Research on the production process and technology of high-quality calcium hydroxide

Calcium hydroxide, commonly known as hydrated lime, has the chemical formula Ca (OH) 2. Generally in powder form, under normal pressure, at 580 ℃, it will lose water and become calcium oxide (quicklime). Calcium hydroxide is slightly soluble in water, and its solubility decreases with increasing temperature. The colorless and transparent solution obtained by dissolving in water is commonly known as clear lime water. The emulsion suspension composed of calcium hydroxide and water is called lime milk.

1. Production process of calcium hydroxide



Raw limestone is cleaned and high-temperature calcined to obtain quicklime. Quicklime, also known as calcium oxide, has the chemical formula CaO. There are two processes for producing calcium hydroxide from quicklime: dry and wet methods.

Dry calcium hydroxide production process: Qualified quicklime is crushed by an E-type crusher. The lime is fed into the lime silo through a bucket elevator and a vibrating conveyor. The lime in the silo is quantitatively added to the hydrated lime pre digester through star shaped feeding, and is initially digested under the strong stirring of the stirring rod, and then enters the digester to complete the digestion process. The digested lime is fed into the lime silo by a lime elevator and a screw conveyor, and then qualified refined lime is obtained by an ash adding screw cyclone separator. Fine hydrated lime is unloaded into the finished hydrated lime warehouse and repackaged according to user needs. During the dry digestion reaction, the organizational structure changes, causing Ca (OH) 2 to form a loose powder with a volume increase of 1.5-2.0 times the original volume. The flowability of the product and raw materials is good, so the dry digestion process can achieve high conversion rate of quicklime under low water cement ratio (water to lime mass ratio) conditions.

Wet production process of calcium hydroxide (lime digestion method): After calcining limestone into calcium oxide, it is selected and digested with water in a certain proportion; The calcium hydroxide (ash calcium powder) material solution is purified and separated to remove slag, then centrifuged and dehydrated, dried at a higher temperature, and screened to obtain the finished product of calcium hydroxide (ash calcium powder).



The reaction process is: CaCO3 → CaO+CO2 ↑ CaO+H2O → Ca (OH) 2.



Although dry digestion technology is currently less commonly used in China, it has the advantages of low investment, short transformation cycle, simple process operation, good treatment effect, and the high activity calcium hydroxide produced has the physical properties of high specific surface area, high pore volume, high effective calcium content, and efficient acid pollutant capture ability. Therefore, choosing high activity calcium hydroxide dry desulfurization as the flue gas desulfurization process has become a focus of research and production utilization in recent years. In addition, studies have shown that calcium hydroxide prepared by dry digestion has a rich pore structure and high desulfurization activity compared to wet digestion products, which can effectively avoid water and heat losses caused by wet digestion processes, and the process is simple and environmentally friendly. At present, China's calcium hydroxide industry is developing rapidly, but compared to developed countries and regions abroad, there is still a significant gap in the development of China's calcium hydroxide industry.

2. Application of Calcium Hydroxide



(1) Flame retardant material



Calcium hydroxide powder has a wide range of applications as a filler in polymer materials. Adding calcium hydroxide to polymer materials can improve the thermal stability and flame retardancy of composite materials; Calcium hydroxide is alkaline and can react with hydrogen chloride (HCl) released during the thermal decomposition of PVC, eliminating the self catalytic effect of HCl on the degradation process of PVC and having a certain degree of thermal stability. At the same time, calcium hydroxide can generate water when heated, which can have cooling, oxygen blocking, and flame retardant effects.



(2) Degradable polymer materials



Calcium hydroxide can be used as an environmental additive for plastic digestion, with dechlorination, cracking, and promoting alkaline degradation effects on plastic decomposition. Xu Jing et al. found that due to the significant absorption of nano calcium hydroxide in the ultraviolet region, compared with ordinary LDPE film, LDPE film with added nano calcium hydroxide showed a significant increase in degradation degree after 5 days of light exposure, indicating that nano calcium hydroxide has a promoting effect on the photodegradation of LDPE film. Meanwhile, nano calcium hydroxide can also enhance and toughen LDPE, improving the mechanical properties of polymer materials.



(3) Wastewater treatment



The role of calcium hydroxide in wastewater can be summarized into four aspects: neutralizing free acids in wastewater, neutralizing acidic salts in wastewater, reacting with metal ions to produce insoluble precipitates, and adjusting the pH value of wastewater. For example, in the exploration of joint treatment methods for fluoride chemical wastewater, the combined use of calcium salts such as hydrated lime and calcium chloride, combined with inorganic coagulants and organic coagulants, can more effectively reduce fluoride ion concentration and achieve good precipitation effect.



(4) Desulfurizer



The wet desulfurization process of calcium hydroxide gypsum involves the large-scale contact of flue gas with Ca (OH) 2 absorption solution, which dissolves SO2 in water and reacts with calcium hydroxide slurry to form calcium sulfite. Under the condition of injecting a large amount of air, calcium sulfite is oxidized to form CaS (V2H2O), thereby achieving the goal of reducing SO2 in flue gas. In the calcium desulfurization process, calcium ions are actually involved in sulfur fixation, and calcium carbonate, calcium oxide, and calcium hydroxide can all be used as desulfurizers.