CONTAINING NICKEL OXIDE MATERIALS

This invention relates to an improved process for the smelting of iron containing nickel oxide materials. The invention has particular application to iron containing nickel laterite material, that is, to material from naturally oxidised nickel sulphide ore bodies, and the description largely is illustrated with reference to such laterite materials. However, the invention also is applicable to iron containing nickel sulphide ore material that has been subjected to an oxidising roast to convert the nickel values to the oxide.

Conventional smelting processes for nickel laterite ores involve the drying of the ore in a pre-smelting drying stage in a rotary kiln, multi-hearth roaster or fluid bed, prior to feeding it to either of two possible smelting furnaces. The first of these smelting furnaces is an electric furnace in which electrodes are submerged in a bed of dried laterite and coke or charcoal to generate high temperatures and strongly reducing conditions. Both nickel and iron present in the ore are reduced, to produce an iron-rich ferro-nickel product with typically about 20% Ni and 75-80% Fe. The ferro-nickel is tapped from the furnace and generally is suitable for sale to stainless steel producers. A slag lean in nickel is produced for discard, after tapping and granulation.

The second type of smelting furnace is a blast furnace in which dried laterite ore, in lump or agglomerated form, is charged together with coke and fluxes to form a porous bed, through which air, or oxygen

enriched air is blown by means of tuyeres. The combustion of the coke produces very high temperatures in a smelting zone of the furnace in front of the tuyeres, where excess coke generates very strongly reducing conditions. Iron and nickel oxides in the ore are reduced, again to produce such iron rich ferro-nickel product. The slag produced is tapped into a granulator and is disposed of to waste.

These furnace systems are very expensive to build and to operate, and use expensive fuels in electricity or coke. Furthermore the conditions are always very strongly reducing, so that the ferro-nickel composition is determined largely by the composition of the feed, and is not able to be controlled substantially by adjusting the operating variables. The present invention is directed to providing an improved process for the smelting of iron containing nickel oxide materials, such as nickel laterite materials. The process of the invention enables recovery of a ferro-nickel product in a system entailing a very much reduced capital cost in plant and equipment compared with the above conventional processes. Also, the invention enables control over the nickel content of the ferro-nickel product such that, if required, the product can be substantially richer in nickel than that provided by those conventional processes.

According to the invention, there is provided a process for smelting of iron containing, nickel oxide materials for the recovery of nickel therefrom, wherein the oxide material is subjected to a smelting operation in a fuel fired furnace of a top-submerged lancing furnace

system, to produce a ferro-nickel product and a slag, and wherein said smelting operation comprises:

(a) a first stage in which the nickel oxide material is fed to the furnace, with injection of an oxygen containing gas and combustion fuel into a molten bath established in the furnace, to produce a slag containing nickel and iron as the respective oxide; and

(b) a second stage in which the slag is subjected to the action of reductant added without further feeding of oxide material, but with injection of oxygen containing gas and combustion fuel into the slag, to reduce the nickel oxide, and a portion of the iron oxide, content of the slag to produce ferro-nickel product and a low-nickel containing slag.

The fuel fired top-submerged lancing furnace system required by the present invention is one which has a very low capital investment requirement compared with the electric or smelting furnace of the prior art, with each of the latter also necessitating a pre-smelting drying stage. The capital investment can be an order of magnitude less than that of the prior art. Also, the process of the invention preferably is operated with relatively low cost coal, with this in particular resulting in lower operating costs compared with the reliance in the prior art processes on electricity or coke.

The nickel oxide material can be smelted in the lancing furnace without the need for recourse to drying in a rotary kiln, multi-hearth roaster or fluid bed, even if its water content is relatively high. Thus, while the

nickel oxide material can, if necessary, be at least partially pre-dried before it is fed to the top-submerged lancing furnace system, the furnace of that system can accommodate a water content at least of the level encountered in feed material used in the pre-smelting drying stage of the conventional processes. However, in a preferred form of the invention where the nickel oxide has a relatively high water content, that material is fed to the lancing furnace system via a section of flue offtake ducting of the furnace, allowing the material to be heated or heated and dried by furnace flue gases from the furnace before passing into the furnace. Most preferably, the ducting section is rotatable to facilitate the material passing through the ducting section into the furnace. A top-submerged lancing system is found to have benefits for the smelting of iron containing nickel oxide material, such as laterite, additional to those detailed above. Such system enables smelting under less strongly reducing conditions than conventional electric and blast furnace processes, and this is found to enable attainment of a ferro-nickel product richer in nickel than is possible with those processes. As indicated, the ferro-nickel product of the conventional processes is determined largely by the composition of the feed material, and those processes essentially are incapable of being operated so as to substantially enhance the nickel content of the product relative to iron.

The process of the invention can utilise any suitable reductant. However, the reductant preferably is coal, such as in lump form, as coal is well suited for use

in the process and is relatively inexpensive.

The fuel may comprise natural gas, fine particulate coal, oil or any other fuel suitable for the furnace system required for the invention. The oxygen-containing gas may comprise air, oxygen enriched air or oxygen. Air is least preferred, as it can make difficult the attainment of an acceptably high flame temperature in a combustion zone established in the furnace. The oxygen containing gas preferably has an oxygen content providing firing or combustion under substantially stoichiometric conditions; the oxygen content of the gas being for example from 90 to 110% of that required for complete combustion of the fuel.

In the process of the invention, the iron containing nickel oxide material is fed to the furnace of the fuel fired top-submerged lancing system, to produce a slag. At least in the first stage, a flux is fed to the furnace if necessary to produce the slag. Reductant is added at least in the second stage to produce a metal phase of ferro-nickel product in that stage. However, most preferably, reductant also is fed to the furnace for the first stage so as to produce a ferro-nickel product metal phase in that stage, additional to such product produced in the second stage. The ferro-nickel product of the first stage may be tapped prior to commencing the second stage, such that the respective ferro-nickel products are separated. Alternatively, the ferro-nickel product of the second stage may be taken up in that of the first stage, or the ferro-nickel product of the first stage may be taken up in ferro-nickel product remaining from the second

stage of the preceding cycle of operation.

The overall feed ratio of reductant-to-nickel oxide material and the combustion stoichiometry of fuel and air are regulated so as to determine the distribution of nickel and iron between the metal phase and slag. That distribution is found to be determined by the relative equilibrium and rates of the overall reduction reactions comprising:

^Ni0 (F or S) ^{+ C = Ni} (M) ^{+ C0} ' ^Fe 2°3(F) ^{+ C = 2Fe0} (s) ^{+ C0,} FeO _(s) + C = Fe _(M) + CO where F, S and M respectively designate feed, slag and metal phase. The exchange coefficient ^C ₍ r _ι ± _/ - _e f° ^{r the} reaction: NiO _(s) + Fe _(M) = Ni _(M) + FeO _(s)

^{is C} (Ni/Fe) ⁼ ∞*. A ^£ C ^E F ^e e ^Q

CFe// CCNNiiO.

This coefficient can be determined by either equilibrium considerations or kinetic factors associated with the particular mechanisms of reactions taking place. However, in practice, it is found that the coefficient is very much greater for a fuel fired top-submerged lancing furnace system than for the above-described conventional processes. This difference is attributable principally to the less strongly reducing conditions obtainable in the lancing furnace system, compared with the strongly reducing conditions prevailing in those conventional processes. The consequence of the difference is that a ferro-nickel product having a substantially higher nickel content is obtainable with the process of the invention,

compared with those conventional processes, for a given nickel oxide feed material. Thus, the present invention has a further, very substantial advantage over the conventional processes. As a practical matter, utilization of the above-detailed reduction reactions is difficult in a single stage operation, due to difficulty in achieving the required feed ratios and combustion stoichiometry on a progressive basis. However, it is found that the desired overall result can be achieved in the two-stage process of the invention. In one form, the two-stage process comprises:

(a) a first stage, in which the nickel oxide material and reductant are fed to the fuel fired top-submerged lancing furnace to produce a ferro-nickel containing product and a slag containing portion of the nickel and iron, as the respective oxides; and

(b) a second stage, in which reductant is fed to the slag, in the absence of further feed of oxide material, to reduce the nickel content of the slag for recovery of further ferro-nickel product, to leave a slag of low nickel content.

In a second form of the invention, the first stage involves the direct feeding of iron containing nickel oxide material, such as laterite ore, and fluxing agent if required to a fuel fired top-submerged lancing furnace to produce a slag, but possibly no ferro-nickel product. In this form, operation is under substantially stoichiometric fuel/air/oxygen firing conditions for the lance, such as

from 90 to 110% for complete combustion of the fuel, and with little or no reductant addition in the first stage.

The nickel content of the slag and of the alloy are determined by the feed rate of the oxide material, the reductant if any and the combustion stoichiometry of the top-submerged lance injection of the fuel-air-oxygen mixture, but these need not be monitored or closely controlled. Feeding preferably is carried out continuously over a sufficiently long time interval to fill the furnace to capacity with slag; the temperature preferably being maintained in the region of 1600°C. Except in so far as metal phase is produced in the first stage, substantially all of the nickel and at least a major part of the iron report as the respective oxide in the slag produced in the first stage. The second stage takes place when feeding of oxide material and flux is halted, whilst reductant such as coal is fed to the furnace to carry out a batch reduction operation on the slag so as to decrease its nickel content to a low level for slag discard. An amount of slag is then tapped from the furnace and granulated for discard. A heel of slag is left in the furnace together with the metal produced during the second stage reduction, and smelting of oxide material is continued in the first stage of another cycle before carrying out a further second stage batch reduction as described. These cycles are continued and metal is tapped only when its volume increases to a sufficient level to require its removal from the furnace. The metal would normally be removed at the conclusion of a first stage smelting of a cycle, before a reduction stage is

commenced .

In a variant of the first and second forms of the invention, the oxide material, such as laterite ore, is fed into a rotating section of flue offtake ducting of the furnace, to allow the oxide material to be heated and dried by the furnace flue gases before dropping into the furnace. Coal is also fed into the furnace and onto the bath surface to reduce the slag, as required by those forms of the invention. Fuel, air and oxygen are injected into the bath to provide heat for the smelting and reduction. The overall operation is carried out in two stages as detailed above in relation to the first or second forms of the invention.

The two forms of the invention detailed above, and the respective variants thereof, illustrate two extremes of the invention in terms of the composition of the resultant ferro-nickel alloy. The alloy produced by the second form and its variant (using little or no reductant in the first stage) has a composition similar to that obtained with the conventional prior art processes. That is, the composition substantially corresponds to the iron to nickel ratio of the oxide material feed, although it can be enriched to a degree in nickel. However, the first form and its variant (using reductant in both stages) produce respective ferro-nickel alloys in each of the first and second stages, with at least that of the first stage being substantially enriched in nickel relative to the nickel to iron ratio of the oxide feed material. It therefore can be of benefit to tap the first stage alloy from the furnace prior to the second stage, so as to avoid

dilution of its nickel content by less nickel-rich alloy from the second stage. Also, by adjustment of the reductant to oxide material feed ratio in the first stage, it is possible to vary the nickel content of the first stage alloy between those two extremes.

In order that the invention may more readily be understood, description now is directed to the accompanying drawings, in which:

Figures 1 to 3 show respective flow charts illustrating alternative processes in accordance with the present invention; and

Figure 4 shows a fuel-fired top-submerged lancing system for use in a process as described with reference to Figure 3. In the case of each of Figures 1 and 2, there is reference to "smelting" and "reduction". Those terms are intended to indicate successive operations conducted in a single furnace of a top-submerged lancing furnace system. Thus, "alloy" depicted as recycled, is in fact simply retained in the furnace on completion of a reduction stage, for use in the first stage of the next one of successive operations conducted in the furnace.

In the arrangement of Figure 1, iron containing nickel oxide material, such as laterite ore, is charged to the furnace of a fuel fired top-submerged lancing furnace system. In the furnace of that system, the oxide material is subjected to smelting in a first stage of one of successive operating cycles, under the action of heat generated by injection of fuel and an oxygen containing gas, via a top-submerged lance, and reductant. The oxygen

containing gas may be air, oxygen or oxygen enriched air.

The fuel preferably is natural gas, fine coal or oil, but other fuels normal for such furnaces may be used. The reductant preferably is coal, such as in lump form, but other reductants for such furnaces, such as fine particulate coal, can be used. Flux, such as coal ash, preferably also is charged to the furnace to assist in forming and establishing a suitable slag.

The first stage typically is conducted over a period of from 2 to 5 hours. However, that period depends in part on the furnace size and the feed ratio of reductant to oxide material, as well as the stoichiometry of the fuel-oxygen containing gas mixture. During the first stage, a ferro-nickel metal phase product forms and progressively increases in volume, principally as a consequence of reduction of nickel oxide in the oxide material. On completion of the first stage, that metal phase contains a major proportion of the nickel content of the oxide material and typically contains a minor proportion of the iron content. However, a portion of the nickel as nickel oxide reports in the slag, along with a substantial part of the iron content as iron oxide. The extent of this separation of nickel and iron between the metal phase product and the slag depends principally on the rate of addition of reductant, the ratio of reactants, and the duration of the first stage. On completion of the first stage, the feeding of oxide material and reductant to the furnace and the injection of fuel and oxygen containing gas is terminated and the ferro-nickel metal phase is tapped from the furnace.

After tapping the ferro-nickel metal phase from the furnace on termination of the first stage, and with continuation of injection of fuel and oxygen containing gas and addition of reductant, but with no further feed of oxide material, a second stage is commenced. The reducing conditions prevailing through that second stage reduces nickel oxide present in the slag and at least part of the iron oxide, to form a ferro-nickel metal phase in the second stage of the process. That metal phase of the second stage is retained in the furnace for the first stage of the next operating stage of the process. The second stage is continued until the slag is substantially depleted of its nickel content and, with a substantial iron content still remaining in the slag at the end of the second stage. All but a heel comprising a minor portion of the slag is tapped from the furnace at the end of the second stage and granulated for discard. The second stage can be continued for from 1 to 3 hours, but its duration depends in part on the conditions previously prevailing in the first stage, the furnace size, and the rate of addition ^" of reductant during the second stage.

On completion of the second stage, the feed of oxide material is resumed for commencement of a first stage of the next cycle of operation, with the slag heel from the previous cycle providing initial slag requirements. The ferro-nickel metal phase remaining from the second stage also is retained in the furnace for the next first stage. Smelting and reduction cycles are repeated in the one furnace and, while tapping of metal from the first stage as indicated above, the metal phase is tapped only after a

first stage at which there is a sufficient quantity to be tapped.

Figure 2 shows a variant on the arrangement of

Figure 1. In this, furnace gas issuing from the furnace offtake flue during a first stage of operation is used to heat incoming feed of iron containing nickel oxide material. The flue gases are at a sufficient temperature to effect drying of the oxide material and can provide roasting of that material prior to it being fed to the furnace. The flue gases may be contacted directly with the oxide material, such as by the oxide material being fed to a section of the offtake duct and caused to pass along that section into the furnace. The section of the duct to which the ore material is fed can be rotated to facilitate passing of the ore material into the furnace. However, as will be appreciated, other arrangements for heating the ore material with flue gases, either directly or indirectly, are possible.

The process as described with reference to each of Figures 1 and 2 is able to be varied, as previously described herein. Thus, the first stage smelting can be conducted with a reduced level or with little if any reductant addition, such that little or no ferro-nickel metal phase is produced in that stage. That is, the first stage is conducted under less reducing or even non-reducing conditions, with the nickel and iron content of the oxide material feed reporting as the respective oxide in the first stage slag. In the second stage, the nickel content of the slag is reduced, along with a portion of the iron content thereof, to produce the

required ferro-nickel metal phase product and a low-nickel slag. The extent to which ferro-nickel product is produced in the second, rather than in the first stage, decreases with decreasing level of reductant used in the first stage.

Figure 3 shows an alternative two stage operation, utilizing a smelting furnace for the first stage, and a separate reduction furnace for the second stage. However, each of the furnaces is a top-submerged lancing furnace system.

In the smelting furnace of Figure 3, a first stage operation is conducted, such as described above in relation to Figure 1 or Figure 2. This produces a ferro-nickel alloy metal phase containing a major portion of the nickel content of the oxide feed material and a minor proportion of the iron content. A resultant slag contains a portion of the nickel of the feed material, as nickel oxide, and a substantial proportion of the iron content as iron oxide. The metal phase is tapped from the smelting furnace, as a required end product. The slag is transferred to the reduction furnace. This transfer may be by a batchwise operation at the end of the first stage. However, it preferably is by means of a continuous flow of the slag to the reduction furnace, during the course of the first stage operation, such as by flow via a launder.

In the reduction furnace of Figure 3, the slag is subjected to a second stage operation, such as described above with reference to Figure 1. Again, ferro-nickel alloy metal phase is produced by reduction of the nickel

oxide content, and at least part of the iron oxide content, of the slag. The ferro-nickel alloy is tapped at intervals, as a required product or intermediate, while the resultant low-nickel slag is discarded. The two stage operation of Figure 3 can, if required, be varied as detailed above in relation to

Figures 1 and 2. That is less or no alloy may be produced in the smelting furnace, with corresponding increase in alloy production in the reduction furnace. Figure 4 shows a top-submerged lancing furnace system for use in accordance with the process of the invention according to Figure 3. The system has a smelting furnace 10, and a reduction furnace 10', each of the top-submerged lancing form. That is, each of furnaces 10,10' is of the form required for the invention, each being illustrative of the type of furnace suitable for the process of Figure 1 or Figure 2.

Furnaces 10,10' are of the same overall form, and description of their features therefore is limited to those of furnace 10. However, as will be apparent, furnace 10' has the same features identified by the same reference numeral primed.

Furnace 10 has a refractory lined vessel 11, typically provided with an external steel shell. Vessel 11 defines a chamber 12 in which, during a pyrometal- lurgical operation therein, there is established a liquid bath 22 comprising slag or having a slag layer on its surface. Gases evolved during the operation pass into the gas space of chamber 12 above bath 22, and discharge via flue off-take 18. Furnace 18 also has a feed chute 16 by

which feed material or solid reactants can be charged to bath 22 under the control of feed valve 14. A tap hole 26 also is provided by which metal phase, or slag if required, can be tapped from the bath. A lance 20 projects down through chamber 12 into bath 22. Adjustment means (not shown), such as an overhead crane, is provided above furnace 10 for raising and lowering lance 20. At its upper end, lance 20 is connectable to a source of oxygen-containing gas, such as air, and to a source of fuel, such as oil or particulate coal, such that the gas and fuel can be injected under pressure into bath 22. Before the lower end of lance 20 is inserted into bath 22, the lance is held in furnace 10 with a jetting tip at its lower end close to the surface of bath 22 and with oxygen-containing gas being jetted onto that surface. This normal operation results in splashing of slag from the bath, so as to cover the lower end portion of lance 20 and to form a protective solidified coating 28 of slag thereon. Coating 28 protects lance 20 from the full extent of the high temperature conditions prevailing in furnace 10, thereby reducing wear of the jetting tip of lance 20. When so protected, lance 20 then is lowered further to insert its lower end into bath 22 for injection of oxygen-containing gas and fuel below the surface of bath 22, and to establish combustion zone 24.

Operation with furnaces 10,10' will be understood by reference to the description of Figure 3 and the variant thereof. Oxide material is fed to bath 22 via chute 16 and valve 14, as preferably also is the reductant coal if

charged to furnace 10; while reductant coal is fed to bath

22' via chute 16' and valve 14'. Ferro-nickel alloy metal phase is able to be tapped from furnace 10, if produced therein, via tap hole 26; while ferro-nickel alloy metal phase, and subsequently low-nickel slag, is able to be tapped from furnace 10' via tap hole 26'. Most preferably slag passes continuously from furnace 10 to furnace 10' via launder 30 provided therebetween.

The process of the present invention has a number of advantages over the conventional processes. Firstly, the lancing furnace system and auxiliary equipment is compact and simple, compared with that required for the conventional processes. Also, oxide material feed preparation can be minimal, with either no drying or minimal drying of that feed. This translates to very much lower capital cost for plants as well as lower operating costs per ton of feed. A second important advantage is that any fuel and reductant source can be used so that fuel and reductant costs can be significantly decreased compared with the requirements for expensive electricity or coke for the conventional processes. With these two advantages, the process of the invention enables the building of a smelting plant for iron containing nickel oxide material, such as nickel laterite, at a much lower capital investment, while the plant can be operated with significantly lower operating costs.

A third important advantage lies in the ratio of recovery of nickel to iron obtainable. The rate of smelting, composition of feed material and rate of reductant addition can be adjusted in the process of the

invention, such that a low-iron, high nickel ferro-nickel product can readily be produced.

EXAMPLE I

A laterite ore of composition 2.31% Ni, 7.7% Fe, 30.8% Si0 ₂ , 19.3% MgO, 0.02% Co, and 1.3% A1 ₂ 0 ₃ was fed into a top-submerged lancing reactor at 111 kg/hr.

Natural gas, air and oxygen were injected into the bath at rates of 83 Nm ³ /hr, 400 Nm ³ /hr and 20 Nm ³ /hr respectively whilst feeding coal (75% fixed carbon, 5.4% volatiles, 8.1% moisture, 12.9% ash) at 31 kg/hr and Fe_0 ₃ ore (11 kg/hr) and limestone (10 kg/hr). After 270 minutes operation the laterite feeding was stopped and the bath reduced by continuing to inject natural gas, air and oxygen at the same rate whilst feeding coal at 50 kg/hr. After 90 minutes of reduction the bath was tapped metal and slag samples assayed as follows:

Metal * Slag * Cu 5.5% EXAMPLE II

This example illustrates the operation of the process in two stages in two separate furnaces connected by a launder, such as described herein with reference to Figures 3 and 4. In the first stage, an air/coal/oxygen mixture injected down the lance of the first stage furnace is designed to be slightly reducing to ensure high fuel efficiency during the smelting stage. In addition a small amount of lump coal is added to the bath to maintain the slightly reducing conditions in that stage. In the second

stage, laterite feed is halted, and reducing conditions are maintained by feeding lump coal to the furnace with the same lance firing conditions to produce ferro nickel from the slag which had been produced in the first stage. 500 kg of laterite ore assaying 2.3% Ni, 11% Fe,

0.1% Co, 4.0% Si0 ₂ , 23% MgO, 0.3% CaO, 1.5% Al^, and with a moisture content of 25% was smelted in the coal fired top submerged lancing furnace of the first stage over a period of 5 hours at a temperature of 1550°C. Over the same period of time, 30 kg of limestone was fed into the furnace as a fluxing agent. The lance of that furnace was fired with 80 kg/hr of fine bituminous coal, together with 365 Nm ³ /hr of air and 30 Nm ³ /hr of oxygen (90% stoichiometry and 27% oxygen enrichment) . Over the last hour of smelting 30 kg of lump coal (5-15 mm) was also fed into the furnace as a reductant. A dip sample of the slag produced was taken and assayed 0.5% Ni, which represents the slag product from the smelting stage.

In the second reduction stage, coal firing was continued with no laterite feed, whilst 30 kg of lump coal was fed over 50 minutes. At the conclusion of this reduction stage the furnace contents were tapped to produce a slag assaying 0.06% Ni and a metal containing 24.6% Ni, 69% Fe, 0.5% Co and minor levels of other metals. It will be appreciated that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.