Upgrading the Nickel Content From Low Grade Nickel Lateritic Iron Ores

The present invention relates to the recovery of nickel in the form of a concentrate from some classes of low grade nickel (from 0.65 to 1% Ni) lateritic iron ore deposits with a relatively high iron content expressed as Fe.sub.2 O.sub.3 (between 30 and 45%) and a silica content more than 40% (free silica and a complex of silicates, mainly serpentines) by a combined process of segregation and magnetic separation or flotation.

These ores cannot be economically treated today by any available method, unless they are concentrated prior to a subsequent processing to make them commercially feasible.

The process according to the invention is directed to the upgrading of nickel content from nickel lateritic iron ore having an iron content by weight of over 10%, silica of over 25%, and a nickel content of at least 0.5%. The ore is ground and the ground ore is thoroughly mixed with a small quantity of calcium carbonate, calcium sulphate and coke and is sprayed with a solution of sodium chloride to make the appropriate pellets. The pellets are gradually heated under a neutral or a slightly reducing atmosphere, to reach 950.degree.-1000.degree. C (1050.degree. C maximum) and are then roasted at this temperature for about 1 hour (90 minutes maximum). During the roasting, the nickel as well as part of the iron and cobalt from their respective oxides, are deposited on a carbon surface of coal under a form of very fine metallic particles through repeated cycles of chloridizations, reductions and hydrogen chloride regenerations. The roasted material is cooled, ground in a water medium and finally subjected to a wet or dry magnetic separation or flotation treatment, to obtain a rich nickel concentrate.

Preferably, the ore must be porous during the roasting, in such a way that the gases have a free access in all the mass of the ore giving the possibility to the involved reactions between solid and gas or simply gas phases to take place simultaneously and the gases to escape evenly from the ore. This role is successfully accomplished by calcium carbonate mainly for feeds in pellet forms. A second function of CaCO.sub.3 is that it acts as a storage of HCL which might have been lost during its formation. Apart from this advantage of the devised process, the addition of small amounts of calcium sulphate, has proved to promote the chloridization of nickel when sodium chloride was used as a chloridizing agent. Sodium chloride apart from its role as a chloridizing agent, acts also as a promoter for hydrogen formation.

Large quantities of water are required in (or for effecting) the process, particularly for the flotation. If soft water is not available in a sufficient amount, sea water has also proved to be suitable.

The iron ore subjected to the present process may be obtained by blending different nickel bearing ores, such as laterites and serpentines, etc., so as to achieve a mixture having the percentages of iron, silica and nickel as mentioned above.

The process may be carried out without forming pellets merely by thoroughly mixing the ground ore with sodium chloride, limestone, gypsum and coke. The amount, by weight, of sodium chloride (which may be cooking salt or unrefined salt) may vary between 1.5 and 7.5%, gypsum from 0.1 to 0.5%, coke from 2 to 5% and limestone from 0 to 10%.

The process for making the appropriate pellets requires special care for improved nickel recoveries. Two steps of mixing are therefore required before pelletizing. Firstly, the ground ore is thoroughly mixed with limestone, gypsum and coke and, secondly, the mixing is continued and is sprayed by about three-fourths the total amount of the sodium chloride solution. The remainder of the solution is left for the pelletizing stage, whereby improved nickel recoveries are achieved.

It should be pointed out that, whenever water is required for the grinding step, pulping adjustments, dilutions, reagent solutions and the magnetic separation, soft water as well as sea water may be used. And, diesel oil may be used as an assisting collector in the flotation of the nickel segregated on the carbon surface of the coal and eventually on the gangue ore for better nickel recoveries.

The single FIGURE of the drawing is a flow chart which summarizes the process according to the invention. The dotted lines shown in the FIGURE indicate the magnetic separation aspect of the process.

A segregation process has been heretofore initially applied to copper oxides using coke and sodium chloride as a chloridizing agent. Over the years, numerous nickel segregation studies have been carried out mainly based on the principle of the copper oxide segregation process. In these studies, sodium choride was replaced by calcium chloride being considered as the most efficient chloridizing agent in the nickel segregation process.

The chemical reactions involved in the segregation process may be summarized as follows:

During the heating stage, the chloride added to the ore, reacts with water vapor to produce hydrochloric acid, while the alkalis and alkali-earth oxides react with the gangue to form complexes of silicates. In its turn, hydrochloric acid reacts with a metal oxide (NiO, FeO, etc.) to produce the respective metal chloride according to the following equation:

where Me may be: Ni, Fe and Co.

Thermodynamically, because of the positive values obtained for the standard free energy changes at all operating temperatures of each metal oxide with HCl, the chloridization step would proceed only when the partial pressure of water is maintained as low as possible (to avoid hydrolysis) and the metal chloride is removed quickly by a subsequent reduction with hydrogen to metal on the coal surface and regeneration of HCl, according to the following equation:

While the reduction of NiCl.sub.2 to nickel metal proceeds quickly, the reduction of FeCl.sub.2 with hydrogen is a slow reaction. Consequently, the process itself gives rise to a better selectivity as far as the grade of the nickel is concerned. The hydrogen is formed by the reaction of water vapor with carbon according to the following equation:

It is interesting to note, that an excess of hydrogen has an adverse effect on nickel segregation, because it would favor the reduction "in situ" and not through the nickel chloride. On the other hand, the FeO which is chloridized easier than NiO to form FeCl.sub.2, has a beneficial effect on the chloridization of NiO (thermodynamically better than HCl), as follows:

Apart from the thermodynamical aspects of the reactions involved in the segregation process, the mineralogical composition of the ore in connection with the new mineral components which might be formed during the heating and roasting stages by the influence of the added reagents plays an important role. Thus the choice of an adequate mixture of reagents would make the nickle oxide more physically accessible to the HCl or FeCl.sub.2 action and consequently would improve the kinetics of chloridization.

First of all, the blended ore with the reagents must be porous during the roasting stage in accordance with the invention. So far, calcium carbonate plays this role, mainly for feeds in pellet form, (as has been observed repeatedly during experiments) since during the gradual heating of the ore, CaCO.sub.3 is decomposed and the carbon dioxide tends to escape evenly from the pellets leaving voids behind it, thus it would allow the reactions of gas with solid phase and gases between them to take place more easily during the roasting stage.

It was also observed that additions of small amounts of calcium sulphate up to 0.25% by volume has a beneficial effect on the segregation of nickel, by comparing the results obtained for the concentrate (by flotation) in the absence of it and it was concluded that it acts as a promoter of nickel oxide chloridization which is considered as the weakest and most delicate point of the process. Apart from the beneficial effect of calcium sulphate as a promoter, it improves the consistency of the pellets, avoiding the cracking during the preheating and roasting stages.

The remainder CaO (from CaCO.sub.3) during the roasting, acts, probably directly, on the lattice of the ore, with some disruptive capacity actions, forming the correspondence silicates and rendering the nickel oxide more amenable to chloridization, presumably by FeCl.sub.2. The presence of a substantial amount of fayalite, as was detected by X-ray diffraction in the roasted products, testifies to the above assumption, since the formation of fayalite is viewed as a continuation of the reaction expressed in equation 4, in which 2 molecules of FeO are constantly removed by a molecule of SiO.sub.2 to form fayalite, thus improving the kinetics of the chloridization of nickel oxide to nickel chloride and the reduction of the latter by hydrogen.

All tests were carried out on a bench scale, namely, in a horizontal electric furnace with a temperature controller and the charges were introduced inside to a 5cm diameter tight ceramic tube. Feeds in the form of pellets were preferred instead of fines. The velocity of the different gases flowing in the tube during the heating was not higher than 0.35 cm/sec and for 200 g of sample. Higher velocities were found to be deterring in the laboratory investigations. Also, suitable gas atmospheres were found to be either nitrogen, or neutral or slightly reducing gases, all without moisture or hydrogen.

For the experiments, the crushed ore was ground to pass a 200 mesh sieve and mixed with coke breeze (-35 mesh), limestone and gypsum. The blended ore was sprayed thoroughly with a 23% weight by volume sodium chloride solution and pelletized (see the FIGURE).

Typical conditions for roasting, flotation magnetic separation as well as the amount of reagents used are given below as follows:

Table 1 below shows typical chemical analyses of an ore deposit as well as a coke breeze, respectively.

Results obtained by a combined process of segregation (under an inert atmosphere of nitrogen) and flotation, according to the already specified conditions with the exception of no gypsum additions.

Comparative results obtained by using different chloridizing agents for segregation through a flotation process under an inert atmosphere and the specified conditions.

Comparative results obtained under the above mentioned specified conditions with the additions of calcium sulphate as a promoter.

Comparative results obtained using different gas atmospheres during the roasting process, maintaining all the remaining factors constant (reagents, temperature, etc.). No gypsum was added.

Comparative results obtained with roasted and ground samples shared in equal parts to recover the nickel either by flotation or by wet magnetic separation.

The results obtained by the devised combined process of segregation (under inert, neutral or slightly reducing atmospheres) through the flotation or magnetic separation have proved satisfactory as far as the grade and the nickel recovery are concerned.

The effect of the porosity is shown by the result obtained by sample No. 662 in Table 3 where calcium chloride was used as a chloridizing agent. It was observed (after cooling) that the roasted product with calcium chloride was harder and less porous than the corresponding roasted ore, with the above mentioned devised chloridizing mixture under the same roasting conditions, e.g. heating rate, retention time, gas flow rate, etc., at least for the type of ore examined. The same phenomenon was observed with sodium chloride when it was used alone, but to a lesser extent than calcium chloride. Again, the grade and the nickel recovery were lower (sample No. 659 in Table 3), but better than in the case of calcium chloride.

Apart from the porosity effect in the segregation process already mentioned, there is also the problem of choosing a proper chloridizing agent with regard to the ores containing combined water. Although CaCl.sub.2 is considered as the best chloridizing agent for the nickel segregation (this is valid only in the presence of very small amounts of water) it has the disadvantage that it cannot be successfully used for nickel ores containing combined water (as has been concluded from the experiments). In an attempt to remove the combined water by preroasting the sample at a temperature around 900.degree. C, no satisfactory results were obtained for the segregation, apparently due to the new mineralogical components formed during the ore preroasting, particularly the forsterite, which presumably includes in its lattice some nickel oxide.

However, the combined water is of importance for non-preroasted ores, at the temperature of their decomposition, since the water would react with a chloridizing agent in the presence of silicates to form HCl. More HCl would be formed by the action of CaCl.sub.2 than by NaCl. Consequently, a greater part of HCl would be lost (together with water) in the case of CaCl.sub.2 use, without reacting with nickel oxide or iron oxide to form the corresponding chlorides.

This is a reasonable explanation for the unsatisfactory results obtained when CaCl.sub.2 was used alone. In the present approach, apart from the use of NaCl as a chloridizing agent, calcium carbonate was chosen to fulfill a double function: firstly, in maintaining an adequate porosity during the ore roasting; and secondly, to store a potential amount of chloride as calcium chloride which is formed by the reaction of CaCO.sub.3 with HCl. Thus CaCl.sub.2 would be able to react more favorably at higher temperatures (in the presence of a minimum amount of water) for nickel chloridization.

The relatively better results obtained with sodium chloride alone, by comparing them with CaCl.sub.2 used, are due to the fact that the former is a weaker chloridizing agent, than the latter. Specifically, during the releasing of combined water, a comparatively less portion of chloride from NaCl is consumed for HCl formation, leaving the remainder for the chloridization of nickel in the presence of a minimum amount of water at higher temperatures.

The beneficial effect of calcium sulphate is shown by sample No. 735 in Table 4 by comparing the results obtained in the absence of it (Table 2). In contrast, increasing amounts of calcium sulphate have an adverse effect on segregation (sample No. 754, Table 4). In the light of the above observations, it was concluded that calcium sulphate in small amounts acts as a promoter, apparently in the chloridization of nickel and iron during the segregation process. Larger amounts would favor the reduction of nickel oxide "in situ" and not through the nickel chloride.

The ambient atmosphere plays an important role in the segregation process. Thus, roastings carried out under an oxidizing or even reducing atmosphere with moisture as shown in Table 5 for samples Nos. 729 and 722 respectively, are unfavorable for this process. The above findings are in full agreement with the predictions in the mechanism of the process and the nickel segregation must be carried out under an indirect heating. There are slight differences between the results obtained for nickel recovery in the roasted product through either flotation or magnetic separation as shown in Table 6. However, it is expected to obtain even better results as far as the grade and the nickel recovery are concerned by using various and more selective magnetic field intensities based always on the same principle as set forth in the present process.

The nickel segregation is an outstanding example of a process in which the ambient atmosphere strongly affects it. Therefore, the process must be carried out under an indirect heating. According to recent developments, such types of heating kilns seem to be available in an industrial scale, capable to work up to a temperature of 1000.degree. C.

The roasting-flotation process makes it possible to treat the concentrate by a hydrometallurigical treatment in view of its easy dissolution in acid or leaching with ammonia. It present also the advantage that the concentrate has a relatively low ratio value of iron to nickel which is approximately 2.2:1 as well as the advantage of low cost of energy, compared with a smelting process. The concentrate obtained by the roast-flotation process should be treated hydrometallurgically in view of the removal of copper from nickel.

On the other hand, the concentrate obtained from the roasting-magnetic separation due to its relatively high ratio value of iron to nickel which is approximately 4.2:1 may be treated by a smelting process to obtain a high grade iron-nickel alloy.

Generally, the present process is an economical one, because of its low cost of reagents used for the segregation and particularly when it is combined with a magnetic separation. Moreover, the weight of the concentrate is only approximately 5% of the initial weight.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.