Pilot plant operations are described for the treatment of manganiferous zinc concentrates from the Gamsberg ore deposit in South Africa via conventional electrolytic zinc processing. Two methods were employed for the control of excess manganese ions in the pilot plant electrolyte involving (1) chemical oxidation and (2) anodic oxidation.
Description of Pilot Plant
The pilot plant was designed to produce a nominal 100 kg of cathode zinc metal per day. This resulted in the treatment of approximately 200 kg of zinc calcine and the production of 1000 liters of impure zinc sulfate leach liquor per day. The individual process steps were carried out batchwise with the exception of zinc electrolysis, which was conducted on a continuous 24 hour basis.
Plant piping was of high density polypropylene. Storage tanks were polypropylene, fiberglass, polyethylene or rubber-lined depending on the temperature of the liquors entering. The cathodes and anodes (7 cathodes and 8 anodes) used were of commercial plant size (1.05 m x 0.6 m) rented from Zincor. The anodes were aged in the Zincor refinery for a period of eight weeks prior to use in the NIM pilot plant. The cathodes were similarly treated at Zincor prior to use at NIM. Total submerged area of the cathodes was approximately 1 m² per cathode
The residue from the neutral leach step was leached with 500 liters of spent electrolyte (170 g/l H2SO4, 60 g/l Zn, 5 g/l Mn) for a period of 2 hours at a temperature of 95°C. The resulting pulp was then settled and filtered, with the filtrate going to jarosite precipitation and the residue washed, sampled and discarded.
Excess manganese was removed from a solution bleed by chemical oxidation with ammonium persulfate and later by anodic oxidation with chemical lead anodes.
Chemical oxidation with ammonium persulfate was performed every second day on 450 liters of purified solution (225 liters from successive lots). The pure solution was first heated to 50°C and then ammonium persulfate added according to the following equation:
(5.4 x volume of solution x g/l Mn) /1000 = kg ammonium persulfate
Anodic oxidation was accomplished by electrolyzing a portion of, the main cell feed electrolyte in a small electrolytic cell. This cell employed five cathodes (210 mm x 360 mm) and six chemical lead anodes. The total submerged cathode area was 0.61 m² with a cathode spacing of 75 mm. Cathode current density was approximately 600 A/m². The anodes were cleaned every third day. Total solution flow through the cell was 1000 liters per day and supplied all of the spent electrolyte required for leaching.
Twenty two days after commencement of the pre-run period, the manganese content of the feed electrolyte had increased approximately 3 g/l, from 1.09 g/l to 3.97 g/l Mn. The amount of manganese removed by the main electrolysis cell (through normal anode deposit) was found to be 28.1% of the total manganese entering the circuit (dissolved during the leaching steps), indicating that approximately 70% of the total manganese entering solution would have to be removed by demanganization. Furthermore, during the pre-run period it was found that manganese dioxide additions, for oxidation of ferrous iron, were not required in either the neutral leach or jarosite precipitation steps as there was sufficient free manganese dioxide present in the Gamsberg calcine to reduce the ferrous iron content of these pulps to less than 1 mg/1 Fe++.
At the start of Phase I, all eight anodes were replaced at the same time with more active anodes. By this action, a large increase in the amount of manganese removed by the main electrolysis cell was observed. As a result, this practice was discontinued and anodes were thereafter replaced at a rate of two per week giving a complete cycle of 4 weeks. This is similar to that practiced in commercial plants, and allows for a constant manganese drop across the electrolysis cell. The use of more active anodes in combination with several chemical demanganizations resulted in a 1.5 g/l drop in manganese content of the feed electrolyte, from 4.0 g/l to 2.5 g/l.
Cathode zinc produced in the anodic cell accounted for 8.6% of the total zinc production. The cathode zinc from the anodic cell contained high concentrations of lend (300 ppm) and had poor physical characteristics (growths on the cathode surface, brittle deposit, difficulties in stripping compared to that of the main cell). The deposit met special high grade zinc standards for the remaining impurities (copper, cadmium, cobalt, iron). The cathode current efficiency for the anodic oxidation cell fluctuated greatly, as compared to the main cell, and averaged only 83.7%.
The accountability of zinc during the pilot plant operation was excellent with less than 1% of the total zinc unaccounted for. The overall zinc extractions (91.8% to 94.1%) obtained in the pilot plant were slightly lower (1%) than would be obtained in a commercial plant due to the limited amount of wash water available for use in the pilot plant and the inefficient type of washing that was possible on vacuum pan filters. The amount of wash water added in the pilot plant was the amount required to make up for the relatively low evaporation losses during leaching and that contained in the leach residues. The total wash water added to the circuit daily was 1.25 to 1.5 tons of water per ton of cathode zinc produced.
The objective of the demanganization step was to control the manganese content of the feed electrolyte at about 5 g/l Mn. In order to accomplish this, it was necessary to remove by demanganization approximately 70% of the manganese introduced into the circuit. Without demanganization, the manganese content of the feed electrolyte would increase rapidly.
The JAR process deviates from and improves the conventional jarosite process in that the entire jarosite pulp is returned to the neutral leach step eliminating the jarosite solid liquid separation step.
This modification has the following advantages over the jarosite process :
- Only one residue is produced that needs to be discarded, which allows for increased washing of the filter cake and thus recovery of more of the water soluble zinc contained in the residue. In the conventional jarosite process two residues are produced, one of which, the acid leach residue, has poor washing characteristics.
- Dissolution of zinc ferrites is increased since the zinc ferrites remaining in the jarosite pulp are recovered in the subsequent acid leach. This, coupled with the increased washing efficiency, results in an increase of approximately 2% overall zinc recovery.
- The JAR process greatly enhances the settling and filtering characteristics of the subsequent neutral and acid leaches because the coarse jarosite particles are now present in both of these leach pulps. As mentioned earlier the neutral leach filtration rate doubled and the acid leach filtration rate was trebled by use of the JAR process.