An integrated limestone/lime process for partial sulphate removal

摘要

This paper will focus on two topics, namely (1) an innovative process for the neutralization and partial sulphate removal of acid streams produced during coal mining and processing and (2) optimization of this integrated neutralization process by adding small amounts of synthetic organic polymers. The Integrated lime and limestone process has been developed to neutralize acid mine water and to remove sulphate (to less than 1200 mg/l), magnesium and metals. Limestone and lime treatment is the most cost-effective technology for neutralization and partial sulphate removal of acidic/sulphate-rich water to sulphate levels of less than 1500 mg/l due to precipitation of magnesium and removal of the associated sulphate fraction (through gypsum crystallization). Neutralized mine water of this quality may be suitable for irrigation. The process consists of the following stages: 1. Limestone (CaCO_3) neutralization to raise the pH to 7 and CO_2-production. 2. Lime (Ca (OH)_2) treatment to pH 12 for Mg(OH)_2 precipitation and gypsum (CaSO_4 centre dot 2H_2O) crystallization. 3. pH adjustment with CO_2 recovered from stage 1 and CaCO_3 precipitation. Retention times of 1 hour, 4 hours and (1/2) an hour were required by stages 1, 2, and 3 respectively. The sulphate level was reduced to 1094 mg/l, which is less than the original aim of 1200 mg/l Chemical costs associated with neutralization, using limestone instead of lime, are reduced by 69 percent in the integrated limestone/lime process. The overflow water from the clarifier of each of the above stages generally contains a wide variety of colloidal impurities that may cause the water to appear turbid or may impart colour. This results in very slow sludge settling rates that cause overflow water with a high degree of turbidity and colour. There are a number of successive or simultaneous stages involved in the agglomeration of particles. To get optimum neutralisation of the acid water and partial sulphate removal to below the saturation level of gypsum (i.e. 1500 mg/l), maximum sludge recovery is needed. The higher the concentration of seed crystals, the larger the surface area. Therefore, effective removal of suspended and colloidal matter from the overflow of each stage is required and can be achieved by coagulation and flocculation. The polymers PAC6 and 3095 were effectively used as coagulant and flocculant respectively. The addition of these polymers resulted in a clear overflow in each of the above stages with a very low turbidity.