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:: Sodium-based Effluent Treatment Trial for Anglo Platinum
In the Anglo Platinum Precious Metals refiners (PMR) facility in Rustenburg the final processing of PGMs to finished products is carried out. After the refining processes the acid effluents generated are neutralised to recover the residual base metals and precious metals from solution, with the barren effluent then disposed of into a storage dam. Lime is used in this neutralisation process. The solid precipitate from the neutralisation process is returned to an existing smelter for further treatment and the barren effluent from the storage dam is processed further through an evaporator to maintain an overall water balance in the PMR process. Of note, the calcium introduced during the neutralisation process contributes to excessive scaling of the evaporator, thereby reducing the capacity throughput of this unit.
As a consequence of the downtime incurred in removing the scale build-up, coupled with the high salinity of the dam liquor minimising the rate of solar evaporation, maintenance of a negative water balance is an ongoing challenge for the PMR operation. To mitigate the problems associated with effluent handling at PMR, various options for conversion of the neutralisation process from a calcium-based to a sodium-based treatment have been under investigation at Anglo Research. Based on laboratory-scale results, the use of sodium hydroxide as a substitute for lime was considered to be most beneficial. Besides reducing the scaling effect in the evaporator, the use of sodium hydroxide as a neutralising reagent presents other advantages. In contrast to the lime process, NaOH can be added in liquid form, involving no undissolved solids, thereby significantly reducing the mass of filter cake produced. This has the knock-on effect of minimising the cake washing requirements and decreasing the transportation costs of returning the filter cake to the smelter. The reagent has a high purity and no contaminants are introduced into the process via this route. Furthermore the reagent is already available on the PMR site and existing storage facilities are conveniently located. The conversion to a liquid reagent would also overcome the operational difficulties associated with the addition of the solid lime, such as blockages, currently experienced on site.
A larger scale, semi-continuous trial using this reagent was subsequently proposed and this trial was carried out at Anglo Research over a period of eight weeks in the fourth quarter of 2006. The primary objective of the trial was to generate the information necessary for sizing of the thickener and the filtering equipment to be included in the proposed circuit design and to allow for an economic evaluation of the modified process to be carried out. The campaign was also designed so as to generate technical information relating to the effects of certain process parameters, including feed composition, pH control in the process, and the concentration of the reagents to be used. During the course of the trial, operating conditions were established that provided for the adequate removal of the base metals from the acid effluent solutions during the neutralisation process, utilising sodium hydroxide. In addition, the slurry produced exhibited enhanced settling characteristics, chiefly attributed to the seeding effects of recycled slurry.The thickened slurry demonstrated good filtration characteristics and the filter cake produced had moisture and chloride contents that were deemed to be within the required specification for recycle of these solids to the existing smelter operation. Settling and filtration specialists were invited to conduct studies on the slurry produced during the trial and provided the information necessary for the sizing of the equipment required for the commercial plant operation, and the data needed for an economic evaluation of the overall proposal.
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