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Milk of Lime Preparation

Milk of lime(MOL), also known as lime slurry or lime milk, is a suspension of calcium hydroxide Ca(OH)2 in water. It is used in the sugar industry for pH correction and as an auxiliary of flocculation in bleaching of the sugar solution. Milk of lime plays an important role in improving the quality and purity of the sugar product. Lime is an essential and inexpensive clarifying agent used in juice clarification processes such as sulphitation, and carbonation.


Preparation of Milk of Lime requires careful selection of raw materials, proper slaking process, adequate mixing and storage conditions, and regular quality control. In this blog post, we will discuss some of the factors that influence the preparation of milk of lime for sugar industries, and provide some tips and best practices to optimize the process.


Lime is available in various forms such as quicklime, limestone, and hydrated lime. The raw materials for preparing milk of lime are quicklime (CaO) and water (H2O). As Quicklime and very reactive compound, it reacts with water and produces hydrated lime. This reaction is exothermic in nature.


CaO + H2O -> Ca(OH)2 + 15.54 Kcal


The quality of quicklime affects the quality of milk of lime. Quicklime should have a high purity, low impurities, high reactivity, and appropriate particle size. Impurities such as magnesium oxide (MgO), silica (SiO2), alumina (Al2O3), iron oxide (Fe2O3), carbon dioxide (CO2), and moisture (H2O) can reduce the effectiveness of milk of lime in bleaching and increase the scaling and corrosion problems in the equipment. Reactivity refers to the ability of quicklime to react with water and form calcium hydroxide. It depends on factors such as time, and atmosphere, as well as storage conditions. Particle size affects the surface area and dissolution rate of quicklime. Smaller particles have higher surface area and faster dissolution rate, but also higher tendency to agglomerate and form lumps.


Water is the other raw material for preparing milk of lime. Water should be clean, soft, and free from organic matter and dissolved gases. Hard water contains calcium and magnesium salts that can precipitate with calcium hydroxide and form insoluble compounds that reduce the efficiency of milk of lime. Organic matter can interfere with the bleaching process and cause color formation in the sugar solution. Dissolved gases such as carbon dioxide and oxygen can react with calcium hydroxide and form carbonates and hydroxides that lower the pH and solubility of milk of lime.


Slaking Process


The slaking process is the reaction of quicklime with water to form calcium hydroxide. It is a critical step in preparing milk of lime, as it determines the final properties and performance of the product. The slaking process involves three main stages: hydration, dispersion, and maturation.


Hydration is the initial stage where quicklime reacts with water and forms calcium hydroxide. It is an exothermic reaction that releases a large amount of heat (15.54 Kcal per mole of CaO). The heat can raise the temperature of the slurry up to 100°C or more, depending on the amount of water added. The ratio of water to quicklime is an important parameter that affects the hydration stage. The theoretical ratio is 0.32 kg of water per kg of CaO, which corresponds to 1.32 kg of Ca(OH)2 per kg of CaO. However, this ratio is too low to ensure a complete reaction and a good dispersion of calcium hydroxide particles. A higher ratio is usually preferred, ranging from 0.5 to 1 kg of water per kg of CaO, depending on the type and quality of quicklime.


Dispersion is the stage where calcium hydroxide particles are separated from each other and distributed evenly in the slurry. It is essential to avoid agglomeration and settling of particles, which can reduce the reactivity and solubility of milk of lime. Dispersion can be achieved by mechanical stirring or by adding dispersing agents such as sodium hexametaphosphate (SHMP) or sodium polyacrylate (SPA). Mechanical stirring should be vigorous enough to create turbulence and shear forces that break up the lumps and prevent sedimentation. Dispersing agents are chemicals that adsorb on the surface of calcium hydroxide particles and increase their repulsion forces, thus preventing them.
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