A steadily increasing demand for nickel has without doubt raised the importance of the worlds nickel reserves and applied more focus on the techniques used for extraction of nickel from its ores.
The Caron Process is one such technique which is effective by reduction of the ore by roasting followed by ammonia leaching of the reduced ores.
Nickel occurs in nature principally as sulphides and laterites (oxides and silicates). Nickel laterite ores, which account for approximately 80% of world's nickel reserves, are now becoming increasingly important for the world's nickel supply as nickel demands have grown in the recent decades.
In general, nickel can be extracted from its ores using smelting, high pressure acid leaching (HPAL) and Caron roast/leach processes.
The Caron process, which involves reduction roasting of the ore followed by ammonia leaching of reduced ore, is one of the ways in which nickel laterite can be treated. It has been successfully used in processing the iron-rich limonitic part of the nickel laterite ore body. Reserves of limonite ores are, however, limited and there are large deposits of saprolitic ores (with high Mg and Si content) that cannot currently be economically processed by the Caron process.
A typical composition of an oxide ore to be treated by Caron process is presented in Table 1. The ore is initially crushed to increase the specific surface area to successfully respond to roast and leach. Usually after mining, the wet ore contains 20 to 50% moisture. It is crushed to less than 1 inch in size and then dried to less than 1 to 3% moisture.
Table 1: Typical composition of an oxide ore
A typical block diagram of the Caron process is presented in Figure 1. The dried ore is fed to reduction roasting stage, where it is gradually heated to a specified temperature under a reducing atmosphere. The aim is to reduce the nickel oxide to metal, making the nickel amenable to ammoniacal leaching. The reduction of nickel and cobalt is a critical step in the Caron process since the co-reduction of iron has to be avoided. A critical evaluation of the efficiency of the process with respect to metal reduction is not straightforward. Most of the iron in the limonitic fraction is converted to magnetite, Fe3O4, whilst the iron in the serpentine fraction is mostly unaffected. The dryer fuel is oil or any other available fuel.
The primary reactions taking place during the roasting are those affecting nickel and iron (equations (1) to (9)). Similar reactions occur for cobalt and copper. Reduction of metal oxides mainly takes place at temperatures in the range of 700-900°C. Temperature has an effect on reaction kinetics, with higher temperatures giving faster reactions and therefore a practical lower limit of about 540°C is used in most operations. An upper limit of about 900°C is observed, primarily to avoid the sintering that occurs at high temperatures.
Figure 1: Block diagram for Caron process
NiO(s) + H2(g) = Ni(s) + H2O(g) … (1)
3 Fe2O3(s) + H2(g) = 2 Fe3O4(s) + H2O(g) … (2)
Fe3O4(s) + H2(g) = 3 FeO(s) + H2O(g) … (3)
FeO(s) + H2(g) = Fe(s) + H2O(g) … (4)
NiO(s) + CO(g) = Ni(s) + CO2(g) … (5)
3 Fe2O3(s) + CO(g) = 2 Fe3O4(s) + CO2(g) … (6)
Fe3O4(s) + CO(g) = 3 FeO(s) + CO2(g) … (7)
FeO(s) + CO(g) = Fe(s) + CO2(g) … (8)
Ni(s) + Fe(s) = FeNi(s) … (9)
In order to obtain a maximum reduction of nickel oxide to metallic nickel, most of the iron must also be reduced, once a substantially large fraction of the nickel in laterite ores is tied up with iron oxides. However, the complete reduction of iron to metallic form is unnecessary and undesirable. After reduction, the resulting material is cooled at 120°C - 150°C under inert atmosphere to prevent re-oxidation of the nickel and other desired metals. A typical composition of a reduced oxide ore after losing 20% of mass is presented in Table 2.
Table 2: Typical composition of the reduced oxide ore
After cooling, the roasted ore is sent for ammoniacal leaching, where metallic nickel is oxidized and dissolved. The amount of metal solubilized is greatly dependent on the reduction roast conditions, which in turn depends on the type of ore. Equations (10) to (13) show the typical overall leach reactions. Ironammine complex is stable under very limited conditions. In oxidizing conditions, this complex is decomposed to form the insoluble hydroxide, as shown in Equation (13). Air is sparged into the leach tanks to provide the required oxygen. Figures 2 and 3 show that at pH between 9 and 10, and under oxidative environment, Ni(NH3)++ is the thermodynamically predominant nickel species whilst that for iron is Fe(OH)3(s).
Figure 2: Eh-pH diagram for Ni-NH3-H2O system at 298 K and 1 atm total pressure. The activities of NH3 above pH = 9.25 and NH4 + below 9.25 are unity. The ionic nickel species is 10-2 mol/l