Patent Publication Number: US-2009217786-A1

Title: Processing of laterite ore

Description:
The present invention relates the processing of laterite ore. 
     BACKGROUND TO THE INVENTION 
     Processing of nickel laterite ore is complicated due to the mineralogically and chemically complex nature of the ore. A technique called high-pressure acid leaching (HPAL) is usually used. The process involves the preparation of the ore into a slurry. The slurry is then contacted with sulfuric acid at temperatures of 250° C.-280° C. and under high pressure for 60 minutes. This process leaches the nickel, cobalt and iron into solution. The resultant nickel cobalt liquor is recovered and processed by solvent extraction to produce separate nickel and cobalt products. 
     The processing of nickel laterites by the above method is complex, costly, time consuming and involves high energy consumption. The present invention attempts to overcome at least in part the aforementioned disadvantages 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a process for the leaching of metals from highly oxidised laterite ores by admixing the ore and an acidic aqueous medium and treating the resulting mixture with a sulfur reducing agent at ambient temperature and pressure so as to reduce mineral species containing cobalt, nickel or manganese, the reduced mineral species being dissolved and subsequently recovered. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Naturally occurring nickel laterites are derived from peridotite rocks containing olivine and serpentine. Under favorable conditions and with abundant rainfall various acids, such as humic and others, are produced as a result of decaying organic matter. These acids leach out the magnesium and silica values, while enriching the residue with iron and nickel. 
     These laterite deposits can be divided into three zones at increasing depth from the surface. The three zones are the limonite zone, the serpentinite zone, and the garnierite zone. 
     The zone of interest in this invention is the limonite zone. Limonite zones are typically highly oxidised and preferably contain asbolite type materials such as asbolane. Asbolane has the general formula: 
       (Co,Ni)1 −y (MnO 2 )2− x (OH)2−2 y÷ 2 x·n (H 2 O). 
     The present invention proposes that highly oxidised laterite ores, preferably containing asbolane, are treated by the reduction of the mineral species by a sulfur reducing agent in an aqueous acidic medium. The present invention is envisaged to be used where the asbolane ore is preferably made up of between 0.1-10 percent cobalt by weight, 40-80 percent manganese oxide by weight and 0.1-20 percent nickel by weight. More preferably the asbolane ore is made up of between 2-5 percent cobalt by weight, 50-70 percent manganese oxide by weight and 5-15 percent nickel by weight. 
     The ore may preferably be ground to a particle size in the range of 50 to 600 mesh, more preferably 100 mesh. This ground ore is admixed with, such as by being added to, an aqueous medium. Preferably the aqueous medium may be a dilute solution of mineral acid, more preferably a dilute solution of sulfuric acid. Preferably the medium has a low pH, more preferably a pH less than 2.0. 
     The sulfur reducing agent may be added directly to the medium in the form of gaseous sulfur dioxide, or an aqueous solution thereof. The amount of sulfur dioxide is preferably in the range of 0.8 to 3.2 mol SO 2 /kg of ore. The sulfur dioxide may be added incrementally or all at once. 
     More preferably the sulfur reducing agent is generated in situ by the reduction of an alkali metal sulfite salt, preferably sodium sulfite through the reaction: 
       SO 3   2− +2H + ⇄SO 2 +H 2 O 
     The amount of sulfite salt may preferably be in the range of 10 to 40 percent by weight of the ore. The sulfite salt may be added to the medium containing the asbolane ore either incrementally or all at once. 
     The nickel, cobalt and manganese in the ore are solubilised in the aqueous medium as a result of a reduction of the mineral components by the sulfur reducing agent. Copper, iron and aluminium may not be leached to an appreciable level and remain in the solid residue. 
     The treatment time may be in the range of 20 minutes to 20 hours, preferably in the range of 30 minutes to 10 hours. The treatment temperature may preferably be in the range of 20 to 50° C. Preferably the pressure is less than 2 atmospheres, more preferably the pressure is ambient. 
     The process of the present invention may be performed in sealed reactors designed to contain sulphur dioxide or any other off gases from venting to atmosphere. The reactors preferably contain a stirring mechanism to maintain the slurry in suspension. Also, the reactors may contain baffles to reduce bypass or short circuiting the reactor residence volume. 
     The nickel, cobalt and manganese can then be recovered by known processes. For example, the soluble metal salts may be recovered by ionic exchange to produce nickel sulphate hexahydrate, cobalt suylphate hexahydrate and manganese sulphate. 
     In an alternative embodiment of the present invention the sulfur reducing agent may preferably be generated in situ by the reduction of an alkali metal metabisulfite salt, preferably sodium metabisulfite through the reaction: 
       S 2 O 5   2− +2H + ⇄2SO 2 ÷H 2 O 
     The amount of metabisulfite salt may preferably be in the range of 5 to 30 percent by weight of the ore. The metabisulfite salt may be added to the medium containing the asbolane ore either incrementally or all at once. 
     The present invention will now be described with reference to the following examples. 
     In the following examples the ore used had the following composition of elements of interest. The percentage of cobalt, manganese, nickel and iron in the highly oxidised laterite ore sample was 0.688, 4.35, 0.996 and 38.1 percent by weight respectively. 
     EXAMPLE 1 
     This example illustrates the rate of leaching over a 24 hour period. 50 g of asbolane ore was admixed with 500 mL of 0.2 M sulfuric acid and 15 g of Na 2 SO 3  added incrementally. The experiment was conducted at 25° C. and ambient pressure. Samples were taken at 0, 6, 9 and 24 hours and analysed for their cobalt, nickel, iron and manganese concentrations. 
     An analysis of the samples taken at six hours showed that 76.7 percent of the cobalt 67.4 percent of the manganese, 43.2 percent of the nickel and 0.8 percent of the iron had been recovered. An analysis of the samples taken at nine hours showed that 79.9 percent of the cobalt, 70.5 percent of the manganese, 45.2 percent of the nickel and 0.9 percent of the iron had been recovered. An analysis of the samples taken at twenty four hours showed that 83.0 percent of the cobalt, 72.6 percent of the manganese, 47.4 percent of the nickel and 1.4 percent of the iron had been recovered. 
     From the data it may be seen that the optimal contact time is under 10 hours. 
     EXAMPLE 2 
     This example illustrates the effect of increasing temperature and decreasing amounts of Na 2 SO 3  in the reaction with respect to Example 1. 50 g of asbolane ore was admixed with 500 mL of 0.2M sulfuric acid and 10 g of Na 2 SO 3  added incrementally. The experiment was conducted at 40° C. and ambient pressure. Samples were taken at 0, 3, 6, 9 and 24 hours and analysed for their cobalt, nickel, iron and manganese concentrations. 
     An analysis of the samples taken at three hours showed that 62.9 percent of the cobalt, 50.1 percent of the manganese, 35.1 percent of the nickel and 0.7 percent of the iron had been recovered. An analysis of the samples taken at 6 hours showed that 74.8 percent of the cobalt, 65.6 percent of the manganese, 43.0 percent of the nickel and 1.0 percent of the iron had been recovered. An analysis of the samples taken at nine hours showed that 81.1 percent of the cobalt 69.7 percent of the manganese, 47.8 percent of the nickel and 1.5 percent of the iron had been recovered. An analysis of the samples taken at twenty four hours showed that 80.8 percent of the cobalt, 69.6 percent of the manganese, 47.3 percent of the nickel and 2.2 percent of the iron had been recovered. 
     From this data it may be seen that the optimal contact time for extraction of the metal values is around 10 hours. 
     After twenty four hours the recovery rates for cobalt and nickel were greater in Example 1 than in Example 2. In the same period the amount of nickel recovered was very similar. The amount of iron however was 1.4 percent in Example 1 compared with 2.2 percent in Example 2. 
     EXAMPLE 3 
     This example illustrates the effect of the concentration of reducing agent on the extraction efficiency. 25 g samples of asbolane ore were admixed with 500 mL of 0.2M sulfuric acid. These samples were then treated with differing concentrations of sulfur dioxide, namely approximately 1.8, 7.2, 10.8, 12.6 and 14.4 mmol/g. The samples were leached for 45 minutes at 25° C. and ambient pressure. At the completion of the leaching period the pulps were filtered dried and analysed. 
     An analysis of the sample treated with 1.8 mmol/g SO 2  showed that 16.2 percent manganese, 8.0 percent nickel, 16.0 percent cobalt and 4.0 percent iron by weight had been recovered from the ore. An analysis of the sample treated with 7.2 mmol/g SO 2  showed that 62.6 percent manganese, 48.6 percent nickel, 25.0 percent cobalt and 4.0 percent iron by weight had been recovered from the ore. An analysis of the sample treated with 10.8 mmol/g SO 2  showed that 82.1 percent manganese, 76.6 percent nickel, 30.0 percent cobalt and 5.8 percent iron by weight had been recovered from the ore. An analysis of the sample treated with 12.6 mmol/g SO 2  showed that 88.6 percent manganese, 88.6 percent nickel, 52.1 percent cobalt and 5.8 percent iron by weight had been recovered from the ore. An analysis of the sample treated with 14.4 mmol/g SO 2  showed that 92.6 percent manganese, 92.0 percent nickel, 88.0 percent cobalt and 5.8 percent iron by weight had been recovered from the ore. 
     It may be seen from Example 3 that a selective recovery of nickel, manganese and cobalt may be achieved by leaching with sulfur dioxide in an acid medium. The efficiency of the leaching increased with the increasing concentration of sulfur dioxide. The quantities of iron leached were minimal and appeared independent of the concentration of sulfur dioxide when leached in this way. 
     EXAMPLE 4 
     This example illustrates the effect of adding Na 2 SO 3  all at once at the start of the reaction rather than incrementally during the reaction, as performed in Example 1. 50 g of asbolane ore was admixed with 500 mL of 0.2M sulfuric acid and 10 g of Na 2 SO 3 , adding all the Na 2 SO 3  at the start of the reaction. The experiment was conducted at 25° C. and ambient pressure. Samples were taken at 0, 30, 60, 110, 150, and 190 minutes and analysed for their cobalt, nickel, iron and manganese concentrations. 
     An analysis of the samples taken at thirty minutes showed that 59.6 percent of the cobalt, 61.4 percent of the manganese, 28.2 percent of the nickel and 0.4 percent of the iron had been recovered. An analysis of the samples taken at sixty minutes showed that 67.3 percent of the cobalt, 71.3 percent of the manganese, 33.3 percent of the nickel and 0.5 percent of the iron had been recovered. An analysis of the samples taken at one hundred and ten minutes showed that 72.3 percent of the cobalt, 76.0 percent of the manganese, 37.1 percent of the nickel and 0.5 percent of the iron had been recovered. An analysis of the samples taken at one hundred and fifty minutes showed that 73.7 percent of the cobalt, 78.6 percent of the manganese, 37.7 percent of the nickel and 0.5 percent of the iron had been recovered. An analysis of the samples taken at one hundred and ninety minutes showed that 74.2 percent of the cobalt , 78.5 percent of the manganese, 39.1 percent of the nickel and 0.6 percent of the iron had been recovered. 
     It is evident from the data in Example 4 that the optimal contact time is approximately 1 hour when the Na 2 SO 3  is added all at once at the start of the reaction. This reaction time is significantly lower than times used in examples 1 and 2, however the recoveries achieved in this example were slightly lower. 
     EXAMPLE 5 
     This examples illustrates the effect that the particle size of the ore has on the efficiency of the leaching process. Samples of the asbolane ore were crushed and separated into sizes ranges, namely 75-104μ, 104-152μ, 152-211μ, 211-295μ, 295-422μ, 422-599μ. Samples of the ore from each size fraction were then admixed with 500 mL of 0.2M sulfuric acid and 10 g of Na 2 SO 3 . The samples were then leached for 45 minutes at 25° C. and ambient pressure. At the completion of the leaching period the pulps were filtered dried and analysed. 
     An analysis of the 75-104μ ore fraction showed that 75.3 percent nickel, 85.6 percent cobalt, 88.0 percent manganese and 5.8 percent by weight of iron had been recovered. 
     An analysis of the 104-152μ ore fraction showed that 70.9 percent nickel, 80.1 percent cobalt, 86.0 percent manganese and 5.8 percent by weight of iron had been recovered. An analysis of the 152-211μ ore fraction showed that 68.1 percent nickel, 76.4 percent cobalt, 84.7 percent manganese and 5.9 percent by weight of iron had been recovered. s An analysis of the 211-295μ ore fraction showed that 59.0 percent nickel, 69.4 percent cobalt, 76.2 percent manganese and 7.6 percent by weight of iron had been recovered. An analysis of the 295-422μ ore fraction showed that 53.2 percent nickel, 59.8 percent cobalt, 60.6 percent manganese and 5.9 percent by weight of iron had been recovered. An analysis of the 422-599μ ore fraction showed that 47.9 percent nickel, 55.6 percent cobalt, 60.2 percent manganese and 5.7 percent by weight of iron had been recovered. 
     It may be seen from this example that the efficiency of the leaching process decreased with the increase in size fraction of the ore. The quantities of iron leached were minimal and appeared independent of the particle size of the ore when leached in this way. 
     Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.