Dolomite

Dolomite

This entry is compiled and applied by "popular science China" Science Encyclopedia

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Dolomite is a kind of sedimentary carbonate rock. It is mainly composed of dolomite, often mixed with quartz, feldspar, calcite and clay minerals. It is grayish white, brittle, hard, and easy to scratch with iron. In case of dilute hydrochloric acid, it foams slowly or not, and its appearance is very similar to limestone. According to the genesis, it can be divided into primary dolomite, diagenetic dolomite and epigenetic dolomite; according to the structure, it can be divided into crystalline dolomite, residual dissimilated particle dolomite, clastic dolomite and microcrystalline dolomite. Dolomite has high magnesium content and forms white stone powder after weathering. It is stronger than limestone. In metallurgical industry, it can be used as flux and refractory, in chemical industry, it can be used to produce calcium magnesium phosphate fertilizer, granular fertilizer, etc. In addition, it is also used as ceramic, glass ingredients and building stone. In addition, Zhangzhou, Fujian Province is famous for its scenic spot of dolomite.

interpretation

Dolomite is a kind of sedimentary carbonate rock. It is mainly composed of dolomite, often mixed with quartz, feldspar, calcite and clay minerals. It is grayish white, brittle, hard, and easy to scratch with iron. In case of dilute hydrochloric acid, it foams slowly or not, and its appearance is very similar to limestone. According to the genesis, it can be divided into primary dolomite, diagenetic dolomite and epigenetic dolomite; according to the structure, it can be divided into crystalline dolomite, residual dissimilated particle dolomite, clastic dolomite and microcrystalline dolomite.

nature

Dolomite is often mixed with calcite, clay minerals, magnesite, gypsum and other impurities. The ore is generally of fine or medium grain structure, with layered, massive, breccia or gravel structure. Dolomite belongs to the cubic crystal system, the crystal is usually saddle shaped rhombohedron, and the aggregate is usually granular or massive. The color is colorless, white or light brown to dark brown. Glass luster.. When calcined to 700-900 ℃, carbon dioxide is lost and becomes a mixture of calcium oxide and magnesium oxide, which is called caustic dolomite; when calcined to 1500 ℃, magnesium oxide turns into periclase and calcium oxide turns into crystalline α - Cao. Dolomite is one of the minerals widely distributed in sedimentary rocks.

features

Main components

Rocks mainly composed of dolomite (more than 50%) but often mixed with calcite and clay minerals. CaMg (CO < sub3 < / sub) < sub2 < / sub (dolomite), containing 30.41% Cao, 21.86% MgO and 47.73% CO < sub2 < / sub, often contains isomorphic mixture of iron and manganese. It belongs to carbonate rock, looks like limestone, and is slightly different from limestone by foaming with hydrochloric acid. The main components are calcium, magnesium and silicon. Dolomite is often light yellow, light yellowish gray, gray white, grayish brown, light flesh red, etc., with grain structure, residual structure, clastic structure or biological structure.

Appearance features

The appearance of dolomite is very similar to that of limestone, but there are dolomite powder and crisscross cleavage ditches on the weathering surface.

This is the most important feature of dolomite in the field.

Common categories

① Micritic dolomite: it is composed of micritic dolomite less than 0.005 mm, with uniform structure, micro bedding, few biological residues, sometimes ostracod fossils, mostly primary dolomite.

② Micro fine crystal dolomite: the crystal size is different, the crystal shape is quite good, and the appearance is similar to granulated sugar. In the field, it can be called granulated sugar dolomite, which is often recrystallized from other types of dolomite.

③ Algal dolostone: similar to algal limestone, that is, dolostone composed of algal fossils. Most of the dolostones in Proterozoic and Sinian strata in China belong to this type, which may be primary dolostone.

④ Biological dolomite and bioclastic dolomite: the fossil remains can be seen, which are mostly formed by dolomitization and metasomatism of bioclastic limestone.

⑤ Intraclastic dolostone: according to the clasts in it

The size can be divided into gravel, sand and silty dolomite. They are often found in the general dolomite in the form of interlayer. It is formed in the upper part of shallow water or in the environment above the intertidal zone, and its debris is formed by the impact of waves or currents.

⑥ Oolitic dolomite: This is a kind of secondary dolomite, which is formed by dolomitization of Oolitic Limestone.

Dolomite is an ideal reservoir for oil or groundwater because of its large porosity.

origin

According to the formation conditions, dolomite can be divided into three types

Primary dolomite: Dolomite deposited in situ is formed by evaporation in a dry and hot climate (28 ℃ ~ 35 ℃). It is formed in salinized lagoons or bays with high salinity, shallow water (0-3m deep tidal zone) and pH value higher than 8.3. It can also be formed in terrestrial saline lakes and is often accompanied by gypsum salt layer.

Diagenetic dolomite: in the process of calcium carbonate precipitation, it is metasomatized by dolomite, and its distribution is usually discontinuous. It is lenticular or patchy in limestone, and sometimes layered, extending for a certain distance.

Secondary dolomite: or epigenetic dolomite, distribution is limited, common in fault structural zone.

application

Dolomite is widely used in building materials, ceramics, welding, rubber, paper, plastic and other industries. In addition, it has been applied in agriculture, environmental protection, energy saving, medicine and health care.

Utilization in metallurgical industry

Dolomite is mainly used in metallurgical industry as flux, refractory, magnesium and magnesium compounds.

As a flux, dolomite can neutralize the acid slag, increase the basicity of the slag, reduce the activity of FeO in the slag, and reduce the erosion of the slag on the lining. Light burned dolomite is mainly used in steel-making, which can improve the fluidity of steel slag and be used as slag forming agent. It can not only prolong the life of converter and improve the fluidity of slag, but also improve the desulfurization and dephosphorization reaction, and save a lot of fluorite. 170 kg dolomite is needed to produce 1 T steel in China. Adding dolomite in ironmaking can dilute slag, reduce slag melting point, reduce fuel consumption and improve pig iron quality.

As refractory, dolomite is mainly used for lining of steel-making Martin furnace and torma rotary furnace, as well as for bottom and slope of basic open hearth furnace and as make-up material in smelting process. Dolomite is often used as a special charge in smelting high-quality ferromanganese alloy and ferrosilicon alloy. At the same time, it is a new method in recent years to add appropriate amount of light burnt dolomite or raw dolomite in the slag making process of steelmaking to replace part of quicklime, so as to inhibit the dissolution of MgO in the lining into the slag. The addition of dolomite can promote the early slag melting and make the MgO in slag reach saturation state, so as to improve the viscosity of slag and have good adhesion to the furnace lining, so as to protect the furnace lining.

Magnesium and magnesium compounds are extracted from dolomite and produced by electrolysis or high temperature metallurgy. In the electrolysis method, magnesium hydroxide is precipitated by calcining dolomite and seawater, and then converted into magnesium oxide by reaction with hydrochloric acid. Finally, the dried magnesium chloride, sodium chloride and calcium chloride are melted in an external heating electrolysis cell. In high temperature metallurgy, dolomite and ferrosilicon are made into pellets, which are melted at 1100 ℃ in a small diameter vacuum furnace to form calcium silicate slag, and magnesium is turned into steam, which is then recovered by coagulation method. Magnesium oxide and magnesium hydroxide can be produced by the same method. Magnesium oxide is a good refractory, while magnesium hydroxide is used to produce a series of other useful magnesium compounds and as plastic filler. Dolomite is also used to produce magnesium carbonate abroad.

Utilization in building materials industry

Dolomite can be processed into dolomite ash after proper calcination. It has white, strong adhesion, solidifying force, good fire resistance and heat insulation performance, and is suitable for interior and exterior wall coatings. After calcining dolomite, it can be used as magnesium chloride cement and magnesium sulfide cement. Because dolomite has good compressive strength, flexural strength, fire resistance and insect resistance, it can play a good waterproof role after adding other fillers, so it can be used as floor material with low price. Dolomite powder can be used for crack treatment, pavement paving and cement mortar sintering slag. Adding 40% dolomite to cement can accelerate the hydration rate of cement. Dolomite can also be directly used as building materials. In addition, dolomite is used as cement, glass and ceramic ingredients in the construction industry, which can increase the strength and luster of glass. It is reported that foreign Magnesium Oxide dolomite has been semi calcined and made into inorganic foam and silicate brick.

Utilization in chemical industry

Dolomite is mainly used in the production of magnesium sulfate, light magnesium carbonate and other chemical raw materials.

Magnesium sulfate (MgSO < sub4 < / sub × 7h < sub2 < / Subo) is obtained by mixing 30% dilute sulfuric acid and dolomite in a certain proportion, reacting, separating and concentrating, precipitating calcium sulfate under the condition of temperature rise, and cooling and crystallization of magnesium sulfate solution.

Mg (OH) < sub2 < / sub was extracted from seawater. When calcined dolomite was used as precipitant, MgO in dolomite was recovered at the same time, which increased the yield.

Dolomite is calcined, digested, carbonized, filtered and separated to obtain heavy magnesium water, which is then heated to decompose and filtered to obtain light magnesium carbonate. Light magnesium carbonate is decomposed into light magnesium oxide, which can be used to produce high purity magnesia.

In agriculture

Chinese PinYin : Bai Yun Yan