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Sulphate Removal Technology

As reported previously (Bratty et al, 2008), the Sulf-IX™ Process which uses ion-exchange technology was developed to remove sulphate from wastewaters and industrial process effluents to meet new environmental discharge regulations for sulphate. The process uses cationic and anionic resins to remove calcium and sulphate, respectively, and employs low cost reagents, namely sulphuric acid and lime, for resin regeneration. The products of the process are clean water, below 250 mg/L sulphate, and clean gypsum solids. A key environmental benefit of the process is that there is no spent regenerant for disposal. The process offers significant technical advantages compared with all other available technologies, including reverse osmosis, and promises to be the most cost effective method for sulphate removal.

A feature of sulphate contaminated waste water in mining is that the metal cations are nearly always balanced by the sulphate anion (SO4 2-). This is due to the fact that most metals are won from ore bodies containing minerals that contain sulphur atoms that oxidize to sulphate during the metal extraction process or due to natural oxidation processes in the waste rock and tailings. Furthermore, sulphur-containing reagents such as sulphuric acid are often added to the metal extraction process. For example, the extraction of copper by leaching with sulphuric acid is widely practiced on a very large scale in many countries. The winning of 1 kg of copper can result in the addition of up to 3 kg of sulphate or more, depending on the degree of additional sulphate-producing reactions taking place in mining wastes due to oxidation of residual sulphide minerals.

The fundamental requirement of any process to remove sulphate from water is that it has the capability to meet the regulatory limits imposed. Traditionally, metals and the associated acidity in wastewater have typically been removed by precipitation and neutralisation by adding lime, normally as calcium hydroxide. Reaction with lime removes metals by precipitation as hydroxides and also causes the removal of sulphate as gypsum dihydrate (CaSO₄.2H₂0). However, the effluent from a lime plant will always contain soluble sulphate, usually in the range 1,600 – 1,800 mg/L, due to the solubility of gypsum. Consequently, lime plants cannot produce effluents with a sulphate concentration lower than 1,600 mg/L, well in excess of current regulated values for sulphate in many jurisdictions. Figure 4 shows a simple schematic of the Sulf-IX™ Process.