gravity spiral concentrator working principle
The gravity spiral circuit is designed to extract and concentrate coarse gold from the recirculating load in the mill grinding circuit and hence prevent a build up within that circuit and the eventual escape of some of that gold into the C.I.L. tanks and thereon into the final tails. (See fig. 4)
For the spirals to work efficiently the feed supply must have consistent characteristics and be of a constant rate. Variations in the flow rate, the feed size distribution and percentage solids will have adverse effects upon separation. Generally the solids tonnage should give adequate loading of the concentrate and middlings areas and the pulp density should be low enough to ensure mobility of particles in these areas. BUY SPIRALS
Feed to the spirals may be adjusted by the moving of two splitter arms on either side of the cyclone underflow discharge box, this altering the volume of the feed passing over the splitter screen. (See fig. 5)
The feed may also be adjusted by varying the speed of the gravity feed pump. This is necessary when the mill feed has been dropped and it is impossible to get sufficient feed for the gravity pump by adjustment of the splitter arms. At such times the speed will need to be dropped and the water additionadjusted to provide optimum feed density.
The pulp density may be altered by the addition of water, before the splitter screen, in the gravity feed pump hopper or to the concentrate launder beneath the primary spirals. The latter option adjusts the density of the feed to the cleaner spiral only.
The static distributor (See fig. 6) at the head of the primary spirals ensures an accurate division of the pulp stream to the spirals. For maximum efficiency a constant head should be maintained in the head pot. The head can be adjusted by either altering the flow rate from the splitter screen and/or altering the annular gap between the head pot and the distributor body, by moving the head pot up or down as required.
Feed from the splitter screen passes down into the gravity feed pump hopper and from there it is pumped to the static distributor above six triplex type primary spirals. As the pulp passes down these spirals; separaration of particles occurs according to specific gravity and the heavier minerals progress to the inner profile while lighter minerals are forced towards the outer profile, along with most of the water and slimes. At the bottom of each spiral layer there are splitters which can be adjusted to ensure the optimum recovery of coarse gold. (See fig. 7)
The middlings and tailings from the primary spirals are directed to both the mill feed and the mill discharge pump. The proportion going to either may be adjusted so as to help achieve optimum grinding conditions.
The concentrate from the inner outlet of the cleaner spiral is fed directly on to the Wilfley table and the middlings and tailings report to the gravity sump pump which feeds into the mill discharge pump feed hopper.
The Humphreys Spiral Concentrator, which was invented by I. B. Humphreys and first used in 1943 for concentrating chromite in Oregon beach sands, consists of five or six spiral turns of a modified semicircular launder which is about the size of a conventional automobile tire. Feed enters the top spiral and the tailing discharges from the bottom one, while concentrate and middlings are cut off by outlet ports regularly spaced at each turn of the spiral, and the products passed through rubber hoses to common launders which run the full length of a bank of spirals. Wash water is supplied from a small wash-water channel paralleling the main channel.
Operating entirely by gravity flow and involving no mechanical parts, the separation of the heavy constituents of the feed is effected by the same centrifugal forces and flow gradients encountered in ordinary river or stream concentration.
A capacity of 38 tons per spiral was obtained in the 1000-ton per 24 hr. Oregon plant operating on about a minus 40-mesh feed and in the 5000-ton plant recently installed near Jacksonville to concentrate ilmenite 174 roughing, and 12 finishing spirals have replaced an installation of tables and flotation cells.
The Humphreys Spiral has been successfully applied to recovery of chromite from chrome sands, rutile, ilmenite, and zircon from sand deposits, tantalum minerals and lepidolite from their ores, gravity concentration of base metal ores and in the cleaning of fine coal.
How it works: Pulp is introduced at the top of the spiral and flows downward. As the pulp follows the spiral channel the light particles in the pulp stream move outward and upward into the fast flowing portion of the stream while the heavy particles move to the inner slow moving portion of the stream, where they are drawn off through concentrate ports.
Adjustable splitters allow any portion to be removed through the ports. Tailing is discharged at the bottom of the spiral.
Spirals are usually installed in double units, two spirals to a frame, in rows of two to twelve. Feed is split evenly to all spirals. At one plant 21 rows of 12 spirals each are fed by one pump.
The Humphreys Spiral Concentrator is a simple, efficient gravity concentrator which effects a separation between minerals of the proper size range that have sufficient difference in their specific gravity.
This concentrator is a spiral conduit of modified semi-circular cross-section, with outlets for removal of concentrate and middling. Pulp is introduced at the top of the spiral. As the pulp follows the spiral channel, lighter particles in the pulp stream move outward and upward into the fast moving part of the pulp stream. The heavy particles move to the inner, slow moving portion of the stream, where they are drawn off through concentrate or middling outlets. Adjustable splitters allow any portion of the concentrate or middling to be diverted through the outlets. Tailing discharges from lower end of spiral. A full- size spiral is used for laboratory testing. Two arrangements are suggested for test work.
In the closed circuit test unit illustrated, although a full-size spiral is used, as little as 20 pounds of material will indicate the possibility of spiral concentration in a batch test. By removing measured quantities of products, and adding like amounts of feed in repeated steps, substantial samples may be taken for analysis and estimates of capacity. Results from this procedure, using 100 to 300 pounds of material, are close to pilot test results.
Another arrangement, also using a full-size spiral, is a small pilot plant, and is suitable for test work where a larger quantity of material can be handled. The storage tanks may be built on the job from drawings supplied. This unit allows continuous feeding of material and permits accumulation of concentrate and tailing in separate tanks, which may then be re-run as feed for second stage concentration or scavenging of tailing.
Spiral concentrators are modem, high capacity, low costunits developed for the concentration of low grade ores. Spirals consist of a single or double helical sluice wrapped around a central support with a wash water channel and a series of concentrate take-off ports placed at regular intervals along the spiral (Figure 17). To increase the amount of material that can be processed by one unit, two or more starts are constructed around one central support. New spirals have been developed that do not use wash water. These new units have modified cross sections and only one concentrate-take-off port, which is
rapid wear of the rubber lining and irregular wash water distribution resulted in major production problems. Although still in use, the Humphreys cast iron spirals have been largely superseded by a variety of other types, notably the fiberglass Reichert spirals and new, lightweight Humphreys spirals.
The processes involved in mineral concentration by spirals are similar for all models. As feed containing 25-35% solids by volume is fed into the channel, minerals immediately begin to settle and classify. Particles with the greatest specific gravity rapidly settle to the bottom of the spiral and form a slow-moving fluid film. Thus the flow divides vertically: one level is a slow-moving fluid film composed of heavy and coarse minerals; the other level, the remainder of the stream, is composed of lighter material and comprises the bulk of the wash water. The slow-moving fluid film, its velocity reduced by friction and drag, flows towards the lowest part of the spiral cross-section (nearest the central support) where removal ports are located. The stream containing the lighter minerals and the wash water develops a high velocity, and is thrust against the outside of the channel (Figure 18). Separation is enhanced by the differences in centrifugal forces between the two: the lighter, faster flowing material is forced outward towards the surface, and the heavier, slower material remains inward towards the bottom.
Spiral concentrators are capable of sustained recoveries of heavy minerals in the size range of 3 mm down to 75 microns (6 to 200 mesh). They are suitable for use as roughers, cleaners, or scavengers. Feed rates may vary from 0.5 to 4 tons per hour per start, depending on the size, shape, and density of the valuable material. Some factors that affect recovery are the diameter and pitch of the spiral, the density of the feed, the location of splitters and take-off points, and the volume and pressure of thewash water. Individual spirals are easily monitored and controlled, but a large bank of spirals requires nearly constant attention.
Advantages of spiral concentrators include low cost, long equipment life, low space requirements, and good recovery of fine material. They can also be checked visually to determine if the material is separating properly. For maximum operating efficiency, feed density should remain constant, the particle-size distribution of the feed should be uniform, and fluctuations in feed volume should be minimized. Spiral concentrators will tolerate minor feed variations without requiring adjustment. Spiral concentrators, like cone concentrators, are efficient, low-maintenance units that should be considered for any large- scale gravity separation system.
The newer Humphreys spirals are capable of recovering particles as small as 270 mesh (53 microns). In a test at CSMRI, a new Mark VII Reichert spiral recovered 91.3% of the free gold contained in the feed in a concentrate representing only 5.4% of the feed weight. The unit showed little decrease in gold recovery efficiency with material down to 325 mesh (45 microns) (Spiller, 1983).
humphreys spiral concentrator
Since it was introduced in 1943 to recover chromite from Oregon beach sands, the Humphreys spiral concentrator has proved successful in several fields of wet mineral beneficiation. By the end of 1957, 9390 Humphreys spirals had been manufactured. Most of these are still in service.
Requirements for Spiral Concentration: Valuable heavy minerals to be recovered must generally be 14 + 200 mesh, but outstanding examples will be discussed later where these size ranges are exceeded in commercial practice at both ends of the size scale.
by these requirements for spiral concentration. The separation of mica and vermiculite from gangue minerals such as quartz, feldspar, and ferromagnesian minerals is dependent on the flaky shape of the micaceous minerals rather than on a significant difference in specific gravity.
Phosphate rock is another application where specific gravity differences are not involved in spiral separation. The phosphate rock pulp, generally 28 mesh, is reagentized at high pulp density with the usual reagents involved in phosphate rock flotation, such as fuel oil and tall oil. Stated simply, in the reagentized pulp the quartz gangue is wetted by water and the oiled phosphate rock is not. The wetted quartz gangue settles to the inside or lower part of the spiral channel where the velocity is low and is removed as a tailing via the normal heavy mineral draw-off port, which on most other ores delivers heavy mineral.
Fine coal below x 0 can be cleaned with Humphreys spirals. The model 24-C spiral used for coal cleaning has six turns of the helix in about the same headroom required for five turns in the model 24-A metallurgical spiral. In other words, the pitch is flatter. Only in the Pennsylvania anthracite region are there commercial plants for cleaning fine coal, but numerous pilot plant tests have demonstrated that many bituminous coals respond well to spiral treatment.
A very simple system has been devised by the Humphreys Investment Co. engineers to avoid such difficulty almost entirely and permit a multi-stage spiral plant to operate on coarse feed. The rougher spiral concentrate is composed of all sizes of heavy minerals present in the feed, but the gangue content is generally finer than 28 mesh. Consequently, treatment of the rougher concentrate in the simple elutriation or single-pocket sizer produces an underflow of very high grade heavy mineral that is finished coarse concentrate. The overflow, stripped of coarse heavy mineral, is ideally suited for the feed to the cleaner spiral section.
Field of Application: Processing Florida beach sands for the recovery of ilmenite, rutile, zircon, stauriolite, and monazite is the largest single type of application of Humphreys spiral concentrators. The ore is ideally suited to spiral concentration, since it completely satisfies the requirements for successful spiraling. The size range is substantially 100 pct 35 mesh + 200 mesh, the minerals are completely liberated, and there is significant difference of specific gravity between the gangue and heavy minerals.
Humphreys Gold Corp., mining affiliate of the Humphrey Investment Co., operates three plants in Florida under various contract agreements with E. I. du Pont de Nemours & Co. and the National Lead Co. There are 3374 spirals35.9 pct of all spirals manufactured up to the end of 1957employed in the three Florida beach sand plants.
The northern U. S. iron ore ranges account for the second largest group application of Humphreys spiral concentrators. On the iron ore ranges there are 1712 spirals, or 18.2 pct of all spirals manufactured to the end of 1957.
It would be worthwhile to discuss in detail some areas where the Humphreys spirals are definitely not suitable. With one exception, spirals have no application in recovery of sulfide minerals, where flotation has been shown to do a satisfactory and superior job.
The installation in 1948 of a Humphreys spiral concentrator section at the Hill-Trumbull plant of The Cleveland-Cliffs Iron Co. is the latest commercial method on the Mesabi Range being used for the recovery of fine iron ore. In two stages of concentration 84 spirals treat approximately 120 long tons per hour of minus 1/8 in. ore. These spirals augment the production of the heavy-density plant which recovers the plus 1/8 in. iron from the plant crude ore. Structure of the ore is such that, when crushed, about 50 pct of the plant crude is minus 1/8 in. size. After desliming in a 66 in. Akins classifier, grinding the classifier product in a ball mill and again desliming in a 78 in. Akins classifier about 15 pct of the crude ore remains to be treated in the spiral plant.
Overall recovery of the ball mill feed in the Hill-Trumbull spiral plant during the 1948 season was about 53 pct of the weight and 66 pct of the total iron content. Of the actual feed delivered to the spirals from the 78 in. classifier 64.9 pct of the weight was recovered containing 73.7 pct of the total iron. The average analysis of the spiral concentrate is 55.05 pct Fe and 14.83 pct SiO2.
Preparation of ore for the heavy-density plant is near standard practice. A slight divergence begins where the minus in. size material leaves the 66 in. Akins classifier as a product and is routed to a 6 by 10 ft Allis-Chalmers ball mill. The ball mill operates in open circuit, is charged with 2 in. diam balls, driven by a 200 hp motor and has a throughput of approximately 150 long tons of solids per hour. Pulp density in the mill is approximately 65 pct solids. The purpose of the ball mill is to subject the ore to a differential grind which liberates silica from the iron ore particles, and middling from the ore particles and from silica. It also assists in breaking down porous iron oxide particles. The ground product flows into a 78 in. Akins classifier which makes a size separation at nearly 200 mesh. The slime or overflow waste analyzes about 25 to 30 pct Fe and 50 to 60 pct SiO2. Classifier product containing about 75 pct solids drops onto a 4 by 8 ft
All 84 spirals are policed by one operator who removes extraneous matter from the spirals, adjusts pulp density of spiral feed, and adjusts splitters and quantity of wash water. He also has time for additional duties including sampling and tending to the ball mill, pumps, classifiers, and screen.
Spiral operators are easily trained and after a reasonable length of time can become proficient. They are required to observe structure of the ore and to note changes which would suddenly increase or decrease the pulp density. When such changes occur, water is added or decreased to maintain the proper pulp density and launder make-up water. Occasionally, the Hill-Trumbull plant crude ore does not provide a sufficient amount of minus 1/8 in. material to adequately feed the spiral section. Before ringing for more feed the spiral operator consults the heavy-density plant foreman to determine how much more feed his section can stand. If the heavy-density plant is operating at maximum capacity and there is not enough orecoming to the spirals, this material is pumped to the 60 in.
The 6 by 10 ft Allis-Chalmers ball mill was fed 150 long tons of solids per hour. Preliminary laboratory tests on this ore showed a distinct advantage in both grade and recovery if the fine ore were given a differential grind. Grinding is fast and in open circuit. Table 5 shows the size distribution before and after abrasion grinding giving analysis of samples from a single shift and illustrating results obtainable from abrasion grinding of this ore.
In spiral concentration of an ore in which a large percentage of the weight and values is in the plus 35 mesh size, it is usually necessary to operate with a feed pulp density of about 40 pct solids and a pulp rate of 25 to 30 gpm.
spiral concentrator | henan deya machinery co., ltd
Spiral concentrators are devices to separate solid components in a slurry, based upon a combination of the solid particle density as well as the particles hydrodynamic properties (e.g. drag). The device consists of a tower, around which is wound a sluice, from which slots or channels are placed in the base of the sluice to extract solid particles that have come out of suspension.
As larger and heavier particles sink to the bottom of the sluice faster and experience more drag from the bottom, they travel slower, and so move towards the center of the spiral. Conversely, light particles stay towards the outside of the spiral, with the water, and quickly reach the bottom. At the bottom, a cut is made with a set of adjustable bars, channels, or slots, separating the low and high density parts. Many things can be done to improve the separation efficiency, including:
-changing the rate of material feed
-changing the grain size of the material
-changing the slurry mass percentage
-adjusting the cutter bar positions
-running the output of one spiral separator (often, a third, intermediate, cut) through a second.
-adding wash water inlets along the length of the spiral, to aid in separating light minerals
-adding multiple outlets along the length, to improve the ability of the spiral to remove heavy contaminants
-adding ridges on the sluice at an angle to the direction of flow.
Typical spiral concentrators will use a slurry from about 20%-40% solids by weight, with a particle size somewhere between 1.5-.075 mm (17-340 mesh), though somewhat larger particle sizes are sometimes used. For good separation, the density difference between the heavy minerals and the light minerals in the feedstock should be at least 1 g/cm3; and because the separation is dependent upon size and density, spiral separators are most effective at purifying ore if its particles are of uniform size and shape. A spiral separator may process a couple tons per hour of ore, per flight, and multiple flights may be stacked in the same space as one, to improve capacity.