Table of Contents
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.
Bullion containing silver is treated in cells charged with a nitric electrolyte. These cells produce fine silver and leave a residue rich in gold.
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 foreman’s office, laboratory, and generator-room, respectively.
The methods here described are those in use, when notes for the present paper were taken.
How to Refine Silver with Electrolysis
An outline of the system is shown by the diagram, Fig. 1, which gives the order of events and the interdependence of the various operations in a brief form.
Silver Refining Equipment
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:
(2 x 8 x 4) + (2 x 4) = 72 surfaces, or 72 x 4 x 8.5/144 = 17 sq. ft.
The total current required is therefore 17 x 8.3 = 141 amperes.
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.
All the products of the silver process can be dried sufficiently in the centrifugals, so that they can be transferred to the crucibles and melted.
Fig. 9, a view of the wash-room, shows the centrifugals with their driving motors.
Operation and Products
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.
So long as the electrolyte contains an ample supply of silver, this is deposited in preference to the base metals.
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 Volhard’s 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.
(In some plants, the anodes are encased in cloth bags, and the black gold is caught before it can drop to the bottom, and is melted for gold anodes, without further 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.
The solution containing the base nitrates is treated as described under the head of Copper-Refining.
Refining Gold with Electricity
The process-tree, Fig. 10, gives an outline of the process of gold-refining, and shows the sequence of events in a graphic form.
Gold Refining Equipment
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 cells are placed in two raws, of 14 each, on a long bench. Those on one side form the first set, and those on the other the second set, for re-treating the cathodes formed in the first.
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.
Operations and Products
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.
Process for the Electrolytic Refining of Copper
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 cells are lead-lined wooden boxes, 3 by 1.5 ft. by 1.5 ft. deep. Each cell contains 23 anodes and 24 cathodes, hanging in alternate rows, 2 in. apart from center to center.
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 current used is direct and has a density of 10 amperes per square foot of cathode-surface, and a potential of 3.6 volts, which is equal to 0.6 volt per cell.
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.