The use of direct amalgamation following stamps or primary mills and the use of barrel amalgamation for treating jig and corduroy  concentrates. Precipitation of gold and silver on zinc dust or on zinc shavings is fully treated, from the clarification and de-aeration of cyanide solutions to the cleanup and melting of the bullion. The use of aluminum dust, sodium sulphide, and charcoal as precipitants receives attention.

Gold Amalgamation Process Principles

The amalgamation process, which has been widely used in a variety of forms from earliest times, depends upon the wetting and alloying of metallic gold and silver with mercury. Because of its high specific gravity and tendency to be collected by larger masses of liquid mercury or mercury-coated surfaces, this alloy, or amalgam, is readily separated from the lighter constituents of the ore pulp. Richards and Lock remind us that:

In milling, three amalgams of gold may be considered. The first is liquid and when filtered contains only about 0.1 per cent gold at 60°F. The second is solid and represents the combination, in some definite chemical proportion, of gold and mercury. The third form consists of nuggets of gold superficially coated with and cemented together by the two other forms of amalgam. Silver amalgam may be divided into three similar classes.

Mercury also unites with a number of other metals, including copper, lead, tin, zinc, sodium, and potassium and in the case of certain metallic compounds, such as the chloride or sulphide of silver, can under favorable conditions cause them to decompose, with the formation of the chloride or sulphide of mercury and silver amalgam.

Direct Gold Amalgamation

Direct amalgamation, which refers to processes which contact the whole of the ore stream with mercury or mercury-covered plates and which has saved so much of the gold of the world in the past, is now largely obsolete. Modem plants almost invariably employ a concentration step ahead of amalgamation and subject only a relatively small bulk of high-grade concentrate to amalgamation treatment. This scheme greatly simplifies the cleanup operations and reduces the chances of gold loss through theft and other means. However, a description of some of the old methods may be of interest.

Amalgamation with Mortar-box and plate amalgamation can save 70% of the gold.

Amalgamation is practiced both inside and outside the 12-stamp batteries. For inside amalgamation, straight-sided, copper-faced, wooden chuck blocks are used. These blocks are equipped with half-round iron strips, spaced 2 in. apart, for their entire length. It was found that the half-round strips assisted amalgam to build to a greater thickness than did the smoother block. If the hourly inspection of the chuck blocks indicates that mercury should be added, it is fed in measured quantities from a horn spoon with the incoming ore at the back of the batteries. The chuck blocks are cleaned twice a month or oftener. The total quicksilver fed is recorded and gives a close estimate of the free-gold content of the ore crushed.

The pulp passing the battery screen falls upon reverse splash plates, the first of which is 5 by 51 in. in size with an area of 1.77 sq. ft. set at a slope of 3 in. per ft., and the second is 8 by 51 in. in size with an area of 2.83 sq. ft. and an inclination of 4 in. per ft. There is a drop of 2″ between these plates. On leaving the reverse splash plates the pulp drops 5″ to the cast-iron lip of the mortar. This drop is variable, depending on the height of the battery discharge. From the mortar lip, the pulp falls 4″ to an amalgamated apron plate, set at a slope of 1¼ in. per ft. This plate is 49 by 58″ in size and has an area of 19.4 sq. ft. An amalgam trap consisting of a wooden box of length equal to the width of the plate and of a uniform depth of 8″ is attached to and forms a part of the apron frame. The pulp flows from this trap through four 2-in. iron nipples, set in the side of the box, on a plane 4″ from the bottom. The pulp, issuing from these nipples, drops through a 10-mesh, woven-wire, brass screen to the sluice plate. If a battery screen is punctured, this 10-mesh screen catches the coarse oversize. The total drop from the discharge nipples of the trap to the sluice plate is 6″. This plate is set at an inclination of 1¼”/ft and is 46.5″ by 16 ft. in size. It has an area of 62.10 sq. ft. The total length of plates per battery is about 21 ft., and the total plate area is 86 sq. ft. On the average tonnage this represents about 4.3 sq. ft. of plate area per ton of ore milled per day. As the pulp leaves the sluice plate, it drops into another amalgam trap which is attached to and forms a part of the plate frame. This trap is a wooden box built the entire width of the plate, but unlike the first trap it has a sloping bottom, and the depth of sand adjacent to the plate discharge is but 4 in., whereas the depth along the trap overflow is 6″. All plates, including the chuck blocks, are made of copper which weighs 5 lb. per sq. ft. and are electroplated with 3 oz. of silver per square foot of contact surface.

When the tube mills operate and grind the sands from the classifier, which treats the tailings of the first two rows of vanners, the ground product is amalgamated on eight shaking plates 4 by 5 ft. in size, which have a combined area of 160 sq. ft. These plates are set upon a slope of ¾ in- per ft- and are oscillated ninety times a minute by a simple strap eccentric.

Each morning the amalgam on the apron and sluice plates is softened with mercury, rubbed with a rag, cleaned with a rubber squeegee, and dressed with a whisk broom for the day’s operation. The cleaning and conditioning of these plates require about 3½ hr. The monthly cleanup, including retorting of amalgam and melting of bullion, occupies 5 days. On the first day of the cleanup all sluice plates are cleaned and scraped with wide-faced putty knives, and in addition one battery is dismantled, cleaned, and reassembled. During the second day six apron plates are cleaned and scraped with a scraper made from an old file, the end of which has been flattened, widened, sharpened, and turned at right angles to its length. Two additional batteries are cleaned on this day. On the third day, the remaining six apron plates are scraped and cleaned, and three more batteries are cleaned out. During the fourth day all splash plates are taken to the cleanup room to be steamed and scraped, and, in addition, four batteries are cleaned up. The two remaining batteries are cleaned on the fifth day.

The amalgam recovered from dressing the plates each day is squeezed into a pellet and stored until retorted with the general cleanup amalgam. Residues taken from the batteries are placed in an amalgam barrel, which contains three pieces of stamp stem and which is rotated for about 12 hr. Mercury, amounting to 350 troy oz., is then placed in the barrel, which is again rotated for an additional hour or two. It is then stopped, opened, cleaned, and the pulp run into a storage box under the barrel. From here it is fed by the cleanup man to a power jig, the bed of which forms on a fine wire screen. The amalgam is found in the hutch of the jig with the iron floating on the amalgam, and the sand over the iron. When all of the barrel charge has passed through the jig, the sand is scooped from the hutch, the iron is removed by a magnet and the liquid amalgam is removed through a spigot into an iron dipper. Here it is further cleaned by mechanical agitation and a water jet which removes any foreign matter present. After squeezing the cleaned amalgam in a canvas cloth by hand and removing most of the liquid mercury, additional mercury is removed by further squeezing the soft amalgam in canvas, using a mold and a hydraulic press.

The final amalgam from the cleanup, placed in trays, is sealed in the retorts during the afternoon of the fifth day. The retorts are heated, and the quicksilver volatilized and condensed during the night. Wood is used for fuel. The next morning, the sixth day, the bullion sponge is removed from the warm retort, placed in graphite pots and, after melting in oil-fired furnaces, is poured in bars which are shipped at once either to the American Smelting and Refining Company at Selby or to the United States mint at San Francisco. The trays into which the amalgam is placed for retorting are painted with a chalk-and-water emulsion which is thoroughly dried before using. This coating assists in a clean removal of the gold sponge after retorting.

During 15 years the bullion has averaged 821 parts gold, 159 parts silver, and 20 parts base metal. The mercury loss is 0.17 troy oz, per ton of ore milled.

Homestake experience favors amalgamation as a cheap method of gold recovery preceding cyanidation. Laboratory tests at mill sizes indicate that upward of 75 per cent of the gold is free. Of this, about 60 per cent is caught by mercury which is fed to the rod mills and ball mills in closed circuit with Clark-Todd amalgamators and Dorr classifiers in the South plant, the only one now operated.

The amalgamator is shown in Fig. 50. It provides new means of increasing the catch of amalgam, by causing the pulp to change direction several times, each change resulting in a retarded velocity of flow and affording opportunity for amalgam to build up on the plate. In this apparatus the launders are fitted with amalgamated trays, which facilitate the attachment of amalgam particles. The trays are deep enough to retain a considerable body of amalgam without loss.

In the amalgamator the amalgam is not caught in a thin film or layer, spread over a large surface of plate; rather, the catch is three dimensional. The amalgam builds upon itself to form bodies of appreciable thickness.

 

The amalgamator also removes scrap iron, fragments of the grinding media, and other material at the point of discharge of the mill, thus keeping clean the recovered amalgam. It does all this in much less floor space than did other systems. The chip screen is removable, but the guard screen below it is not.

The second launder is lined with a removable amalgamated-copper box. The bottom of the third launder is covered with an amalgamated silver-plated copper plate. This plate is also removable and is taken out and cleaned at more frequent intervals than is desirable for the trays.

Homestake amalgam yields 43 per cent gold; mercury consumption is 1/8 oz. troy per ton of ore crushed, the cost of which was (in 1936) 1.25 of the 1.4 cents total cost for amalgamation.

Formerly, copper plates in front of the stamp mortar boxes and in the tube-mill circuit, also mercury wells with baffle boards below the plates, saved 78 to 89 per cent of the gold. Now, by blanket, concentration below the stamp mortar boxes and after the tube-mills, and subsequent concentrate treatment, the recovery is approximately the same as with all-amalgamation. In the blanket machines the blanket or carpet is attached to jute cloth or to discarded filter cloth and is washed by spraying every 1 to 2 hr. Tilting tables to which blankets are fixed have been adopted at Champion Reef.

The treatment of the concentrate for the recovery of gold values varies on the four mines. At Ooregum the concentrate is treated by amalgamation on copper plates; at Mysore by a process of reconcentration and tabling, yielding gold dust containing 90 per cent fine metals which is smelted direct to bullion; and at Nundydroog and Champion Reef by agitating with strong cyanide solution followed by zinc-box precipitation.

TREATMENT OF GRAVITY CONCENTRATES

Barrel amalgamation is the simplest and most common method of treating the rich concentrates caught by the corduroy, jigs, or other gravity means. A heavy, cast-iron barrel with manhole, revolving slowly, is used. In it are placed a charge of concentrates, steel balls or a rod or two, some water, lime, and mercury. The whole may be run 2 to 12 hr. The pulp is discharged; the amalgam is then caught in riffles, and the fine pulp in boxes or tubs, from which it may be fed slowly into the mill circuit for cyaniding. The amalgam is retorted in the ordinary manner.

Grinding concentrates in batches is the usual practice. Small gold particles are released, and consequently the recovery is greater. However, gold particles smaller than 700 mesh are amalgamated with difficulty, probably because they remain suspended in the pulp. The best grinding practice must be determined by trial, so as to release gold but not comminute the released particles.

If the gold is coated with rust, grinding a sandy concentrate usually scratches the gold particles so that they will amalgamate. It is sometimes expedient to mix jig and flotation concentrates for amalgamation. Grinding a sandy concentrate with mercury in an alkaline lime or sodium hydroxide solution will usually give good recovery in spite of the popular belief that flotation concentrate will not amalgamate.

It is recommended that two speeds be provided for barrels where grinding is used, for then it is possible to continue with the amalgamation step without removing the grinding balls.

The time of grinding should be considerably reduced where arsenic and other minerals are present that “sicken” the mercury (see below for “Use of Chemicals”). The capacity of grinding barrels is about as follows:

The barrel is usually discharged into a hydraulic, vertical-flow separator, which collects the mercury and allows the fine ore and slime particles to pass on to waste or into the mill circuit.

A Berdan pan is effective for cleaning mercury or amalgam or for amalgamating corduroy gold. The pan of this type, made by the Mine and Smelter Supply Company, is suspended at an angle from a guarded, gear-driven spindle within a frame of angle iron standing 4¾ ft. high. The bowl or pan is 24 in. in diameter and runs at 20 r.p.m., taking ¾ hp. Grinding is done by means of an 8-in. steel ball running loose in the bowl. Material as coarse as ¼ in. can be fed to the pan. A Berdan pan can be fed continuously or intermittently. If the latter method is employed, when the material has been ground, a supply of water will wash out the slime and leave clean amalgam. This pan can be used for the cleanup, for grinding concentrates at a small mine, or for grinding and amalgamating rich ore.

The Wheeler pan consists essentially of a cast-iron tub, usually about 5 ft. diameter and 2½ to 3 ft. deep, carrying a broad annular die ring on the bottom, on which heavy shoes are dragged by means of a yoke; this, in turn, is driven by a spindle from bevel gears and a belt-driven countershaft below the pan bottom. Shoes and dies are ordinarily of gray cast iron which wears down with a rough scored surface. White iron and alloy steels are unsuitable because the wearing faces become smooth and polished, with accompanying reduction in capacity. An adjusting screw with locking wheel is provided for adjustment of the height of shoes. Mullers, which carry the shoes, should be attached to the yoke arms by a flexible fitting in the nature of a universal joint; if a rigid joint like that in an amalgamating or cleanup pan is used, the shoes will often chatter, capacity be reduced, and breakage increased. The die ring and shoe circle are sometimes continuous, but ordinary short spaces are left between both the shoes and the die segments. These form channels into which pulp flows and from which the crushing faces are fed. New shoes weigh 75 to 200 lb., and the crushing force is limited to that exerted by their weight when dragged over the die. Compensating weights are sometimes used to keep the crushing force up to normal as the shoes wear.

Use of Chemicals

Mercury losses occur through the formation on the fine metallic globules of a tough skin composed of base-metal amalgams or foreign matter. It is also known that acids liberated from decomposing sulphides attack the mercury, but the latest work seems to indicate that the unsaturated surface of fresh fractures react with the mercury to produce minute films that prevent coalescence.

The particular combination of reagents found by A. E. Flynn of the Nova Scotia Technical College to be most effective on an arsenopyrite concentrate from Canada was as follows:

Arsenious oxide………………….0.85% weight of charge
White lead…………………………1.50
Sodium hydroxide………………0.70

It is stated that the lead remains in some insoluble form which evidently plays the desired part. Some 99 per cent of the mercury was recovered.

Other reagents recommended by operators include lime, lye, cyanide, sal ammoniac, litharge, and even soap, depending upon the particular conditions, but their use is generally the result of trial-and-error methods.

Amalgamation at Pickle Crowe

Concentrates from the cones and blankets are stored in a box measuring 3 by 6 by 3 ft. The contents of this box at 80 per cent solids, with 5 lb. lime and 5 lb. sodium cyanide, are emptied daily under normal operating conditions into the amalgam barrel. This is of cast iron with rubber lining. The inside dimensions are 3 by 4 ft. It is belt driven at 20 r.p.m., and at one time a load of 300 lb. of 2-in. balls was used with grinding continued for 16 hr. Weight of pulp discharge is approximately 600 lb. After grinding and agitation the concentrates are approximately 80 per cent minus 200 mesh. Then 30 lb. mercury is added, and agitation continued for another hour. The use of grinding balls was later discontinued.

The door of the barrel is then opened, and the pulp and amalgam allowed to flow slowly over an amalgam plate as the barrel continues to revolve. The plate is silver-plated copper, 3 by 4 ft., with built-up sides to avoid splash. It is set at a slope of 3 in. per ft. The pulp running- off the plate is caught in a 12- by 8- by 6- in. trap which holds the free mercury and amalgam not caught on the plate. Overflow from this trap passes over a second plate and a second trap and finally to a strip of corduroy blanket. Blank tails are pumped to the primary thickeners.

All traps and plates are cleaned daily. The amalgam is spread thinly over the second plate and washed with water to remove steel, etc. The corduroy blanket is washed in the storage box. The amalgam is separated from free mercury by squeezing it in a canvas cloth and then retorted in a 2-qt. cast-iron retort. The sponge gold from the retort is melted on refining days, and the condensed mercury added to the working stock. Mercury consumption is 0.5 lb. per charge.

Bullion Parting

The Miller or chlorine process of parting was adopted following the abandonment of the gold standard in the United States and the subsequent increase in the price of gold. Because 60 per cent of Homestake bullion comes from amalgamation and the balance from cyanide precipitate, this company had a choice of three methods by which the combined bullion could be treated, viz., the Miller, electrolytic, and acid parting, and they chose the first. The following analysis is representative of the bullions to be treated. The cyanide precipitate is melted in a Monach furnace, and a double pour is made: one-half the slag in one pot and the balance in a second pot. After 5 min. the slag is poured, leaving a scull. The bullion is set in a special divider placed in the pot.

The accompanying flow sheet (Fig. 54) gives a good description of the parting and refining operation by which all Homestake bullion is now being treated. All refined bullion is shipped to the United States mint.

Silver is melted into bars averaging about 980, although a process was developed whereby silver of standard purity, 999 fine, was made. After the process had been developed, new Treasury Department rules made it unnecessary to refine silver to this degree. With ordinary care, gold bars, as shipped, are about 997 fine.

Treatment of Gold Precipitates Containing Copper