The Recovery of Cadmium from Cadmiummcopper Precipitate, Electrolytic Zinc Co. of Australasia, Risdon, Tasmania

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1 The Recovery of Cadmium from Cadmiummcopper Precipitate, Electrolytic Zinc Co. of Australasia, Risdon, Tasmania G. H. ANDERSON* CADMIUM-COPPER precipitate, a byproduct of the purification stage of the zinc plant, is composed mainly of zinc, cadmium and copper in varying amounts depending on the efficiency of precipitation and the cadmium and copper contents of the impure solution treated. The composition range is approximately: cadmium pct, copper 6-8 pct, and zinc pct. As received at the cadmium plant, the precipitate is a dark gray to black press cake produced by filter pressing the flocculent cadmium-copper precipitate formed when impure zinc solution is agitated with zinc dust. The details of the cadmiumcopper precipitation are to be found in the paper "Electrolytic Zinc at Risdon, Tasmania," by W. C. Snow. Transactions AIME, 121, 501. The treatment, in addition to recovering cadmium as metal, yields a copper product as a residue for realization and recovers in solution for return to the zinc plant, the excess of zinc dust used during purification. The recovery of cadmium metal is tpproximately 200 tonst per year, but, vith the amount of copper residue t Throughout this paper the long ton of and the Imperial gallon are used. 4ARCH 1949 recovered, varies with the zinc production tonnage, composition of the original concentrates, degree of roasting and other operational factors. The operations involved in the recovery of the foregoing materials are carried out in a building adjacent to the purifkation section of the zinc plant and can best be described by dividing the process into the following stages: I. Oxidation and Grinding of the cadmiumcopper precipitate. 2. Leaching and Filtering. 3. Precipitation of cadmium. 4. Oxidation and Grinding of the precipitated cadmium. 5. Leaching oxidized cadmium precipitate and Purification of leach solution. 6. Electrolysis. 7. Melting, Casting and Packing. A feature of the process is the use of two distinct solution circuits. Spent electrolyte from the zinc plant circuit containing about 10 pct sulphuric acid is used as the primary solvent of the San Francisco Meeting, February TP 2525 D. Discussion of this paper (2 copies) may be eant to Traneaclions AIME before June 15, Manuscript received October 1, * Aesistant Operating Superintandent. Electrolytic Zinc Co. of Australasia Ltd., Hobart. Tasmania. cadmium and zinc present in the oxidized precipitate. When the cadmium is later separated by precipitation and filtering, the filtrate, originally zinc plant spent. electrolyte and now fortified in zinc, is returned to the zinc plant. The precipitated cadmium, after oxidation, is redissolved in spent electrolyte from cadmium electrolysis which solution is in closed circuit within the cadmium plant except for discards to the zinc plant circuit as mentioned in Section 5 hereinafter. (Fig 1.) Details of the various operations follow. 1. Oxidation and Grinding of the Cadmium-copper Preeipi tate The precipitate having had a preliminary drying by eomprgssed air in Dehne filter presses in the zinc plant is trucked to a storage platform of sufficient length to store 3 to 4 days' production separately. After 48 hr exposure to atmosphere, oxidation is sufficient to enable all cadmium and zinc but only a portion of the copper to be dissolved in dilute sulphuric acid. The oxidized precipitate is shovelled into trucks and, after weighing and sampling, is broken in size by means of Metals Transactions... PO5

2 1 PRECIPITATE FROM ZlliC PLANT k C 0 CU PRECIPITATE STORAGE i MIXER TUBE MILL I STORAGE ZlNC PLANT SPENT 7 DECANT-, UNDERFLOW ZlNC DUST DUST FlLTER CAKE FILTRATE COPPER RESIDUE FOR REALISATION 4 STORAGE 1 RETURN TO ZlNC PLANT OXlDlJlNG HEARTH '=3 BALL MlLL CD SPENT LIMEROCK SOLIDS >TTDRos I t ROSIN DISTILLATE GLUE CELLS FILTRATE C CD SHEETS 7-PRILLS I MOULDS PACKED CADMIUM TO ZINC PLANT ROASTING FIG 1-Flowsheat. right and left handed blades rotating in a shallow mild steel trough. Return solution (see Stage 3) is added to form a pulp which passes to a tube mill 5 ft long and 3 ft diarn having cast steel liners and loaded with flint pebbles. The use of return solution is restricted to produce a thick pulp which is elevated to a wooden storage tank of 3200 gal capacity and provided with air agitation. 2. Leaching and Filtering Leaching is carried out in a wooden tank of approximately 14,000 gal capacity, having a conical bottom and provided with both mechanical and air agitation. Discharge is through a plug cock but, in addition, the tank has a side discharge to which is fitted, inside the tank, an armored rubber hose for decanting clear solution after settlement of the solids. The tank is covered and ventilated to atmosphere. Approximately gal of zinc plant spent electrolyte containing g sulphuric acid and g zinc per liter are run into the leachiug tank and after agitation is started the ground cadmium-copper precipitate is charged until the acidity of the solution falls to 5-6 g sulphuric PO6... Metals Transactions MARCH 1949

acid per liter. Agitation is continued for another 5-6 hr by which time the acidity is reduced to 1-2 g sulphuric acid per liter, then ground limestone is added slowly until a ph 3-4 is reached.

3 acid per liter. Agitation is continued for another 5-6 hr by which time the acidity is reduced to 1-2 g sulphuric acid per liter, then ground limestone is added slowly until a ph 3-4 is reached. Approximately 30 min. later a sample is taken, tested for copper and the calculated quantity of zinc dust required to remove copper in excess of 0.5 g per liter, is added. (See Appendix 1.) The zinc dust containing pct of minus 250 mesh material, used in this and other stages of the recovery of cadmium, is produced at Risdon by atomizing molten high grade zinc with compressed air at 200 psi pressure. Control of the copper cont>ent of the solution is important, and is considered to have three distinct advantages. First, its subsequent precipitation with the cadmium in Stage 3 prevents coagulation of the cadmium into metallic balls and facilitates filtering; second, during the cadmium circuit leach (Section 5) arsenic is precipitated or otherwise eliminated, and third, its presence insures a minimum of zinc and cadmium in the copper residue. If no copper is in the leach solution, sufficient sulphated copper residue is added to give the desired content. Sulphated copper residue is made by mixing copper residue and sulphuric acid and storing in heaps for one or two days. Approximately 30 min. after the addition of the calculated amount of zinc dust, a further test is taken to determine copper in solution, and if necessary further additions of zinc dust are made. Agitation is continued for another hour after which solids are allowed to settle for 3 to 4 hr when clear solution is decanted and pumped to the cadmium precipitating tank. (See Section 3.) The remaining pulp is agitated, air lifted to a wooden storage tank and filtered in a small Moore filter unit. A cake of about in. thickness is formed in 20 min. and washed with water until relatively free of zinc and cadmium. The filtrate and washings are pumped by wet vacuum pumps to the cadmium precipitation tank containing the decanted leach solution. The filter cake is discharged to a hopper and fed to an oil Ered rotary drier 15 ft long and 3 ft 9 in. diam consisting of a mild steel shell lined with 10 ga stainless steel sheet and fitted with suitable angle lifters. The dried copper residue is bagged for shipment to a copper refinery at Port Kembla in New South Wales. Metallurgical and Average figures for year ending June 30, CALIMIUM-COPPER PRECIPITATE 6.85 tons treated per day. ASSAY Cd per ceut Cu per cent Zn percent Pb per cent Ag oz per ton. Cadmium extracted by leaching per cent Copper solubility per cent Zinc dust used per ton ppte. leached Ih Limestone uaed per ton ppte. leached lb Copper residue factor per cent Copper ~esidue made per filter cake lb Wash displacement COPPER RESIDUE ASSAY Cu ,6.5 per cent Cd per cent Zn per cent Pb per cent Ag oz per ton. 3. Precipitation of Cadmium The precipitation tank is identical in construction to the leach tank previously described except that no decantation hose is fitted and air agitation is not provided. To the decanted solution, filtrate, and washings derived from the 8000 gal leaching operation described in Section 2 ab6ut 600 lb of zinc dust is added after agitation has commenced. Filtered samples of the solution are tested with hydrogen sulphide from time to time and further smaller additions of zinc dust are made until only a trace of cadmium remains. Approximately min. later, if a similar test indicates a satisfactory cadmium content, the pulp is filtered in two 27 in. Dehne filter presses and the filtrate, known as "Return Solution," after by-passing for a few minutes, passes to a storage tank prior to its return to the zinc plant circuit for the recovery of its zinc and cadmium contents. When the filter presses are full of precipitate, pumping is stopped and a water wash given until a specific gravity of shows that the precipitate is almost free of zinc sulphate. After drying with compressed air the washed precipitate is discharged to hoppers over a drying and oxidizing hearth. Metallurgical and Average figures for year ending June 30, Zinc dust efticiency pct Cadmium in return solution gpl 4. Oxidation and Grinding of the Precipitated Cadmium The oxidizing hearth is of brick construction with a hearth of cast iron plates approximately 11 by 9 ft mounted so that gases from an oil burner first pass under the hearth and then over the precipitate which, when dropped from the hoppers above, is raked evenly over the hearth to a depth of 4 to 6 in. As the precipitate oxidizes readily, a maximum gas temperature of 300 C is set at the rising flue from under the hearth to above the charge but rarely is this limit necessary. Hand rabbling proceeds at intervals until the color of the precipitate indicates that oxidation is complete, at which stage the material is discharged by hand rakes to a ventilated hopper from which the precipitate is screw fed to a mild steel ball mill 4 ft long and 2 ft 6 in, diam loaded with 1% and in. steel balls. Cadmium cell feed solution is added to produce a reasonably thick mill discharge which is stored in a wooden tank ready for air lifting to the leach tank (Stage 5) when required. For ventilat'ion purposes, a fan handling 10,000-15,000 cfpm is connected to the hood over the oxidized precipitate hopper and to a hood over the mill feed screw discharge point. The dust laden air is delivered to the base of a water irrigated wooden scrubbing tower mounted over a concrete sump from which the water is recirculated by means of a pump while the settled pulp is withdrawn at intervals and pumped to the leach tank (Stage 5). Metallurgical and Average figures for year ending June 30, CADMIUM PRECIPITATE Pcr Totalcadmium Acid sol. cadmium Zinc Oxidation of cadmium in ppte Metals Transactions

Sulphuric acid used per ton cadmium dissolved..... 0.287 tons Zinc dust used per ton cadmium dissolved.... 2470 Ib ton cadmium dissolved..... 56.3 lb Hydrated lime used per FEED SOLUTION ASSAY Cd.

5 Sulphuric acid used per ton cadmium dissolved tons Zinc dust used per ton cadmium dissolved Ib ton cadmium dissolved lb Hydrated lime used per FEED SOLUTION ASSAY Cd gpl Zn ~1 Fe gpl Mn gpl Cl mgpl Cu mgpl As... <0.1 rngpl The cell room contains 20 cells arranged in five rows of four cells, each of which is of 3 in. timber, lined first with 8 lb lead and then with 2 in. of a 40 pct sulphur-60 pct sand mixture which has given good service as only 2 linings have failed in 25 yr. Internal dimensions of the cells, which have a volume of 208 gal, are 5 ft square and 2 ft 6 in. deep while the cross-section is semioctagonal. The lead of the 10 half-round, 23 in. radius anodes contains 0.01 pct silver to give added rigidity and extend the 4 yr life to 5 yr. Mounted on a 3 in. diam mild steel shaft and separated at 5% in. spacing by rubber covered aluminum sleeves are 9 aluminum cathodes, 4 ft diam and in. thick, each of which has a submerged surface area of 8 sq ft and lasts 7 to 8 yr. To enable the stripped cadmium sheet to be readily broken into four conveniently sized quadrants, each side of the cathode is radially grooved at 90" spacing to a depth of JQz in. Before assembly, each cathode is given a matte surface by scrubbing with a steel wire brush and sandstone. Once each year the cathode assembly is dismantled, inspected, cleaned and, if necessary, faulty parts replaced. Cathodes are rotated at 10 rpm by suitably driven link chains through gearing from which the cathode assembly may be disengaged without stopping the driving mechanism. Electrically the cells are arranged in series and current is supplied by two motor driven parallel connected generators rotating at 960 rpm and each capable of supplying 650 amp over a range of 25 to 70 volts. Anode bus bars of 139 in. square copper are arranged to give the anode head bars an edge contact and between adjacent cells is a hinged section for short circuiting a cell when cathode assemblies are removed for stripping. The series circuit is made complete by means of a 9 in. diam brass collector keyed to the cathode shaft and con- MARCH 1949 tacting brushes connected by copper cable to the following anode bus bar. Cell voltage is approximately 3.5 volts per cell. Feed solution containing 150 g cadmium per liter gravitates from the feed storage tanks through iron pipes of uniform length flanged together for ease of cleaning. Each cell has a separate feed pipe through which the rate of feed is controlled to hold the acidity of the cell solution at g sulphuric acid per liter. Titrations with caustic soda using Congo Red indicator are made every four hours. Spent overflows from each cell to a launder common to each row and gravitates to a storage tank. Glue solution containing 0.14 lb glue per gal is added every four hours at a rate equivalent to 2% to 3 lb per ton cadmium deposited. During deposition, the cadmium surface tends to roughen by the formation of "trees" which are wiped off at intervals with a slotted wooden tool to which rubber strips are attached to make contact with each face of the cathode simultaneously. Trees are cleaned from the cells monthly and returned to the tube mill grinding the original cadmium-copper precipitate. Stripping periods are either 48 or 96 hr depending on the current density demanded by production. When the current density on the immersed cathode surface is between 10 and 12 amp per sq ft quite a reasonable current efficiency is obtained and the deposit adheres to the cathode, but, when the current density is higher, the stripping period is reduced to 48 hr as beyond this time the current efficiency is found to decrease and the deposited cadmium sheets tend to become detached. The production of cadmium is not regular owing to variation in the amount of incoming cadmium, consequently both the number of cells used and the current density are varied, but it is aimed to operate at 10 to 12 amp per sq ft; however, at times a higher density is necessary. When the deposit is about to be stripped, the cell concerned is short circited by means of the hinged section of the anode bus bar, hooks suspended from a bridle of convenient length are attached to fittings on the cathode shaft and the whole cathode assembly is lifted by means of a travelling chain block and lowered on to trestles at the end of the cell row. The deposit is removed by means of stripping knives, broken into quadrants, stacked almost vertically in portable racks, weighed and transferred to the melting section. Before returning the assembly to the cell, each aluminum cathode is hosed, scrubbed with a s t3 bristle brush and, if necessary, given a light rubbing with sandstone. (Fig 2.) Metallnrgioal m d Average figures for year ending June 30, Average current amp Current e5ciency.. Current density Cadmium "trees" produced Number of cells in use DISCARD SOL. ASSAY pct 11.0 amp per sq f* *" 6.11 pct of cathode deposited G PER ~ T X R Cd Zn 37.2 H~SOI Meltlng, Castlng and Packing The cadmium sheets are melted, under a cover of molten rosin, in an oil fired cast iron pot, having a capacity of approximately 2500 lb cadmium. The molten cadmium is handled from the pot to the molds in a mild steel ladle, the pouring spout of which is in the form of a pipe reaching almost to the bottom to prevent dross or flux entering the mold. Cadmium is cast in various shapes for commercial purposes. (See Appendix 3 and Fig 3.) Molds are either lubricated with stearin or No. 3 shell grease or sprayed with a fine mist of dilute sulphuric acid in order to give a clean bright surface to the castings which after inspection are packed in Tasmanian oak cases in which are inserted sheets of waxed paper and cardboard to keep the castings from contact with the wood. Each case is banded with in. wide banding iron and stamped with particulars of type of casting, gross and net weights, lot and case numbers. Periodically dross is ladled from the surface of the molten cadmium in the melting pot into mild steel trays and after cooling and weighing is ground in a Berdan grinding pan from which a stream of water carries off the slime to a settling pond, the overflow from which passes into the cadmium-copper precipitate pulp circuit. The dross slime, collected in a settler, Metals Transactions... PO9

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