A process for beneficiation of low-grade manganese ore and synchronous preparation of calcium sulfate whiskers during hydrochloric acid regeneration

In order to economically utilize low-grade manganese ores in Indonesia, a process is proposed to produce high-grade manganese concentrate and prepare calcium sulfate whiskers. Low-grade manganese ore is beneficiated with hydrochloric acid as a leaching agent to obtain a manganese concentrate. Hydrochloric acid can then be regenerated using cheaper sulfuric acid via contact with the leach liquor (a CaCl2 solution), whereas calcium sulfate whiskers can also be precipitated during this regeneration. Herein, the effects of pH and reaction time on the leaching of manganese ore were evaluated, and the effects of reaction temperature, stirring speed, precipitation time, calcium ion concentration and sulfuric acid concentrate on the morphology of the calcium sulfate whiskers were experimentally investigated. The products were analyzed by means of scanning electron microscopy (SEM), thermogravimetric-differential scanning calorimetry (TG-DSC), and X-ray diffraction (XRD). The results show that control pH to 3 and leaching duration is 1.5 h, the grade of manganese ore increases from 16.4% to more than 51.6%. The CaSO4·2H2O whiskers prepared have good crystallinity, uniform morphology, high aspect ratio and good reproducibility under the optimum conditions: 50 °C, stirring speed of 250 r·min−1, precipitation time of 1 h, calcium ion initial concentration of 30 g·L−1, and sulfuric acid concentration is 30.8%. In addition, CaSO4·0.5H2O whiskers with high aspect ratio can also be prepared under atmospheric pressure. The phase transformation of CaSO4·2H2O is also studied. In low concentration hydrochloric acid (1 mol·L−1), the main product is CaSO4·2H2O. In high concentration hydrochloric acid (2.5 mol·L−1), when the reaction time is less than 2 h, CaSO4·0.5H2O occurs only when the temperature reaches 102 °C. When the reaction time is not less than 8 h, and the temperature is higher than 95 °C, CaSO4 is produced. A manganese concentrate (Mn, more than 51%) can be obtained via the regenerated hydrochloric acid leaching.