HYDRATED LIME

Lime is a calcium-containing inorganic mineral composed primarily of oxides, and hydroxide, usually calcium oxide and/ or calcium hydroxide. It is also the name for calcium oxide which occurs as a product of coal-seam fires and in altered limestone xenoliths in volcanic ejecta. The word lime originates with its earliest use as building mortar and has the sense of sticking or adhering.

These materials are still used in large quantities as building and engineering materials (including limestone products, cement, concrete, and mortar), as chemical feedstocks, and for sugar refining, among other uses. Lime industries and the use of many of the resulting products date from prehistoric times in both the Old World and the New World. Lime is used extensively for wastewater treatment with ferrous sulfate.

The rocks and minerals from which these materials are derived, typically limestone or chalk, are composed primarily of calcium carbonate. They may be cut, crushed, or pulverized and chemically altered. Burning (calcination) of these minerals in a lime kiln converts them into the highly caustic material burnt lime, unslaked lime or quicklime (calcium oxide) and, through subsequent addition of water, into the less caustic (but still strongly alkaline) slaked lime or hydrated lime (calcium hydroxide, Ca(OH)2), the process of which is called slaking of lime.

When the term is encountered in an agricultural context, it usually refers to agricultural lime, which today is usually crushed limestone, not a product of a lime kiln. Otherwise it most commonly means slaked lime, as the more dangerous form is usually described more specifically as quicklime or burnt lime.

In the lime industry, limestone is a general term for rocks that contain 80% or more of calcium or magnesium carbonate, including marble, chalk, oolite, and marl. Further classification is done by composition as high calcium, argillaceous (clayey), silicious, conglomerate, magnesian, dolomite, and other limestones. Uncommon sources of lime include coral, sea shells, calcite and ankerite.

Limestone is extracted from quarries or mines. Part of the extracted stone, selected according to its chemical composition and optical granulometry, is calcinated at about 1,000°C (1,830°F) in different types of lime kilns to produce quicklime according to the reaction:

Before use, quicklime is hydrated, that is combined with water, called slaking, so hydrated lime is also known as slaked lime, and is produced according to the reaction:

Dry slaking is when quicklime is slaked with just enough water to hydrate the quicklime, but remain as a powder and is referred to as hydrated lime. In wet slaking, a slight excess of water is added to hydrate the quicklime to a form referred to as lime putty.

Because lime has an adhesive property with bricks and stones, it is often used as binding material in masonry works. It is also used in whitewashing as wall coat to adhere the whitewash onto the wall.

Cycle

The lime cycle for high-calcium lime

The process by which limestone (calcium carbonate) is converted to quicklime by heating, then to slaked lime by hydration, and naturally reverts to calcium carbonate by carbonation is called the lime cycle. The conditions and compounds present during each step of the lime cycle have a strong influence of the end product, thus the complex and varied physical nature of lime products.

An example is when slaked lime (calcium hydroxide) is mixed into a thick slurry with sand and water to form mortar for building purposes. When the masonry has been laid, the slaked lime in the mortar slowly begins to react with carbon dioxide to form calcium carbonate (limestone) according to the reaction:

Ca(OH)2 + CO2 → CaCO3 + H2O.

The carbon dioxide that takes part in this reaction is principally available in the air or dissolved in rainwater so pure lime mortar will not recarbonate under water or inside a thick masonry wall.

The lime cycle for dolomitic and magnesium lime is not well understood but more complex because the magnesium compounds also slake to periclase which slake more slowly than calcium oxide and when hydrated produce several other compounds thus these limes contain inclusions of portlandite, brucite, magnesite, and other magnesium hydroxycarbonate compounds. These magnesium compounds have very limited, contradictory research which questions whether they “…may be significantly reactive with acid rain, which could lead to the formation of magnesium sulfate salts.”Magnesium sulfate salts may damage the mortar when they dry and recrystalize due to expansion of the crystals as they form which is known as sulfate attack.

Hydrated lime is a stable, dry, fine powder produced by the chemical combination of quicklime with water. This is the most concentrated form of lime.

Hydrated lime, apart from building industries, is also largely used in chemical industries as a cheapest alkali available. It is consumed in hundreds of solid liquid phase reactions. Hydrated lime requirements definitely exceed the vast tonnage of quick lime required in dry, thermal processes such as sintering, smelting and fluxing etc. Hydrated lime is used for neutralization, coagulation, canticization, dehydration, hydrolyzation and absorption. It is also used as a flux in metallurgy, as a specified lubricant, as a bonding agent, as a filler, as a raw material and also in the manufacture of refectories. The present existing manufacturing capacity in the country for the manufacture of hydrated lime is not sufficient to meet the growing demand of its dependent chemical and other industries as also in building industry. Raw materials, process equipments and the necessary know-how involved in the manufacture of hydrated lime are available indigenously.

The term lime has broad connotation and frequently is used in referring to limestone. According to precise definition lime can only be a burned form:

Quick lime and hydrated lime.

These products are oxides or hydroxides of calcium and magnesium, expect hydraulic types in which the CaO & MgO are chemically combined with impurities. The oxide is converted to a hydroxide by slaking on exothermic reaction in which the water combines chemically with the lime.

The term lime is generally used to cover all aspects of the element calcium. The term lime includes limestone (calcium carbonate – CaCO3), burnt lime (calcium oxide – CaO), and slaked or hydrated lime (calcium hydroxide – Ca(OH)2). All of these materials are used in large quantities.

Limestone is sparingly water soluble:

However, most natural water contains high levels of carbon dioxide. This increases the solublity of the limestone as follows. Firstly, the dissolved carbon dioxide reacts quite readily with water to form bicarbonate and hydronium ions:

In these acid conditions, the carbonate then reacts readily to form bicarbonate:

Thus when carbon dioxide is dissolved in the water, the following equilibrium exists:

Quick Lime
CaCO3+ heat <=============> CaO + CO2

High Calcium high calcium

lime stone quick lime.

CaCO3. MgCO3+ heat <========> CaO.MgO + 2CO2

Dolomitic Dolimitic

Lime stone quick lime

Hydrated lime
CaO + H2O <===============> Ca (OH)2 + heat

high calcium high calcium

quick lime hydrate

In most types of dolomite quick limes, when hydrated under atmospheric conditions, all the CaO component readily hydrates, but very little of the MgO slakes. The result is a solomitic monohydrate or a combination of hydroxide and oxide However when dolomitic quicklime is hydrated under pressure or is subjected to long retention periods, most of the Mgo hydrates to form a so called highly hydrated dolomitic lime.

Hydrated lime requirements definitely exceed the vast tonnage of quick lime required in dry, thermal processes such as sintering, smelting and fluxing etc. Hydrated lime is used for neutralization, coagulation, causticization, dehydration, hydrolyzation. It is also used a flux in metallurgy, as a specified lubricant as a bonding agent, as a filler, as a raw material and also in the manufacture of refractories.

The present existing manufacturing capacity in the country for the manufacture of hydrated lime is not sufficient to meet the growing demand of its dependent industries. Raw materials, process equipments and the machining involved in the manufacture of hydrated lime are available indigenously.