FIELD OF THE INVENTION

The present invention relates to a method for removing sodium from chloride containing solutions in a hydrometallurgical metal recovery processes. More specifically the invention relates to removal of sodium from hydrometallurgical process solutions resulting from chloride based leaching of nickel containing ores and/or concentrates.

BACKGROUND OF THE INVENTION

In chloride based leaching of ores and/or concentrates, in particular of nickel containing ores and/or concentrates, the leaching solution is typically recirculated within the process. This leads to build-up of elements such as sodium and potassium. Sodium is typically present in the solution as sodium chloride. In low concentrations, i.e. less than about 10 g/L, this is not a problem. However, the sodium will make the solution more concentrated and even- tually more solution is needed to leach same amount of metals if sodium is not removed.

Traditional way to remove impurities such as sodium is to take out a small bleed stream from the process. In concentrated chloride solutions this is not an economical option since the solution needs to be treated and the chlo- rides need to be replaced.

The invention is based on the fact that the sodium chloride crystallizes before the other salts present in the solution for example calcium chloride or ammonium chloride.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is thus to provide a method so as to alleviate the above disadvantages. The objects of the invention are achieved by a method, which is characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention based on the realization that sodium can be removed from the hydrometallurgical process streams simultaneously to adjusting the water content of the process stream. Accordingly taking out only the impurity that needs to be removed the volume of the bleed stream can be minimized and the process can be made more economical and usually more environmental friendly. Since the sodium input to the process is usually quite small and process can tolerate some sodium, all sodium is not needed to be removed from the process stream. This enables crystallization from the bleed stream instead of the main process stream. This means smaller equipment and lower investment cost. Since the water is needed to be evaporated due to the closed solution circulation in the process the saturated solution can be produced for the crystallization when the evaporation is also put to the bleed stream. Crystallization from the saturated solution is easier since it starts immediately when more water is evaporated or temperature is lowered.

Sodium chloride can be taken with this invention as almost pure product since some sodium is left to the depleted sodium solution. Sodium chloride is the first salt that crystallizes from the solution and therefore amount of solid impurities in the sodium chloride product is small. To further purify sodium chloride product it can be washed. BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which

Figure 1 is a flow diagram illustrating an embodiment of the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for removing sodium from a chloride-based hydrometallurgical process stream comprising the steps of:

(a) withdrawing a bleed stream of hydrometallurgical process liquors and/or slurries from the chloride-based hydrometallurgical process stream;

(b) evaporating water from the bleed stream to obtain a saturated sodium containing solution;

(c) crystallizing a part of the sodium chloride comprised in the saturated sodium containing solution from the said solution to obtain a sodium de- pleted solution; and

(d) returning the sodium depleted solution to the process stream.

The method of the present invention is suitable for treating any hydrometallurgical process streams resulting from leaching of metal containing ores and/or concentrates with chloride based leaching solutions, such as a concentrated CaC based solution. The said hydrometallurgical process stream further comprises ammonium chloride, nickel chloride, copper chloride, cobalt chloride, and/or magnesium chloride. Typically the said hydrometallurgical process stream comprises ammonium chloride. The hydrometallurgical process stream typically comprises significant amounts of sodium. In particular the hydrometallurgical process stream comprises 1 to 50 g/L, more particularly 5 to 25 g/L, sodium. Further preferably the hydrometallurgical process stream comprises 10 to 100 g/L Ca and/or 0.1 to 10 g/L NH ₄ ^" .

The method of the present invention is particularly suitable for treatment of leaching solution resulting from leaching of nickel containing ore and/or concentrate. The term "concentrate" as used herein refers to any product generally produced from metal containing ore for concentrating the desired metal(s) in the product and/or removing impurities from the mined ore for further treatment of the product for the recovery of one or more of said metal(s). Such concentrate can be produced by any suitable methods known to a per- son skilled in the art.

Preferably the method of the present invention provides for removal of at least 30%, more preferably at least 50%, typically 30 to 90%, more typically 60 to 80%, of sodium comprised in the hydrometallurgical process stream before the withdrawal of the bleed stream. Typically the sodium depleted solu- tion returned into the main process stream, i.e. the hydrometallurgical process stream, comprises less than 4 wt%, preferably less than 2 wt%, sodium. In some cases where evaporation need of the process is big the sodium concentration is higher in the main process stream after the evaporation and sodium removal than before those process steps due to the decreased total amount of the process stream.

Figure 1 shows an example of a process flow of the method of the present invention. Referring to Figure 1 , a bleed stream 2 is withdrawn from a hydrometallurgical process stream 1 . The bleed stream can be withdrawn at any process stage after leaching of the ore and/or concentrate. Preferably the bleed stream is withdrawn at a process stage where the concentration of any valuable metals in the said process stream is as low as possible to prevent loss said valuable metals during the removal of sodium. Advantageously the bleed stream is withdrawn after a process stage whereby the acid concentration of the said process stream is under 5 g/L. Such process stages include precipitation, solvent extraction and neutralization stages. In nickel leaching such process stages are in particularly iron precipitation, solvent extraction and/or ammonia regeneration. Suitable locations for the withdrawal include after iron precipitation, after one of the solvent extraction stages, and/or after regeneration of ammonia. In a preferred aspect of the present invention the bleed stream is withdrawn after regeneration of ammonia.

The volume of the bleed stream is dependent of the amount of water that needs to be removed from the process stream to maintain water balance. If water balance is taken care in other parts of the process the bleed stream is dependent of the amount of sodium needed to be removed, but typically the water balance determine the bleed stream amount. The amount of bleed stream can be calculated from the amount of water needed to be removed from the process at this stage, from the salt concentration of the process stream and the salt concentration of the saturated sodium containing solution, e.g. a solution containing 50 g/L CaC^ and 5 g/L NaCI has a salt concentration of 55 g/L. Unless otherwise denoted, the term "salt concentration" as used herein and hereaf- ter refers to the amount of all dissolved salts in the particular stream. The salt concentration of a solution is dependent on temperature and the nature of salts in the said solution. The process stream typically comprises 5 to 35 wt% salts, more typically 10 to 30 wt% of the total weight of the said stream. The saturated sodium containing solution typically contains 35 to 70 wt% of salts more typically 40 to 65 wt% of the total weight of the saturated solution. Typically a variation of ±10%, more typically ±5%, is tolerated for the amount of required bleed stream. If the salt concentration in the process solution, evaporation need or salt concentration the saturated sodium containing solution changes, also the amount of bleed stream will change. Examples how the amount of the bleed stream can be calculated are presented in examples 1 and 2.

The bleed stream is preferably withdrawn continuously from the hy- drometallurgical process stream i.e. as a continuous bleed stream of the hy- drometallurgical process stream. The continuous operation of sodium chloride crystallization enables smaller equipment and more even sodium concentration in the process. The other option is to withdraw a continuous bleed stream for evaporation of water in step (b) and operate the crystallization of sodium chloride in step (c) as a batch process.

The bleed stream 2 is then subjected to evaporation 10 in the evaporation step (b) to remove water 1 1 from the bleed stream and to concentrate sodium in the bleed stream. This results in a saturated or nearly saturated sodium containing solution 3 which is subjected to crystallization 20 in the crystal- Iization step (c). The term "saturated or nearly saturated" as used in context of the saturated sodium containing solution refers to the fact that further evaporation of water from the solution would cause crystallization of sodium chloride thus enabling the crystallization of the same in the next step.

The evaporation step (b) 10 is typically performed in a multi stage evaporation equipment. In a multi stage evaporation the first evaporation step is typically performed atmospherically or near atmospherically. Further evaporation step(s) are typically performed under reduced pressure. Advantageously steam formed in the first evaporation stage is utilized for indirect heating of the further evaporation step(s).

Depending on the process sodium balance, a part of the saturated or nearly saturated sodium containing solution 3 can be fed back to the main process between evaporation of the water and crystallization of the sodium chloride as a concentrated stream 5. Thus in an embodiment of the present invention a part of the saturated sodium containing solution (3) obtained in step (b) is returned to the hydrometallurgical process stream before crystallization step (c). Alternatively or additionally part of a partly concentrated sodium containing solution i.e. part of the bleed stream being concentrated in step (b) can be returned to the hydrometallurgical process stream before it is saturated or nearly saturated. This is possible when a multi stage evaporation equipment is utilized and/or the evaporation step is otherwise performed in multiple sub- steps. In the above mentioned embodiments the bleed stream of hydrometallurgical process liquors and/or slurries withdrawn from the chloride-based hydrometallurgical process stream may be 100% of the hydrometallurgical pro- cess stream.

The concentrated stream 5 is preferably returned continuously to the hydrometallurgical process stream 1 . The amount of such concentrated stream is dependent on the sodium amount in the stream, sodium removal need in the process and the amount of sodium at the concentrated stream af- ter crystallization, stream 4. For example if the sodium amount in the stream 3 is 100 kg/h and sodium is needed to be removed from the process 50 kg/h and the stream 4 after crystallization contains 10 kg/h sodium, 40% of the stream 3 is fed back to the main process stream in stream 5. Typically the variation of stream split percentage value is ±10%, more typically ±5%. The saturated sodium containing solution 3 is then fed to crystallization step (c) 20 to remove sodium as sodium chloride 22 from the saturated sodium containing solution. Crystallization can be achieved by evaporation of water thus causing the sodium chloride to crystallize from the saturated solu- tion or decreasing the temperature to cause the sodium chloride to crystallize. In evaporation crystallization option the evaporated water is removed from the process with stream 21 . Typically crystallization in step (c) 20 is achieved in a separate crystallization device. Alternatively crystallization step (c) 20 can be achieved in the same equipment as the evaporation step (b) 10. Further, part of the evaporation step can be achieved in a first evaporation equipment to concentrate the stream and the final saturation and the crystallization step is achieved in an another equipment.

Typically a slurry comprising sodium chloride crystals is obtained from the crystallization step. The slurry is then mostly dried in a centrifuge to obtain mostly dry or dry crystals. The dried crystals may then be washed with water to remove remaining mother liquor and to further purify the crystals. The washing solution is preferably recirculated to the crystallization step (c) to improve the recovery of sodium chloride.

Preferably at least 30 wt%, preferably from 30 to 100 wt%, more preferably from 50 to 95 wt%, of the sodium chloride contained in the originally withdrawn bleed stream is crystallized. Removal of sodium chloride in step (c) results in a sodium depleted solution 4 that is returned to the process stream to keep the salts in the process in a closed circulation.

The sodium depleted solution 4 is returned to the hydrometallurgical process stream 1 typically at the same process stage where the bleed stream 2 was withdrawn from the process stream 1 . This enables continuation of the following process steps in a conventional manner.

The sodium depleted solution 4 is preferably returned to the hydro- metallurgical process stream i.e. as a continuous sodium depleted stream. This enables a steady salt concentration in the main process stream. Even if the crystallization is operated as a batch process a continuous stream of sodium depleted solution to the process stream is needed since big differences in the salt concentrations in other parts of the process make it difficult to operate the process. The hydrometallurgical process stream 1 typically contains from 1 to 50 g/L, more typically 5 to 25 g/L sodium after the sodium depleted solution is mixed with the said process stream. EXAMPLES Example 1

50 t/h water is removed from a 100 t/h process solution. The flow of the needed bleed stream amount is calculated followingly: The process solu- tion comprises 20 wt% of salts. 60 wt% of salts in the saturated solution is used. The 100 t/h process solution flow then contains 20 t/h salts, 80 t/h water and produced saturated solution contains 20 t/h salts and 13.3 t/h water. Therefore in the bleed stream is 50 t/h + 13.3 t/h = 63.3 t/h water. This means that water needed to be put to the bleed stream is 63.3 t/h / 80 t/h = 0.791 times the water in the initial process stream.

Example 2

When process stream is 100 t/h, water evaporation need is 10 t/h, salt concentration of the process solution is 25 wt% and saturated solution salt concentration is 60 wt%, the amount of bleed stream is 33.3%.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.