Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound – Iron group metal
Reexamination Certificate
2000-10-31
2002-04-30
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
Iron group metal
C423S150100, C423S140000, C423S142000, C423S146000, C423S147000
Reexamination Certificate
active
06379637
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method of processing highly-serpentinized saprolitic fractions of nickel laterite deposits, by atmospheric leaching with sulphuric acid, to obtain high degrees of extractions of their nickel and cobalt values.
BACKGROUND OF THE INVENTION
In the general practice of mining nickel laterite ores, that portion of the laterite ore profile that is saprolitic in nature, usually containing between 7% and 15% iron, between 15% and 20% magnesium, less than 0.05% cobalt, and between 1.7% and 2.2% nickel, is largely neglected. This stems from the fact that this fraction is found not to be ideally suited for economical extraction of its nickel and cobalt values by established conventional processing technologies, nor by the more recently developed high pressure acid leach technology. In some major nickel laterite mining practices, this segment of the laterite profile has normally been left behind, which is the case in two countries which host the world's largest known deposits of nickel laterite ores, namely Cuba and New Caledonia. (In New Caledonia, these remnant nickel saprolites are referred to as “petits minerais”, being too “small” in nickel values to be of economic interest.)
For over a century, nickel laterite ores high in magnesia, relatively low in iron, and enriched in nickel usually containing over 2.2% nickel and more commonly at least 2.5% nickel, referred to either as garnierite ores or saprolite ores, have been processed by pyrometallurgical means to produce either a ferronickel, a Class II nickel product that could go directly to market for the production of stainless steels, or to produce an intermediate nickel sulphide product that could go to refineries for conversion to either Class I or Class II marketable nickel products. A good portion of the cobalt would be lost, some in the slag during the smelting stage, and in the case of ferronickel a good percentage of the cobalt would be present as an impurity of no value. Such metallurgical processes involve drying the humid ores, preheating them with or without effecting a partial reduction and subsequent reduction smelting at high temperatures in electric furnaces. It is axiomatic that such metallurgical processes consume high amounts of energy per unit of nickel production, and in most cases result in complete loss of value of the cobalt that accompanied the nickel in the ore.
About half a century ago, an ammoniacal leaching process was developed and commercialized which could treat lateritic ores high in iron and of low nickel content, commonly containing between 1.2% and 1.5% of nickel. It employed a combination of pyrometallurgical and hydrometallurgical technologies. The laterite ore is first dried and then subjected to partial reduction in Herreschoff furnaces or rotary kilns, at elevated temperatures but well below smelting temperatures, to selectively reduce the nickel and cobalt but only partially reduce the iron. This partially reduced calcine is then quenched and leached in ammoniacal carbonate solutions to dissolve nickel and cobalt; and the nickel is subsequently recovered from the ammoniacal leach solution as a nickel hydroxide/carbonate which would then be converted to a Class II nickel oxide or utility-grade nickel. In some cases the nickel leach solution would proceed to electrolytic refining for the production of electrically pure Class I nickel. Nickel recovery seldom exceeds 80% and recovery of by-product cobalt seldom exceeds 45%. While the hybrid pyrometallurgical-hydrometallurgical process could treat the high-iron low-magnesium and low-nickel laterite ores, often referred to as limonite ores, and is less demanding of energy requirements than the smelting process, the nickel recovery in actual continuous practice often drops to below 75% and the cobalt recovery drops to below 40%.
Research in the early 1950's demonstrated that by subjecting the high-iron, low-magnesium and low-nickel laterite ores, that is the limonites normally containing between 1.2% and 1.6% of nickel and between 0.1% and 0.25% of cobalt, directly in their humid state to sulphuric acid at elevated temperatures and pressures, that nickel and cobalt extractions of over 90% could be achieved with the energy requirement only a fraction of that required by the smelting or ammoniacal leaching processes. While this technology heralded a new era for the production of nickel and cobalt, only one commercial plant was built at Moa Bay in Cuba. This plant confined itself to the processing of limonites very low in magnesia contents, i.e., with les than 1% of magnesium oxide, and operated at around 240° C. and 475 psig. The product at the Moa Bay plant is an intermediate nickel-cobalt sulphide which is sent overseas for refining to marketable nickel and cobalt end products. This new hydrometallurgical technology has been further developed to operate at temperatures as high as 270° C. and 810 psig pressure. In recent years three pressure acid leach plants have been constructed in Australia, and were being commissioned in 1999/2000.
Saprolitic laterite ores from different sources, while possessing similarity in chemical composition, are prone to yielding very different results particularly when subjected to atmospheric leaching with sulphuric acid. The main difference between the saprolites is the degree of serpentinization. It has been observed that two deposits of saprolites situated side-by-side (usually separated by a geological fault) could be serpentinized to different degrees. It has also been observed that the saprolites from one region of a country hosting nickel laterite deposits can be very different from those in another region of that same country. These regions would normally be separated by one or more geological faults, by different geological structures or by mountain ranges. What distinguishes these two regions is that the rock remnants found in the lower profiles of a nickel laterite deposit in one case will not be highly serpentinized and will be low in nickel values, while the rock remnants in the other case will be highly-serpentinized and will contain higher concentrations of nickel and will be much softer and more readily broken.
An extensive laboratory study of atmospheric acid leaching of a large variety of laterites was carried out, see John H. Canterford “Leaching of some Australian nickeliferous laterites with sulphuric acid at atmospheric pressure”, Australasian Institute of Mining and Metallurgy, No. 265, March 1978. He worked at a low pulp density of about 9% solids, at boiling point with reflux temperatures up to 107° C., with various strengths of sulphuric acid and for leaching times between 1 hour and 26 hours. With 3 out of 13 of his samples, he was able to achieve 90% or better extractions of nickel in seven hours of leaching, while employing about 1 part of acid per 1 part of laterite. By doubling the acid addition, 2 out of 13 samples yielded nickel extractions of 90% or better in one hour of leaching. He noted “the differences in reactivity of all the samples”, as well as “the different behaviour of samples from within the same ore body ” He concluded that “the results presented clearly indicate the problems involved in developing a process for treating these complex ores” Canterford suggested that atmospheric leaching should be considered where “there is a local high volume source of cheap sulphuric acid” and went on to say that “the economic viability of this approach cannot be assessed at this stage”.
A process patent devoted to atmospheric acid leaching of saprolitic ore fractions, U.S. Pat. No.4,410,498, assigned to Falconbridge Nickel Mines Limited teaches the use of SO
2
as a reducing agent to maintain a low redox potential so as to enhance the atmospheric leachability of the saprolite being processed, Even after 4 hours of leaching at 85° C., the maximum nickel extraction achieved was 80%.
Therefore it would be very advantageous to provide an economical, rapid method of processing highly-serpentinized saprolitic fractions of nic
Bos Steven
Curlook Walter
Hill & Schumacher
Schumacher Lynn C.
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