Focus on Separation Technologies

01 Introduction: Bottlenecks and Breakthroughs in Comprehensive Dolomite Utilization

Dolomite is considered a double salt composed of magnesite and calcite, with the main chemical component CaMg(CO₃)₂. Its theoretical composition (ω/%) is 21.7% MgO, 30.4% CaO, and 47.90% CO₂, often associated with impurities such as quartz and feldspar [1]. Pure dolomite is white, while iron-bearing varieties appear gray or dark brown, with weathered surfaces turning brown.

Currently, the utilization of magnesium from dolomite across various industries is relatively well-developed, while the development and utilization of calcium remain insufficient. Calcium is often used to produce low-value-added building materials or fillers, resulting in significant waste of calcium resources. Therefore, while ensuring the supply of raw materials for bulk metal industries such as steelmaking and magnesium smelting, fully utilizing both calcium and magnesium resources to develop high-purity, high-value-added calcium and magnesium products has become a research hotspot in the deep processing and comprehensive development of dolomite mineral resources [3].

The key to fully utilizing calcium and magnesium resources in dolomite lies in the efficient separation of calcium and magnesium, as well as the effective removal of impurity components.

02 Overview of Major Calcium-Magnesium Separation Technologies

Current technologies for calcium-magnesium separation and impurity removal from dolomite mainly include the carbonization method, acid dissolution method, ammonium leaching method, brine-dolomite method, and complexation leaching method [3].

2.1 Carbonization Method

The carbonization method is the most commonly used industrial approach due to its simple process, low cost, and ease of industrial implementation. This technology relies on the difference in solubility between CaCO₃ and Mg(HCO₃)₂ in aqueous solutions, achieving calcium-magnesium separation by controlling the endpoint pH of the carbonization process [3].

The process flow is as follows: dolomite is calcined and digested to produce dolomite lime milk, which is then reacted with purified CO₂ from kiln gas. Under controlled process conditions, CaCO₃ precipitate and Mg(HCO₃)₂ heavy magnesium water are formed. After solid-liquid separation, lightweight CaCO₃ is obtained. The filtrate, heavy magnesium water, is thermally decomposed to produce a basic magnesium carbonate intermediate, which is then calcined to produce MgO [3].

The carbonization method offers the advantages of a simple process and low production cost. However, this method relies solely on process control during carbonization to achieve separation, making precise control difficult in actual production. Consequently, the resulting calcium and magnesium products often exhibit relatively low purity [3]. To address this, various improved technologies have been developed, including batch recovery, secondary carbonization, pressurized carbonization, and additive-assisted carbonization.

2.2 Ammonium Leaching Method

The ammonium leaching method uses the weak acidity of ammonium salt solutions ((NH₄)₂SO₄, NH₄Cl, NH₄NO₃) to react with digested dolomite lime, producing solutions of calcium and magnesium salts. Depending on requirements, NH₃ or CO₂ is then introduced to obtain the corresponding calcium and magnesium products [5]. This method involves mild reactions, is easy to operate, effectively separates calcium and magnesium from dolomite, and yields high-purity products.

2.3 Brine-Dolomite Method

The brine-dolomite method involves adding digested dolomite lime dropwise into brine (MgCl₂) to produce magnesium hydroxide. After filtration and drying, Mg(OH)₂ powder is obtained, while the filtrate is further processed to prepare CaCO₃ [5]. The advantages of this method include low process cost and minimal pollution. It effectively utilizes magnesium resources from both brine and dolomite lime, achieving relatively complete separation of calcium and magnesium. However, a significant drawback is the generation of large quantities of calcium chloride solution as a byproduct, which is difficult to handle [5].

03 Research Progress in Separation Technologies

3.1 Improvements in Carbonization Method Separation

Wang Wenze et al. [6] prepared lightweight CaCO₃ using a phase-transfer carbonization method with calcined dolomite powder as raw material. Through single-factor and orthogonal experiments, the optimized phase-transfer conditions were determined: liquid-solid ratio of 20 mL/g, n(ammonium citrate):n(CaO) = 4:3, reaction temperature of 20°C, and reaction time of 10 min. The optimized carbonization conditions were: endpoint pH of 7.6, CO₂ flow rate of 0.6 L/min, reaction temperature of 65°C, and stirring speed of 550 r/min. Under these conditions, calcium carbonate with a purity of 98.18% was produced, exhibiting uniform particle size and good dispersibility. After two cycles of calcium-magnesium separation from the calcined dolomite powder, "calcium free of magnesium" was essentially achieved, significantly improving the purity of the calcium product.

Wang Xin et al. [11] investigated the extraction of calcium from calcined dolomite powder using a citric acid-ammonium solution. The extraction rates for Ca²⁺ and Mg²⁺ reached 99.34% and 6.11%, respectively. Carbonization of the calcium citrate yielded a calcite-type lightweight CaCO₃ with ω(CaCO₃) = 98.2% and a spindle-like morphology. The filter cake, after subsequent processing, produced an MgO sample with ω(MgO) = 99.2% and a short rod-like shape.

3.2 Progress in Ammonium Leaching Separation

Fan Yuanyang et al. [5] used dolomite lime and a recycled ammonia solution as raw materials to prepare calcium carbonate whiskers and magnesium hydroxide through a cyclic process of ammonia distillation, magnesium precipitation, and calcium precipitation. In cyclic experiments, an optimal Ca/Mg to ammonium salt molar ratio of 1:2 was identified, achieving extraction rates of 91.32% for Ca²⁺ and 90.95% for Mg²⁺. The study confirmed that three cycles were optimal for preparing calcium carbonate whiskers, with the product achieving a calcium carbonate content of 98%.

Deng Xiaoyang et al. [7] used lightly calcined dolomite powder, ammonium chloride, and carbon dioxide as raw materials to prepare well-shaped, uniformly distributed cubic-like calcium carbonate crystals with an average particle size of 5–10 μm via an ammonia distillation and calcium precipitation process without the use of crystal morphology control agents.

Jia Xiaohui et al. [8] proposed a two-step separation method using an ammonium chloride solution, first extracting calcium followed by magnesium. The calcium extraction rate from dolomite exceeded 95%.

Wang Dongyi et al. [10] prepared magnesium hydroxide and calcium carbonate whiskers through a process of calcination, ammonia distillation, and precipitation. The prepared calcium carbonate whisker product achieved an aspect ratio of 20, a whiteness of 98.7, and a Ca²⁺ conversion rate of 80.75%, with material recycling throughout the entire process.

04 Conclusion

Current technologies for calcium-magnesium separation from dolomite are advancing rapidly. The carbonization method, ammonium leaching method, and brine-dolomite method each offer distinct advantages and limitations. The carbonization method features mature technology and low cost but faces challenges in purity control. The ammonium leaching method provides superior separation efficiency and high product purity but involves relatively complex processes. The brine-dolomite method enables synergistic utilization of magnesium resources but struggles with the disposal of byproduct calcium chloride. Under laboratory conditions, calcium extraction rates approach 99%, essentially achieving "calcium free of magnesium" and laying a solid foundation for the subsequent preparation of high-purity calcium carbonate.