IMPROVED INSTRUMENTATION FOR URANIUM PLANTS

Extensive use was made of control instrumentation in the design of South African uranium plants, particularly those constructed in 1970—1971. However, many of these instruments were supplied without adequate back-up information on their application, engineering design, installation and commissioning, and consequently they did not always perform satisfactorily.

During the past five years, much progress has been made in South Africa in the development of new instruments and in the adaptation of existing commercial devices for control purposes on uranium plants [25]. Thus, for example, a much improved instrument, based on the principle of electrodeless conductivity measurement, was developed for the control of sulphuric acid addition to the leach. At one of the plants where this instrument was subjected to full-scale trials, previous practice was for the leach operators to maintain the free acid in the leach pachucas between 2 and 10 g/1 by manual methods of measurement and control. The new automatic control system using the electrodeless conductivity meter succeeded in maintaining the free acid level at 6.8 ± 0.4 g/1 with a considerable reduction in sulphuric acid consumption. Other developments include much improved instruments for controlling the addition of manganese dioxide to the leach by measuring the redox potential as an indicator of the ratio of ferric to ferrous ions and for measuring the pH in organic continuous emulsions in Purlex plants.

All the existing uranium producers have now installed these new instruments with considerable savings in the costs of reagents and labour and improvements in operating efficiency.

IAEA-AG/33-5 35 PACHUCAS

(Acid leach with MnO2 or ferric)

THICKENERS (CCD: murky solutions)

i

OR

Strong-base resin

Elution with 10% H2S O4

M I X E R - S E T T L E R S (Bufflex liquid-liquid extraction)

HYDROCYCLONES (Partial solid—liquid separation:

dilute slurries with up to 10 mass % solids)

CONTINUOUS COUNTERCURRENT

ION-EXCHANGE COLUMNS

Weak-base resin

Nitrate elution including use of NHg for nitrate recovery

ADU PRECIPITATOR ADU PRECIPITATOR

FIG.6. Proposed new uranium recovery flowsheets based on the treatment of unclarified solutions or dilute slurries by CIX.

10. CONCLUSIONS

(a) The rapid increases in the price of gold and uranium in recent years have coincided with an unprecedented increase in working costs at South African gold mines. The economic feasibility of an expansion in uranium production capacity, particularly for the treatment of low-grade slimes dams, is strongly dependent on the development of processes that are cheaper than those employed at present.

36 JAMES

(b) Provided that laboratory results can be reproduced on an industrial scale, wet high-intensity magnetic separation may well prove to be a major factor in the development of economic methods for the recovery of uranium from very low-grade ores and gold-plant tailings previously considered not to be worth treating for uranium.

(c) The trend in South Africa is towards higher overall extractions of gold and uranium by a changeover to reverse leaching and by the introduction of more intensive conditions (high con-centrations of ferric ion and high temperatures) in the uranium leach.

(d) A reassessment of the economics of pressure leaching in the light of current technology and predicted economic trends has resulted in a collaborative industry project to test it on a demonstration-plant scale. The extent of the improvement achieved in the recovery of gold when pressure leaching is used in a reverse leaching process will be a major factor in any decision to adopt this technique at existing or future uranium plants.

(e) The successful development and pilot-plant application of the NIM CIX Contactor to the recovery of uranium has opened up the possibility of a new generation oí uranium recovery flowsheets, which offer a reduction in the overall capital cost of plants by the elimination of the clarification step and the substitution of cheaper and simpler solid/liquid separation for the filtration step.

(0 The successful development of improved instrumentation has resulted in considerable savings in the costs of reagents and labour and an improvement in operating efficiency at South African uranium plants.

ACKNOWLEDGEMENTS

Thanks are due to the many staff members of the National Institute for Metallurgy, the Extraction Metallurgy Division of the Atomic Energy Board, and the uranium industry, who have been responsible for the work described here and who have assisted the author in compiling this paper.

The permission of the Atomic Energy Board and the Nuclear Fuels Corporation of South Africa to publish this paper is gratefully acknowledged.

REFERENCES

[1] The Recovery of Uranium (Proc. Symp. Sâo Paulo, 1970), IAEA, Vienna (1971).

[2] Uranium in South Africa 1946—1956, Symp. Associated Scientific and Technical Societies of South Africa, Johannesburg, 1957.

[3] WORROLL, R.E., "Uranium mining and marketing in South Africa", Natural Uranium Supply (Proc. Symp.

Mainz, 1974), AED-CONF-74-600-013.

[4] Quarterly review of South African gold shares, Min. J., Lond. 19 2 (1975).

[5] NUCLEAR EXCHANGE CORPORATION, Rep. No.84, Menlo Park, California (Jul. 1975).

[6] BOTHA, 1.С., RUSSELL, B.G., A Review of the Present and Future Development of Sulphur Production in South Africa, National Institute for Metallurgy, Johannesburg, Rep. No.1750 (Jul. 1975).

[7] CHAMBER OF MINES OF SOUTH AFRICA, Annual Report (1974).

[8] LAXEN, P.A., "Some developments in gold extraction from Witwatersrand ores", 104th AIME Annual Meeting, New York City, Feb. 1975, Paper No. A75-56.

[9] FAURE, A., et al., The production of high-purity uranium at a South African gold mine, J.S. Afr. Inst.

Min. Metall. 66 8(1966)319.

[10] FAURE, A., TUNLEY, Т.Н., "Uranium recovery by liquid-liquid extraction in South Africa", The Recovery of Uranium (Proc. Symp. Sato Paulo, 1970), IAEA, Vienna (1971) 241.

[11] TUTT, D.J., "Review of the application of beneficiation processes to uranium ores before leaching", The Recovery of Uranium (Proc. Symp. Sao Paulo, 1970), IAEA, Vienna (1971) 33.

[12] LIEBENBERG, W.R., Mineralogy and the metallurgist, Miner. Sci. Eng. 2 4 ( 1970).

IAEA-AG/33-S 37

[13] LEVIN, J. (National institute for Metallurgy, Johannesburg), personal communication.

[14] BRITTEN, H., DE КОК, S.K., Uranium m South Africa, 1946-1956, Vol.1, p.462.

[15] NEYTZELL-DE WILDE, F.G., Developments in the Auto-oxidation Process for the Individual or Simultaneous Production and Regeneration of Dilute Sulphuric Acid and Ferric Sulphate with Special Reference to the Application of the Liquors Produced to Metallurgical Leaching Practice, PhD Thesis, University of the Witwatersrand (1950).

[16] DE КОК, S.K. (Anglo-Transvaal Consolidated Investment Co. Ltd., Johannesburg), personal communication.

[17] LAXEN, P.A., A Fundamental Study of the Dissolution in Acid Solutions of Uranium Minerals from South African Ores, National Institute for Metallurgy, Johannesburg, Rep. No. 1550 (May 1973).

[18] FAURE, A., Atomic Energy Board, Unpublished confidential report, 1971.

[19] COLBORN, R.P., NICOL, D.I., JOCHENS, P.R., "The determination of heat-transfer coefficients for slurries in a spiral heat exchanger", Symp. Heat Transfer and Design and Operation of Heat Exchangers, Johannesburg, 1974.

[20] GOLDBLATT, E.L., Improvements in the Continuous Production of Ferric Sulphate, General Mining and Finance Corporation Limited, S. Afr. Patent No. 73/0722 (19 Dec. 1972).

[21] JACKSON, E.J., ROBINSON, R.E., The Application of Pressure Leaching for the Extraction of Uranium, Anglo American Corporation of South Africa, Central Metallurgical Laboratory, Research Programme C.M.L. 6(1956).

[22] HAINES, A.K., PAYNTER, J.C., The Development of Multistage Fluidized-Bed Continuous Ion-Exchange Columns for Hydrometallurgical Applications, National Institute for Metallurgy, Johannesburg, Rep. No.1317 (Jul. 1971) (restricted).

[23] HAINES, A.К., et al., "The use of weak-base resins in the recovery of uranium from unclarified leach liquors", 1 lth Int. Mineral Processing Congress, Cagliari, Apr. 1975.

[24] LLOYD, P.J., "Economic recovery of low-grade uranium values from Witwatersrand ores - A review of progress", The Recovery of Uranium (Proc. Symp. Sâo Paulo, 1970), IAEA, Vienna (1971) 437.

[25] SOMMER, G., Less hands in uranium extraction, Pelindaba, Atomic Energy Board, Nucl. Act. (Jul. 1973).

DISCUSSION

F.E. McGINLEY: With regard to the reverse leach procedure: what pH do you have to attain before you can subject the uranium tailings to gold cyanidation?

M.G. ATMORE: The final pH will be 11 -12 for cyanidation, but before transferring the acid-leached pulp it is neutralized to a pH in excess of 7 and conditioned for 2—3 hours to ensure that there is no residual acidity.

K. PINKAS: I would like to know whether you noticed a poisoning of ion exchange resins;

I know that silica and other impurities can cause trouble.

H.E. JAMES: In the early days, when we had the normal leach procedure, there was a lot of poisoning of resins with cobaltic cyanides resulting from the prior cyanidation step. In our work on the application of weak-base resins we certainly have experienced considerable problems with silica pick-up on these resins. This work was very adequately described by Haines et al. at the 1 lth International Mineral Processing Congress (Ref. [23] of the paper) and I shall not go into details here. We are fairly confident that the problems associated with silica pick-up can be overcome. I must point out that the use of weak-base resins is not currently being considered for commercial application in South Africa and a lot of work still needs to be done.

F.R. HARTLEY: First, I should like to follow up the comments just made on silica poisoning in relation to our experience in Australia some years ago with the Mary Kathleen operation. The resin at this plant was particularly prone to silica pick-up. Subsequent experimental work showed quite conclusively that the major portion of the silica was picked up whilst the barren solution was passing through the resin in the trailing column. In fact, at Mary Kathleen, the problem was exacerbated by the fact that a three-column operation was adopted. As a result of that study we believe that the application of continuous moving-bed ion exchange operation would alleviate the problem because it minimizes the volume of barren liquor in contact with the resin. I should like to ask whether you have found that silica pick-up depends very strongly on the barren solution which is passed through the resin.

38 JAMES

H.E. JAMES: In the adsorption period a number of competitive mechanisms are in operation of which silica uptake is the slowest. The major influences on silica uptake are the pH changes accompanying the passage of solution through the column, and the total contact time. In a continuous ion exchange column, adsorption contact time is minimized but some silica take-up does occur.

S. AJURIA-GARZA: In your pressure leaching, at what temperature and pressure do you operate the autoclaves?

H.E. JAMES: This particular development is being undertaken by the Anglo-American Cor-poration and I should like Mr. Atmore to answer the question.

M.G. ATMORE: I think that this depends to a large extent on the type of refractory minerals you are trying to attack. Our aims in pressure leaching have been two-fold: first, to get the best possible uranium recovery when treating the usual South African ores associated with the Witwatersrand gold deposits, and second, to improve the overall recoveries of uranium in those cases where they are very much lower, as with refractory uranium ores which often contain titanium, such as brannerites. The maximum temperatures used are 180-200°C, but some good results have been obtained at lower temperatures, down to 140-150°C, which is very interesting.

F.C. LENDRUM: Was there any indication, in your experience, of increased losses of uranium in tailings when grinding extremely finely? Have you seen indications of excessive or increased losses in the very fine slimes, as if the clay minerals acted as ion exchangers?

H.E. JAMES: Work carried out at the National Institute for Metallurgy about five years ago showed that adsorption of both uranyl and ferric complex species onto quartz and pyro-phyllite does occur but is very low, even with very finely ground material. The results indicated that losses of uranium through adsorption in the repulping stage at a typical South African uranium plant amounted to less than 0.5% of the uranium present in the ore.

F.C. LENDRUM: I should like to thank Mr. James for his very enlightened paper. In Canada we feel that if there is to be an intensive investigation in the field of uranium ore processing, it must be one aimed at elimination of clarification and a simplification of the liquid/solid separation steps by the use of upflow ion exchange.

One further question: You mentioned the production of ferric iron by bacteria. I gathered that this is done outside the leaching circuit, i.e. you subject a ferrous solution to bacterial oxidation and then put it back into the leach circuit. Is this correct?

H.E. JAMES: Yes. The idea is to carry out the operation in separate tanks where ferrous solution is passed over plastic packing material covered with a thin film of bacteria. This procedure is the basis for the patent by E.L. Goldblatt of General Mining and Finance Corporation (Ref.[20]

of the paper).

F.R. HARTLEY: Not knowing too much about biological systems, I should like to ask whether you have to add an organic nutrient of any sort to the bacterial system for producing ferric sulphate?

H.E. JAMES: No. The ions which already exist in the solution act as nutrients for the bacteria and they stand up to quite severe conditions.

F.R. HARTLEY: Your description of the gravity concentration split brings to mind the commercial application of heavy-media separation at one plant in South Africa. I wonder if you could report about the long-term results of that particular operation.

H.E. JAMES: Again, this is an Anglo-American plant and Mr. Atmore is better qualified to deal with that question.

M.G. ATMORE: At different times we actually had two plants using heavy-media separation.

The first was at Free State Saaiplaas where the use of heavy media was primarily aimed at increasing the throughput of a plant which was heavily loaded and operating at designed capacity, without seriously reducing gold recovery. It was an attempt to squeeze the plant. Naturally the effect of heavy media on uranium recovery was investigated and interesting results were obtained. However, great troubles, particularly from abrasion and medium loss, were encountered. In spite of these it was decided to incorporate a heavy-medium stage in an extension being designed at the time for

lAEA-AG/33-5 39

one of our Vaal Reefs plants. This ran for about two years. The indications were that it was marginally more successful technically but substantially less successful economically, so it was discontinued and we are at present not using this technique on uranium, although it is widely used in the Group elsewhere.

J. CAMERON: You mentioned both notation and wet high-intensity magnetic separation as potential methods for recovery of uranium from old tailings dumps. Is there a judgement on which of these is likely to be the more productive and what are the different approaches to this?

H.E. JAMES: In my paper I mentioned the work being done by the Anglo-American Cor-poration at one of the old mines in the eastern Witwatersrand where they are installing a flotation plant with a capacity of one million tonnes of slime per month. This plant is primarily for the production of a pyrite concentrate for sulphuric acid manufacture but gold and uranium will be recovered as by-products. A number of old dumps will be processed through the plant and the residues re-deposited in another area as one large dump. It is only when you take into account pyrite, gold, uranium and property values that this type of operation begins to look economic for the very low-grade dumps. With regard to both flotation and wet high-intensity magnetic separation we have the same experience as in Canada, namely that these techniques do not produce a low enough grade of uranium in the tailings to allow one to discard it. These techniques cannot there-fore be used for preconcentration of run-of-mine ores in the classical sense. We are, however, considering their use for the recovery of some of the uranium and gold left in tailings, but applying them at the end of the circuit. With a view to the very long-term future we are hoping that the efficiency of the wet high-intensity magnetic separator and particularly the engineering of large-scale equipment will be improved, allowing us to treat ores with lower uranium grades than are currently being considered economically treatable.

M.G. ATMORE: In relation to the comparison made by Mr. James, I should like to make one point. The wet high-intensity magnetic separation is new as far as we are concerned. On the other hand, the flotation of residues has been practised by us since about 1952. This has been a standard procedure and in our operations we have not been unduly concerned at the low recoveries of uranium, which were 30—35%, since we have been able to obtain high sulphide recoveries and appreciable additional gold. At various times we have used modified procedures but we have always reverted to the straightforward circuit and reagents. It is these that will be used in the Rioden project. This project is located east of Johannesburg in an area where gold mining commenced at an early stage. In many of the old dumps to be reclaimed and treated, the residual gold deposited was higher than it would be now.

F.C. LENDRUM: What are the economics of pyrite in South Africa?

M.G. ATMORE: The economics of recovering sulphur-bearing materials are about the same as they are elsewhere. However, for a number of years the cost of pyrite recovery was very low because other costs were borne predominantly by the gold recovery process or by uranium production itself. Until recently, this very cheap pyrite resulted in an extremely low sulphuric acid production cost, and until two or three years ago this would have been 12 rand or perhaps 13 rand per tonne of sulphuric acid. More recently, this has increased substantially, as have all other operating costs, and in our opinion the price of sulphuric acid now available in South Africa is very similar to that charged almost anywhere else.

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