THE DOMINATING IMPORTANCE OF LATERITIC ORES FOR THE FUTURE SUPPLY OF NICKEL BECOMES OBVIOUS WHEN ONE CONSIDERS THAT ABOUT 80% OF THE PRESENTLY KNOWN NICKEL RESERVES ARE ASSOCIATED WITH LATERITIC TYPES OF ORE.
CONSIDERING THAT NO VALUABLE METALS ARE CONTAINED IN THE LATERITES, NICKEL PRODUCTION USING CONVENTIONAL METALLURGICAL METHODS IS COMPARATIVELY DIFFICULT AND COSTLY. PYROMETALLURGICAL METHODS REQUIRE EITHER LARGE AMOUNTS OF ENERGY OR LESS ENERGY BUT HIGH TECHNOLOGICAL OUTLAY. THE OBJECTIF IS TO DEVELOP A LOW TEMPERATURE PROCESS FOR THE DEVELOPMENT OF MARGINAL ORE-BODIES.
The application of microbially assisted leaching to the recovery of nickel from low grade laterites (nonsulphide ores) was examined. The mineralogy of the nickel ores was studied and leaching using acids, commercial chelating agents and biologically produced acid mixtures were compared. Leaching with acids produced in situ (by growth of the microorganism in the presence of the ore) was then examined.
The aim of the work was to examine the application of microbially assisted leaching to the recovery of nickel from low grade laterites (nonsulphide ores).
It was found that Greek laterites were generally amenable to low temperature leaching, with nickel recoveries of greater than 70% achievable in column leaching experiments with sulphuric acid as the leaching agent, but there was considerable variability in the ease of nickel leaching from the various ores.
Sulphuric and citric acid leaching, especially in columns, showed a strong selectivity for nickel compared to iron. Mineralogical analysis of selected ores showed that nickel was generally present as garnierite or chlorite and electron microprobe analysis of the litharakia ore showed that the nickel was dispersed throughout particles, usually associated with magnesium and aluminium, but critically showing little association with the bulk of the iron. Leaching of nickel was always accompanied by release of magnesium and it is assumed that the observed selectivity resulted from channelling along nickel and magnesium rich veins.
Of the potential chelating acids produced by the microorganisms, citric acid proved to be the most effective for nickel leaching. However problems of calcium citrate precipitation were encountered in column leaching experiments.
Oxalic acid, a common fungal metabolite, should be excluded from the leaching process as plugging can occur.
In a 2-stage method where the chelating acids were produced (biologically) separately and then applied to the ore, the extent of leaching was generally consistent with the levels of available citric acid. Also, it was found that higher levels of nickel release were obtained when some fo the organisms were grown in the presence of the ore ('in situ') than found in the comparable 2-stage methods suggesting contact between the ore and the organism is beneficial.
THE OVERALL PROJECT IS DIVIDED INTO TWO PARTS:
I) AN INVESTIGATION INTO THE MICROORGANISMS WHICH CARRY OUT SUCH A DISSOLUTION RESEARCH DURING THE LATERISATION PROCESS BY WHICH LATERITIC NICKEL DEPOSITS ARE FORMED IN NATURE, THE MEANS BY WHICH THE ACTIVITY OF THE MICROORGANISM CAN BE OPTIMISED.
THIS PART WILL BE EXECUTED IN THE IMPERIAL COLLEGE, UNIVERSITY OF LONDON, AND PROPOSAL IS UNDER SUBMISSION BY PROFESSOR A.R. BURKIN WHO WILL BE THE OVERALL PROJECT MANAGER.
II) USE OF THE INFORMATION ALREADY IN THE LITERATURE AND THAT TO BE OBTAINED IN PART I) OF THE PROJECT TO DEVELOP A PROCESS FOR THE ECONOMIC RECOVERY OF NICKEL AND COBALT FROM THE GREEK LATERITIC ORES AND ALSO FROM SOURCES AROUND THE WORLD AS WELL AS FROM NICKELIFEROUS PERIODOTITES ASSOCIATED WITH SULPHIDIC NICKEL ORES.
Funding SchemeCSC - Cost-sharing contracts