First, the basic knowledge of 
    magnesium hydroxide, the chemical formula Mg (OH) 2, the formula amount of 58.32. White amorphous powder. The name is caustic soda, light burned magnesia, etc., the suspension of magnesium hydroxide in water is called magnesium hydroxide emulsion, referred to as magnesium milk, the English name is Magnesium hydroxide. Magnesium hydroxide is a colorless hexagonal column crystal or white powder, hardly soluble in water and alcohol, soluble in dilute acid and ammonium salt solution, and the aqueous solution is weakly alkaline. The solubility in water is small, but the water-soluble fraction is completely ionized. The concentration of the saturated aqueous solution was 1.9 mg/liter (18 ° C). Heating to 350 ° C loses water to form magnesium oxide. Natural mineral brucite of magnesium hydroxide. Can be used in sugar and magnesium oxide. Because magnesium hydroxide is abundant in nature and its chemical properties are similar to aluminum, users have begun to use magnesium hydroxide instead of aluminum chloride for fragrance products. Used as an analytical reagent, it is also used in the pharmaceutical industry. 
    Magnesium hydroxide is a medium-strong base (the solubility of magnesium hydroxide is very small, the solution is very alkaline, sometimes treated as a weak base), and heated to 623K (350 ° C), ie dehydration decomposition: Mg (OH) 2 →MgO+H2O, soluble in acid or ammonium salt solution. As with magnesium oxide, it absorbs carbon dioxide in the air and gradually forms a basic carbonate having a composition of 5MgO·4CO2·xH2O. Decomposes to magnesium oxide and water at temperatures above 350 ° C, but only at temperatures above 1800 ° C can be completely dehydrated. 
    Second, the classification is     
    divided into light and heavy. 
    III. Preparation method 
    1. Brine-lime method The lime water prepared by the purification and refining brine and the lime treated by digesting and slag is precipitated in the sedimentation tank. The obtained slurry is added with a flocculating agent, and after fully mixing, it is separated into a settling tank for separation, and then filtered, washed, dried, and pulverized to obtain a finished magnesium hydroxide. The reaction equation is as follows:
    2. The brine-ammonia method is a raw material that removes impurities such as sulfate, carbon dioxide, and a small amount of boron by purification, and uses ammonia water as a precipitant to carry out a precipitation reaction in a reaction vessel, and a certain amount of seed crystals are supplied before the reaction. Stir. The ratio of brine to ammonia is 1: (0.9 to 0.93) and the temperature is controlled at 40 °C. After the reaction is completed, a flocculant is added, and the precipitate is filtered, washed, dried, and pulverized to obtain a finished magnesium hydroxide. The reaction equation is as follows: 
The test method needs to improve the yield, shorten the washing cycle, and improve and perfect the production process. 
    3. Magnesite-hydrochloric acid-ammonia method Magnesite ore and anthracite or coke are calcined in a shaft kiln to form magnesium oxide and carbon dioxide. The bitter soil powder is adjusted to a slurry and then reacted with a predetermined concentration of hydrochloric acid to prepare a magnesium chloride solution. The magnesium chloride solution is reacted with a certain concentration of ammonia water in a reactor, and the product is washed, sedimented, separated by filtration, dried, and pulverized to obtain a magnesium hydroxide product. A surface treatment agent may be added for surface treatment as needed. 
    4. The magnesium hydroxide of the microcrystals 
    was prepared by preparing a saturated solution of MgCl2·6H2O at room temperature, and an excessive amount of concentrated aqueous ammonia was introduced under stirring at 70 ° C, and magnesium hydroxide was gradually precipitated. 
    Note 1: The distilled water and ammonia water used should not contain CO2. The distilled water should be fully boiled. The CO2 in NH3 should be removed by KOH. 
    Device for preparing magnesium hydroxide Note 2: The reaction vessel should be made of hard glass to avoid the reaction of Mg(OH)2 with silicic acid. 
    Fourth, the application of 
    one or three functional flame retardants

    Magnesium hydroxide is currently recognized as an excellent flame retardant with three functions of flame retardant, smoke suppression and filling in the rubber and plastics industry. Its molecular formula is Mg(OH)2, its molecular weight is 58.32, its density is 2.39g/cm 3 (25 °C ), and its bulk density is 0.15-0.40g/cm 3 (related to particle size). It is white powder, has the characteristics of non-toxic, odorless, good stability, non-volatile, high decomposition temperature, non-corrosive equipment, etc. It is the first choice material for organic polymer to achieve halogen-free flame retardant.

    Magnesium hydroxide is a new type of filled flame retardant. Its flame retardant mechanism is: releasing the bound water during the thermal decomposition process, absorbing a large amount of latent heat, thereby reducing the surface temperature of the filled synthetic material in the flame. It has the effect of inhibiting decomposition of the polymer and cooling the combustible gas produced.

    2. Halogen-free is the general trend. Magnesium hydroxide is a halogen-free and inorganic-added flame retardant. It is a kind of inorganic-added flame retardant which is better in comprehensive function and technical economic evaluation. attention. In view of the serious lessons of many major foreign fires and the needs of environment, safety and other aspects, the halogen-free flame retardant has become a common problem faced by the worldwide flame retardant industry. Halogen-free is the general trend. During the period of the Eighth Five-Year Plan of China, the development of halogen-free flame retardants for cables was included in the Torch Plan and the relevant ministries and commissions were issued, but so far there is no commercial production. In 1994, the "Code for Design of Power Engineering Cables" (GB50217-94) stipulated that halogen-free flame-retardant cables should be selected in coal-fired, fuel-fired systems and other flammable environments; the Ministry of Coal (MT386-95) standard stipulates that underground coal mines Halogen-free flame-retardant cables are specified for safety problems such as fire prevention and smoke suppression. In some new large-scale projects in China, such as Shanghai Pudong International Airport Co., Ltd., in the cable procurement tenders, the use of halogen-free flame-retardant cables is strictly regulated; the underground cables used in Shanghai Metro Line 3 are all halogen-free flame-retardant cables. Among the halogen-free flame retardant materials used, magnesium hydroxide is the preferred variety in terms of superior performance, wide application, cost-effective, environmentally friendly, etc., whether used alone or in combination (with ATH). .

    Third, the advantages compared with ATH 
In addition to the halogen-free characteristics, the initial decomposition temperature of magnesium hydroxide is 340 ° C ~ 490 ° C decomposition complete, the heat absorption is 187cal / g, about 17% higher than ATH, the temperature is 140 ° C higher, The smoke suppression ability is better than ATH, the hardness is lower than ATH, and it has good flame retardant and smoke elimination effect, and is especially suitable for processing various engineering plastics such as polyolefin (PO) with higher temperature. When magnesium hydroxide is used for PE and PP, its flame retardant effect is higher than that of ATH. This is because magnesium hydroxide has a certain carbonization effect on the polymer in addition to dehydration reaction during combustion decomposition, forming a protective layer and acting as a resistance. Burning effect. In the case of the same filling amount, magnesium hydroxide and ATH can be used together to complement each other, and the flame retardant and smoke eliminating performance is better than the single effect. The PP magnesium hydroxide is a highly effective smoke-reducing filler. All of these have further promoted the application of magnesium hydroxide flame retardants in different types of polymers. 
Like ATH, domestic magnesium resources are abundant, magnesite is 3.1 billion tons, dolomite is 1.4 billion tons, brucite is 27Mt, seawater salt industry by-product magnesium chloride brine 15 million m 3 , magnesium sulfate 1000?kt, Chaer Khan Salt lake magnesium chloride 2.7 billion tons, Yuncheng salt lake magnesium sulfate 10500kt and so on. These resources provide a solid raw material basis for the development and production of magnesium hydroxide. At the same time, in the process of research and development of magnesium hydroxide, according to the principle of advanced technology, economical rationality and environmental acceptability, different alternative routes are proposed.

    In the fire, more than 80% of the victims were not burned, but died of suffocation. The addition of magnesium hydroxide flame retardant can greatly reduce the smoke density during combustion and reduce the risk factor of sudden death. Secondly, the polymer material will have an insulating effect after the addition of magnesium hydroxide. Once burned, magnesium hydroxide forms a thin film of magnesium oxide. This layer of magnesium oxide film is resistant to high temperatures, prevents further burning, and isolates air from other flammable materials, increasing the chances of survival in the event of a fire. 
Magnesium hydroxide is an excellent flame retardant for plastics and rubber products. As a flue gas desulfurizer in environmental protection, it can replace caustic soda and lime as a neutralizing agent for acid-containing wastewater. Magnesium hydroxide flame retardant is colorless, non-toxic, odorless, non-corrosive and other properties, and is a green environmentally friendly polymer material additive. 
Used in the electronics industry, pharmaceuticals, sugar refining, as insulation materials and in the manufacture of other magnesium salt products. 
The pharmaceutical industry does. antacids, osmotic laxatives. 
As an important chemical product, magnesium hydroxide micro-nano materials have been widely used in ceramics, flame retardant, catalysis, medicine and environmental protection.