AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION Innovation Patent Applicantss: MINTEK Invention Title: Regeneration of magnesium hydroxide The following statement is a full description of this invention, including the best method for performing it known to me/us: -2 REGENERATION OF MAGNESIUM HYDROXIDE BACKGROUND OF THE INVENTION This invention relates, in a general sense, to the recovery of Mg(OH) 2 from barren liquors, formed by base metal precipitation, that normally 5 accumulate in a tailings dam. The use of magnesium oxide as a neutralising agent in hydrometallurgical circuits, for the recovery of base metals such as Ni and Co, has gained popularity in the last decade. A higher quality mixed hydroxide product, of about 42% Ni or Co, can be obtained, compared to a product which io results from the use of lime, which produces a quality range of about 15-17% Ni or Co. Another advantage of using MgO during precipitation processes is that it does not form gypsum. Gypsum formation, arising from the use of lime, necessitates the use of a larger base metal precipitation circuit. Furthermore, 15 the use of lime increases wash-water requirements which in turn adversely affect the water balance of base metal circuits. The use of MgO is however more expensive than the use of lime. Furthermore, for some installations, MgO must be imported and logistical issues can be encountered. 20 An object of the present invention is to address, at least to some extent, the aforementioned aspect. SUMMARY OF THE INVENTION The invention provides a method of recovering Mg(OH) 2 from a liquor formed in a base metal precipitation circuit, which includes the steps of: 3781422_1 (GHMatters) P91640.AU 18/10/12 -3 a) using Mg(OH) 2 to precipitate base metal from a liquor which contains a base metal-sulphate, thereby producing a solution which contains Mg2+; b) precipitating Mg 2 . by the addition of lime to the solution, thereby 5 producing a stream which contains Mg(OH) 2 and gypsum; and c) separating Mg(OH) 2 from gypsum in the stream. The stream may be passed through a thickener to remove excess water, leaving behind wet solids consisting primarily of gypsum and Mg(OH) 2 .The use of a thickener may not be needed if the density of the solids 1o in the stream allows for the stream to be fed directly to a separating device. The wet solids may be passed through a hydrocyclone or similar device to separate the gypsum from the Mg(OH) 2 , thereby producing a secondary stream which is rich in Mg(OH) 2 A portion of the wet solids may be recycled to step (b) to grow gypsum 15 particles preferentially to Mg(OH) 2 particles. The secondary stream which is rich in Mg(OH) 2 may be recycled to step (a). Fresh Mg(OH) 2 may be added to supplement the recycled Mg(OH) 2 stream. Step (a) may be carried out at ambient temperature. However an increase in efficiency is achieved if this step is carried out at an elevated 20 temperature of, say, 45*C to 55*C. Similarly step (b) may be carried out at pH 8.5 to pH 9.6 at ambient temperature but an increase in efficiency is achieved if precipitation is effected at a higher temperature e.g. of, say, 45 0 C to 550C. BRIEF DESCRIPTION OF THE DRAWING 25 The invention is further described by way of example with reference to the accompanying drawing which is a schematic representation of a base metal precipitation circuit in which the method of the invention is implemented. 3781422_1 (GHMatters) P91640.AU 18/10/12 -4 DESCRIPTION OF PREFERRED EMBODIMENT The accompanying drawing illustrates a process 10 in which the method of the invention is utilized to process a solution 12 which includes a base metal sulphate. 5 Mg(OH) 2 is added to the solution 12 during a step 14, thereby to precipitate a base metal such nickel or cobalt. The base metal is precipitated in the form of a solid base metal-hydroxide in a solution 16 which also contains about 5 g/L of Mg 2 + as MgSO 4 . The solution 16 is subjected to a solid/liquid separation step 18 which 10 produces a precipitate 20 of the base metal hydroxide, and a MgSO 4 solution 22. The step 14 may be carried out at an ambient temperature but, preferably, is carried out at an elevated temperature of about 50 0 C for the higher temperature results in an increase in the efficiency of precipitation. 15 In a step 24 lime is added to the solution 22 to generate a stream 26 which contains precipitated Mg(OH) 2 and gypsum. The step 24 may be carried out at ambient temperature but, preferably, is carried out at an elevated temperature of about 50*C and a pH of about 9,15. The elevated temperature increases the efficiency of precipitation. 20 Excess water 28 is removed from the stream 26 by a thickener 30. A stream 32 emerging from the thickener contains wet solids consisting primarily of Mg(OH) 2 and gypsum. In a step 34 the wet solids are separated into gypsum 36 and a stream 38 which is rich in Mg(OH) 2 . This separation, which is effected by the use of a 25 hydrocyclone or any other suitable classification device, is possible due to the size difference of the particles, namely that the gypsum particles are larger than the Mg(OH) 2 particles. 3781422_1 (GHMatters) P91640.AU 18/10/12 - 5 A dotted line 40 indicates that the stream 26, if it has the correct solid density, can be sent directly to the classification device for separation into gypsum 36 and the stream 38 i.e. the use of the thickener 30 is, in this instance, not required. 5 The Mg(OH) 2 rich stream 38 is recycled (step 42) to the precipitation step 14 and is added to incoming solution 12. Some of the Mg(OH) 2 , initially used in the step 14, is lost during subsequent steps in the process. Thus the recycling (step 42) of the stream 38 returns a reduced amount of Mg(OH) 2 to the start of the process. To make up 10 the deficit, fresh Mg(OH) 2 should be added to the stream 38. The invention holds a number of advantages compared to conventional base metal precipitation circuits. These include the following: a) lime is not used to precipitate the base metal - thus the final product (Co/Ni) is not diluted by the formation of gypsum; 15 b) a smaller precipitation circuit can be used; c) a higher grade product can be achieved with Mg(OH) 2 compared to the product which is produced when lime is used; d) the utilization of the recovered Mg(OH) 2 is similar to that which is obtained with commercially available MgO; 20 e) importation of fresh MgO is significantly reduced; and f) reagent cost is reduced. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as 25 "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication 3781422_1 (GHMatters) P91640.AU 18/10/12 -6 forms a part of the common general knowledge in the art, in Australia or any other country. 3781422_1 (GHMatters) P91640.AU 18/10/12