2. DESCRIPTION OF THE RELATED ART Some zinc (Zn) ores, for example sphalerite, contain high levels of manganese (Mn) that cannot be separated using conventional mineral processing techniques, since the Mn is present in the crystal lattice. When Zn sulphides containing Mn impurities are roasted and then leached, they can, in some cases, produce leach solutions or electrolyte solutions containing unreasonably high levels of Mn.

Although some relatively small amounts of Mn are generally acceptable in a Zn electrolyte, for example, excessive quantities can create problems, since the Mn is oxidized at the anode in the form of MnO 2, some of which falls at the bottom of the cell and must be periodically removed. Greater concentrations of MnO, can lead to significant reductions in electrolytic efficiency.

It is generally known in the prior art that precipitating zinc and manganese together, using sodium carbonate or lime as naturalizing agent, can be carried out at 70°C and at pH values between 5 and 7. However, in this case, zinc begins to precipitate before manganese and therefore most of the zinc would be precipitated with manganese which is obviously not acceptable, as shown in figure 1.

Another solution proposed by the prior art is to oxidize Mn 2* to Mn"so that MnO2 can be removed by precipitation at a pH where Zanzis soluble. Air and oxygen gas are typically used as oxidants in this case but they are generally uneconomically slow. Peroxide or stronger oxidants, such as Caro's acid or ozone, are in many cases too expensive.

US Patent 2,816,819 to Wallis et al. discloses a system which uses SO2/Air to precipitate iron from a cobalt-or a nickel-bearing solution. Canadian Patent 935,650 discloses a technique by which a mixture of SO2/Air is used to precipitate a number of impurities from a cobalt or a nickel solution. However, neither reference is concerned with techniques for reducing impurities from Zn-bearing materials.

It is an object of the present invention to provide an improved method to remove at least a portion of Mn from Zn-bearing materials.

SUMMARY OF THE INVENTION Briefly stated, the invention involves a process for removing at least a portion of a manganese constituent from a zinc-bearing material, comprising the step of subjecting the material to mixture of SO2 and oxygen, at conditions sufficient to oxidize the manganese constituent.

Preferably, the material is maintained at a pH sufficient to precipitate manganese while minimizing precipitation of the zinc constituent.

BRIEF DESCRIPTION OF THE DRAWINGS Several preferred embodiments of the present invention will now be described, by way of example only, with reference to the appended drawing in which: Figure 1 is a plot of precipitation for Mn and Zn according to pH; Figure 2 is a plot of precipitation using SOZ/OZ mixture as an oxidant; DESCRIPTION OF THE PREFERRED EMBODIMENTS Briefly stated, the invention involves a process for removing at least a portion of a manganese constituent from a zinc-bearing material, comprising the step of subjecting the material to mixture of SO2 and oxygen, at conditions sufficient to oxidize the manganese constituent.

Preferably, the material is maintained at a pH sufficient to precipitate manganese while minimizing precipitation of the zinc constituent.

Preferably, the process is carried out at a pH between about 3 and about 5, more preferably between 3 and 4. Still more preferably, the pH is 3.

If the pH is higher than 5, for example up to 7, at least some residual Zn may be precipitated with the Mn. The higher the pH in this range, the greater the quantity of Zn being precipitated with Mn. In this case, it may be feasible either to process the Mn subsequently with the residual Zn in place or alternatively to subject the co-precipitate to a mildly acidic solution (such as at a pH of 3 to 4) to re-dissolve the residual zinc.

The pH limit of 4 is significant because, as the following examples illustrate, residual Zn has been found to appear in the precipitate at a pH value above 4 while there appears to be no Zn co-precipitate at pH values below 4. Therefore, it may be desirable, in some circumstances, to maintain the reaction at the lower end of the pH range, that is in the vicinity of pH 3 in order to minimize the likelihood of a Zn co-precipitate. This route may also be enhanced by maintaining a distribution of nucleation sites in the reaction, such as MnO2 crystals.

The oxygen may be in the form of 02 or air or a mixture of both.

Preferably, the process occurs at a temperature ranging from about 40 to about 80°C, more preferably at a temperature ranging from 50 to 80°C, still more preferably at a temperature ranging from 58 to 78°C. For example, the process may be carried out at about 70°C or, alternatively, at about 60°C.

It may also be desirable, in some cases, to raise the temperature above 80°C, for example to an upper limit of about 130°C or higher in order to increase the reaction kinetics of the process, though this would need to be done under pressure, depending on the chosen temperature. For example, a temperature of 130°C would require a pressure of about 50 psi.

Preferably, in the case where the oxygen is present in the form ouf ou gas, the SO2 is at a concentration from about 0.5% to 10%, with the balance 02 gas, more preferably from 1 to 8%, still more preferably at a concentration from 2 to 3%.

In the case where the oxygen is present in air, the SO2 is preferably at a concentration ranging from about 0.1% to 2%, with the balance being Air, more preferably from about 0.2 to 1.4%, still more preferably from about 0.4 to 0.6%. For example, the concentration may be about 0.5%.

The zinc material may be in a number of forms including an aqueous solution, such as a leach solution or an electrolyte solution.

The present process is beneficial in that it makes use of a relatively inexpensive and plentiful oxidant, a gas mixture of O2/SO2, or alternatively Air/SO2, or still alternatively 100% pure Air can be used together with equivalent amounts of SO2, preferably added as SO2 in a gaseous or liquid form, or added as a constituent in a solution containing, for example, sodium metabisulphite, ammonium metabisulphite, potassium metabisulphite or other suitable forms of metabisulphite.

Embodiments of the present invention will be described with reference to the following Examples which are presented for illustrative purposes only and are not intended to limit the scope of the invention.

EXAMPLE 1 A solution containing 5 g/L Mn as MnSO4 was sparged with a mixture of SO2 and 02 at different pH levels. The amount of Mn removed at each pH is shown in figure 2.

A comparison can be made between the precipitation of pH's of manganese oxidized with SO2/O2 as shown in figure 2 with the precipitation of zinc as shown in figure 1. The Mn is removed from solution at pH levels ranging from about 3 to 5. However, in the region from about 3 to 4, the Mn is removed from solution while the Zn is soluble and therefore remains in solution.

EXAMPLE 2 A solution containing 18 g/L Mn as MnSO4 was sparged at 60°C and at a pH of 6.5 with SOz/O2 mixtures containing 2% (v/v) SO2 After one hour, 25% of the manganese had precipitated. After 2 hours, 57% of the Mn had precipitated and after 4 hours, 99.5% of the Mn had precipitated.