoksin is sponsored by

Cupellation is the technique that forms the first part of the fire assay process, in which lead is added to the unrefined gold material. The mixture is heated in air to between 1,830F and 2,010F (1,000C and 1,100C), at which point the gold-containing metal dissolves in the lead. All base metals, including the lead, are oxidized to form a lead oxide slag. A gold-silver bullion, which also contains any platinum group metals (PGMs) present, remains. If pure gold is required, additional refining steps are necessary to separate out the gold.

While this procedure can be used on the very small scale (roughly up to 10 grams) such as in fire assay, its use on a small to medium scale (roughly 100 grams to 10 kg) is not recommended because it emits copious quantities of toxic lead oxide fumes. These fumes give rise to environmental pollution unless expensive fume abatement systems, also known as gas scrubbers, are installed.

In the inquartation and parting process, the refinable material is melted with additional silver or copper to produce an alloy containing 25 percent or less gold. The dilution ensures that all the base metals and silver can be dissolved out in nitric acid. Ganoksin is sponsored by

Next, the molten alloy should be grained to maximize surface area. The grained alloy is attacked with nitric acid to dissolve out all the base metals and silver, leaving behind a gold sludge. This sludge is then washed, filtered, and dried.

Any platinum and palladium present will also be dissolved out (although the process may need to be performed twice to ensure their complete removal), but insoluble PGMs will remain. In such cases, further refining is necessary if pure gold is needed.

When used for refining material that doesnt contain PGMs, the inquartation and parting process is capable of producing gold of up to 99.99 percent purity. The process is particularly suited for treatment of low karat gold scrap, since large additions of copper or silver are unnecessary to achieve the desired 25 percent-or-less gold content. On the contrary, this process may not be as desirable for an operation making predominantly medium to high karat gold jewelry, as scraps from production may need to be substantially diluted with copper or silver. Ganoksin is sponsored by

In addition, inquartation and parting can be used as a preliminary step to reduce the silver content of silver-rich refinable materials from 40 to 50 percent to below 10 percent prior to refining by the Aqua Regia process, which is explained below.

A pyrometallurgical chlorination process, the Miller process is one of the oldest and most widely used processes in large scale gold refining. It involves bubbling chlorine gas through molten bullion. The base metals and silver are removed as chlorides, which either volatilize or form a molten slag on the surface of the melt. The process is complete when purple fumes of gold chloride start to form, usually when the gold content reaches a purity of 99.6 to 99.7 percent. Any PGMs present are not removed, and further refining is necessary if pure gold is required.

The typical gold purity achieved by this process is 99.5 percent, with silver as the main impurity. The process has the advantage of being quick and is widely used for primary refining of gold dor from the mines. Ganoksin is sponsored by

Considerable technical skills are required for this process, and there are a number of health and safety implications in the use of chlorine gas. Expensive fume extraction and treatment facilities are essential. Consequently, this process is not suited for small to medium scale refining by jewelers.

An old and well-established process, the Wohlwill method is widely used in major gold refineries, often in conjunction with the Miller process. (For typical jewelers scraps and wastes, a preliminary refining step, such as the Miller or inquartation process, is required.) An electrolytic refining technique, it entails the electrolytic dissolution of an impure gold anode in a hydrochloric acid-based electrolyte. The process results in a deposition of 99.99 percent pure gold at the cathode. The silver and insoluble PGMs (along with a little gold) fall out as anode slimes, with the silver precipitated out as silver chloride, and all are recovered later. Any base metals, platinum, and palladium remain in solution, and can be treated later to recover the PGMs.

Gold of a purity of at least 98.5 percent is normally required for the anode, as too much silver will result in silver chloride building up on the anode surface and preventing dissolution of the gold. Typically, the input material for the anode is the gold from the Miller process, described previously. Ganoksin is sponsored by

A variant of the Wohlwill electrolytic process, the Fizzer cell process is suitable for jewelers small-scale refining operations. In the electrolytic cell, the cathode is contained within a porous ceramic pot, which acts as a semi-permeable membrane; it prevents gold dissolved in the electrolyte on the anode side of the wall from passing through and depositing on the cathode. Thus, gold and other soluble metal chlorides build up, and insoluble chlorides, such as those of silver and the insoluble PGMs, drop to the bottom of the cell.

Periodically, the cell is drained and filtered, and the gold in the electrolyte is precipitated with a selective reducing agent, as in the Aqua Regia process described later. In this way, the dissolved PGMs are separated from the gold, which can reach a purity of 99.99 percent. Ganoksin is sponsored by

Unlike the Wohlwill process, the Fizzer cell can treat anodes containing up to 10 percent silver, and up to 20 percent silver if an imposed alternating current is added. The surface of the anode may need to be scraped free of silver chloride at regular intervals.

The Aqua Regia process can produce gold of up to 99.99 percent purity. It is based on the fact that Aqua Regia (a mixture of hydrochloric and nitric acids in a 4.5:1 ratio) can dissolve gold into soluble gold chloride. The process is most suited to medium- to large-scale operations. A typical batch size is 4 kg of scrap, and equipment in a range of capacities is commercially available from several suppliers.

The main limitation of the process is that the feed material should have a silver content of 10 percent or less to avoid blocking up the dissolution of the scrap. Because of this, pretreatment by the inquartation process to reduce the silver content may be necessary. Alternatively, the low silver content may be achieved by a judicious blending of batches of scrap. Thus, the process is more suited for medium to high karat gold scrap refining. Ganoksin is sponsored by

Copious brown fumes of nitrogen oxide are emitted while the gold is being dissolved. Fume abatement systems are required to stop emission of these toxic fumes and to comply with pollution laws. It is also worth noting that these strong acids require suitable storage and safety procedures.

Once the gold is dissolved, the yellow-green solution must be filtered to remove the insoluble silver chloride, the insoluble PGMs, and any non-metallics, such as abrasives and inclusions. The gold can then be selectively precipitated using a number of reducing agents, such as ferrous sulphate (also known as Copperas), sodium bisulphite, and sulphur dioxide gas. Other less frequently used agents include hydrazine, formaldehyde, oxalic acid, and hydroquinone. Some emit copious quantities of gas and some are carcinogenic.

One refining expert, Roland Loewen of Alchemy Gold Refining in Baytown, Texas, favors an aqueous solution of sodium bisulphite over ferrous sulphate. This is added slowly until the yellow color of the solution disappears. He notes that a smell of sulphur dioxide may be apparent at this point. Completion of the reaction can be ascertained with the stannous chloride test. (In this test, drops of stannous chloride are added to the solution. If gold is present, the colloidal gold formation will create a purple coloration in the solution.)

After precipitation, the solution should stand overnight to allow the fine gold particles to settle as a sludge on the bottom. Most of the liquid can be decanted off and the remaining portion with the gold can be filtered. To ensure that all other metals are dissolved away, the filtrate is washed with hydrochloric acid and then water. It is then dried and placed in a crucible for melting and graining.

Occasionally, jewelers who try this process complain that they have lost a considerable amount of the gold. This suggests that either they are not fully dissolving all the gold in the first stage or, more probably, not precipitating all the gold in the reducing step. To see if you are guilty of the latter, analyze the liquid for gold content using the stannous chloride test.

As with some of the other refining methods, the dangers of handling strong acids are present in the Aqua Regia process. Anyone using this process must be aware of the risks and ensure that they have trained chemists and safe facilities.

In fact, all of the refining processes described in this article require technical expertise and safe implementation. Each process entails permit licensing and regulation by the EPA. For all of these reasons, as well as the safety requirements described in this article, it is often safer-and more cost-effective-to leave refining to the experts.

In any strategy to recover precious metals, there is no sense in spending more on processing costs than the value of metal recovered. Compare the overall cost of in-house refining to the recovery efficiency (the amount of gold and other precious metals) achieved by an outside refiner. You may find that low-grade scraps and wastes are not economic to recover in-house and are best treated by a commercial refiner. Silver and platinum group metal (PGM) recovery will also play a part in determining the economic viability of in-house processing.

The gold purity obtained will vary depending on refining technique and operating skill. If the gold is being used for re-alloying in-house and you have access to analytical facilities to obtain gold purity, this may not be important.

If you are re-using the gold for new alloy production, be aware that some impurities may not be removed in the refining process. For example, PGMs are not removed by some techniques, and they can affect the new alloys color or properties.

Think about health, safety, and environmental pollution. Local legislation on disposing of effluents and release of toxic fumes may restrict your choice of technique. Also, many refining techniques require the use of strong acids; the safe storage and handling of these chemicals may restrict your choice.

The award-winning Journal is published monthly by MJSA, the trade association for professional jewelry makers, designers, and related suppliers. It offers design ideas, fabrication and production techniques, bench tips, business and marketing insights, and trend and technology updatesthe information crucial for business success. More than other publications, MJSA Journal is oriented toward people like me: those trying to earn a living by designing and making jewelry, says Jim Binnion of James Binnion Metal Arts.

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high purity gold and the miller process - manhattan gold & silver

high purity gold and the miller process - manhattan gold & silver

Here at the MGS blog, weve gone on and on about the importance of gold purity from how it is measured, to record setting levels of gold purity. But how are certain levels of purity actually achieved? You may think that one just needs to be lucky enough to find an untainted vein of gold and mine it. However, such happenstance would be impractical and perhaps even impossible. Certain stores of gold become very pure because refiners make them that way. One such method of doing so is known as the Miller Process.

After gold is mined, it is usually riddled with impurities typically lesser metals like zinc, copper, iron, and silver. These metals are obviously all very different from one another. But, they do have one key property in common: they will readily combine with pure chlorine gas to form chlorides. To take advantage of this trait, the impure gold sample is melted down in a crucible. Once the sample is liquefied, pure chlorine gas is blown across it. This causes the impurities to form chlorides which rise to the top of the crucible. The chlorides can be skimmed off of the top of the crucible, leaving pure gold behind. With that, the Miller Process is complete and the gold is then refined in the manner specific to its intended use (jewelry, industry, bullion, etc.).

Invented by Dr. Francis Bowyer Miller, the Miller Process was a game changer in the world of gold refining. It is popular among metal refiners all over the world because in a nut shell its cheap, easy, and produces high-purity gold samples about 99.95% pure gold. While that level is considered very pure by many standards, some industries require even purer gold samples. Naturally, a higher purity sample requires a more expensive and complex process. Well discuss that in a future blog post.

what is the miller process?

what is the miller process?

The Miller process is a gold refining process that produces gold of approximately 99.95% purity, sufficient for many applications. It is faster and less costly than other refining options used to produce purer gold, which makes it a popular choice at some refineries. This technique involves passing chlorine gas through molten told to trigger a chemical reaction that separates impurities. If a refinery needs gold of a higher purity, it can send the processed bullion to a facility with other refining techniques available for additional treatment.

Refinery starts with basic smelting to extract gold and remove as many impurities as possible. Smelted gold can be poured into a crucible where it is kept hot while chlorine gas bubbles through it. The gas reacts with impurities, causing them to precipitate to the surface in the form of chlorides which can be skimmed away. Some also form gases that vent from the top of the crucible.

After approximately an hour and a half, the gold is pure enough that it can be poured into solid bullion or other storage formats. Gold treated with the Miller process is assayed to confirm the purity so the final product can be stamped to record it. For industrial processes like gold contacts on electrical equipment, gold of 99.95% purity is often acceptable. Using gold produced by Miller process can be less expensive than 99.99% gold mixtures, which require more time and money.

Environmental controls can be important during gold processing. Mining to smelting and final purification can generate a number of harmful chemicals that need to be controlled. Techniques for managing the environmental impact of gold production can include filtering industrial waste, storing chemicals in hazardous material containers for disposal, and making refinery processes more efficient to reduce overall waste and usage. Firms using the Miller process may control their chlorine to prevent environmental contamination and worker injury while also limiting waste, because chlorine can get expensive when used on an industrial scale.

Markets where gold is traded typically have strict regulations on the content and weight of gold products. This ensures consistency and prevents attempts to take advantage of traders and consumers with products that may not contain pure gold or could be underweight. Bullion can be randomly assayed and weighed to confirm that it meets the advertised specifications. Substitution of Miller process gold for other golds with a higher purity rate can be grounds for penalties.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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