With the social and economic development, industrial waste burning coal releases sulfur dioxide, carbon dioxide, sulfur fuel release compounds, nitrogen compounds and mining, metallurgy, printing and dyeing, chemical, pharmaceutical and other industries rely on human emissions of pollution of the environment to survive Increasingly, how to effectively deal with these pollution factors to reduce the huge losses they bring to humanity has become one of the most important global issues that need urgent solution.

Based on the need for environmental protection, the treatment of these pollutions must use chemical products with the following characteristics: non-toxic, mild, non-corrosive treatment equipment, cheap and easy to obtain, low processing cost, high efficiency, strong ability, easy to operate, and easy Recycling or comprehensive utilization, without causing secondary pollution.

Magnesium hydroxide slurry state is consistent with one of the best features of all of the above raw materials, which are mainly used in liquid inorganic bases environmental protection products, active, a large specific surface area, adsorption capacity, cushioning, and Neutralizing ability, non-precipitating, good fluidity, convenient to use and adjust, mild, safe, non-toxic, harmless, less corrosive, easy to operate, easy to recycle by-products or comprehensive utilization, etc. Type “Green Safe Neutralizer”, which is applied to the neutralization of acidic wastewater and the removal of heavy metal ions (Ni ²⁺ , Mn ²⁺ , Cd ²⁺ , Cu ²⁺ , Cr ³⁺ , Cr ^{6+ ,} etc.) in waste liquid. Flue gas desulfurization, printing and dyeing waste liquid treatment and other environmental protection fields, with unparalleled superiority of other alkaline substances (calcium oxide, calcium hydroxide, sodium hydroxide, sodium carbonate, etc.), used in acid industrial wastewater, sulfur-containing smoke The use of some strong alkaline substances in the field of gas treatment, such as lime, caustic soda, soda ash, etc., is gradually limited and replaced by the rise of weak base magnesium hydroxide.

Due to the many advantages of slurry magnesium hydroxide in the field of environmental protection, in the late 1990s, the production and application of foreign slurry magnesium hydroxide materials developed rapidly; although China has abundant magnesium resources, magnesium hydroxide Production and application have not attracted enough attention, mainly in the research and development stage. In recent years, although some pilot or production facilities have been built in China, the scale is small, the variety is small, the product quality is low, and the technical level is low. It is urgent to improve the overall level of the industry.

First, the main production method of the existing slurry magnesium hydroxide

According to the use and morphology of magnesium hydroxide, it can be divided into powder, filter cake and slurry. The purity of the slurry magnesium hydroxide used in the environmental protection field is not very high, generally about 30%, mainly requiring no serious impurities such as heavy metals, and the production method is relatively simple, mainly including crude magnesium oxide ( Magnesium sand, crude industrial magnesium oxide, etc.) hydration method, seawater or brine-alkaline substance (ammonia, lime, calcium hydroxide, sodium hydroxide, etc.) precipitation method.

The magnesia hydration method is a very old production process, mainly in which the lightly burned magnesia powder obtained by light burning of magnesite is placed in a reaction tank containing hot water, stirred while being added, and the heat sinking is performed after the feeding is completed. After about 2 hours, the solid-liquid separation and dehydration were carried out to obtain a cake-like and slurry-like magnesium hydroxide. This process basically does not have the function of removing impurities. The quality of the product is affected by the purity and activity of the raw material magnesium oxide. The impurities in the magnesium oxide are basically brought into the product except for the trace soluble salts. Therefore, only low-grade hydrogen can be produced. Magnesium oxide.

Seawater or brine-alkaline substances (ammonia, lime, calcium hydroxide, sodium hydroxide, etc.) are precipitated by simply purifying seawater or brine, adding an alkaline precipitant to produce magnesium hydroxide precipitate, filtering and washing. Dehydration to obtain filter cake and slurry magnesium hydroxide. Although the principle is simple, the volatility of ammonia is strong, it is easy to pollute the environment, and the operation is difficult; lime and calcium hydroxide are easy to form calcium sulfate, which precipitates with magnesium hydroxide, resulting in high impurity content and poor quality; sodium hydroxide is strong The alkali easily causes the formed magnesium hydroxide to form a colloidal precipitate, which brings difficulties to the performance control of the product, and at the same time easily brings in more Na ⁺ and Cl ^- and other impurities, and also causes high impurity content of the product, and the purity is difficult to be guaranteed.

Second, seawater, brine - light burnt dolomite precipitation method

Magnesium hydroxide has unparalleled superiority in other environmentally friendly fields. It has been widely produced and widely used in foreign countries. However, China's magnesium hydroxide production method is backward, cost is high, and impurities are high. The quality is poor, and there are very few applications in the field of environmental protection. In view of this, we have developed a seawater, brine-light burnt dolomite precipitation method for the production of environmentally friendly magnesium hydroxide.

The method belongs to a kind of precipitation method. The seawater, the brine and the light burned dolomite are used as raw materials, and the magnesium hydroxide is controlled by a one-step synthesis process of controlled crystallization, which overcomes the disadvantages of the previous production methods, and has high purity and low impurity content. ,stable quality.

(1) Basic principles

The hydrated dolomite is hydrated to form calcined dolomite milk containing calcium hydroxide and magnesium hydroxide, and the calcined calcium hydroxide in the dolomite milk and the raw material seawater and the magnesium ion in the brine are continuously synthesized and separated into an integrated reactor. The reaction takes place to form magnesium hydroxide. The process adopts a self-developed continuous synthesis and separation integrated reactor, which always maintains a certain amount of seed crystals in the reactor, simplifies the traditional seed crystal addition process, and generates magnesium hydroxide and impurities in the reactor. The separation is carried out effectively, the magnesium hydroxide complete solution is sedimented, washed, separated and dehydrated to obtain filter cake-like magnesium hydroxide, the filter cake is added with water, and the dispersant is added in proportion to avoid agglomeration of magnesium hydroxide. The slurry magnesium hydroxide with different concentration and stable performance is obtained, and the reaction equation is:

(2) Process flow (see Figure 1)

Figure 1 Process flow chart of seawater, brine-light burnt dolomite precipitation

First, using a new treatment technology different from that mentioned by Han Lihua, the impurities in the raw material water affecting the quality of the product are removed, and the purified raw material water is obtained. The light burned dolomite is added with an appropriate amount of purified raw material water to be hydrated and digested, and the water is added to meet the synthesis requirements. Light burnt dolomite milk.

Then, the purified raw material water and the light burned dolomite milk are proportionally injected into the continuous synthesis and separation integrated reactor with stirring, and the reaction time and the reaction end point are controlled to make the two fully contact and completely react. Due to the difference in the sedimentation rate of magnesium hydroxide and the insoluble larger particle size impurities, the insoluble larger particle size impurities are first precipitated to the bottom of the reactor and discharged from the bottom of the reactor. The complete solution rich in magnesium hydroxide enters the primary settler from the upper part of the reactor for solid-liquid separation, and the solid phase is washed by purified water to remove most of the soluble impurities and then enters the secondary precipitator for secondary solid-liquid separation. Dehydration to obtain a cake-like product, the filter cake is added with water, and the dispersant is added in proportion to avoid agglomeration of magnesium hydroxide, thereby producing slurry magnesium hydroxide of different concentrations and stable performance.

(3) Product quality

The technical indicators of magnesium hydroxide are various, but the slurry-like and filter cake-like magnesium hydroxide used in the environmental protection field has not seen special quality standards in China. In order to meet the needs of users, foreign manufacturers have regard to slurry and filter cake. The magnesium hydroxide products have established corporate standards, as shown in Table 1.

Table 1 Foreign slurry, filter cake magnesium hydroxide enterprise standard

The main indicators of magnesium hydroxide produced by this process are: Mg(OH) ₂ 30%～35%, CaO 0.5%～0.6%, Cl ^- ≤0.1%, although the content of impurity calcium oxide is slightly higher than the quality of Japanese and American products. Indicators, but far below the Swiss quality indicators. And the quality of magnesium hydroxide is sufficient to meet the quality requirements of environmental protection in wastewater treatment, flue gas desulfurization, etc. We will further improve the process on this basis, improve product quality, to meet the needs of more industries with higher use requirements.

(four) process characteristics

The main raw materials of the process are seawater, brine and light burnt dolomite, which are widely available and inexpensive.

The process reaction is carried out at normal temperature, and the whole process does not require pressurization and heating, and the production is energy-saving and low in cost.

The process has no toxic, harmful and corrosive materials input and output, and there are no special requirements for the production equipment. The main equipments are filter presses, ordinary industrial pumps, carbon steel tanks such as reactors and settlers, and the equipment investment is low. ,easy to use.

In the process, the pretreatment of the raw material water effectively reduces the impurity content in the product, and the product quality is obviously superior to the domestic similar process products, and the requirement of producing high quality magnesium hydroxide by the precipitation method is achieved.

In this process, the development and application of continuous synthesis and separation integrated reactors effectively control the crystallization of the product, retaining a sufficient amount of seed crystals in the reactor, avoiding the addition of the seed crystals, achieving continuous synthesis, and realizing The effective separation of the target product and impurities, the product quality is less than the traditional method of production of impurities, high quality.

Third, the conclusion

Pollution is causing enormous harm to human beings and causing huge losses to the economy. As for the sulfur dioxide emitted by China, the acid rain caused by it has caused losses of about 110 billion yuan per year in China's economy. Environmental management is urgently needed.

China's treatment processes in acid wastewater neutralization, heavy metal ion removal and flue gas desulfurization are relatively backward, complicated in operation, high in raw material consumption, high in operating cost, and incomplete treatment, and by-products cause secondary pollution.

With the implementation of China's sustainable development strategy, the accession of the WTO, the enhancement of environmental awareness and the gradual improvement and improvement of environmental laws and regulations, new technologies and processes applied in the field of environmental protection have also received increasing attention. More and more efficient, non-toxic, high-quality new or alternative products are gaining more and more attention.

China's seawater, brine resources, dolomite, magnesite, brucite and other magnesium-rich resources are quite rich, should make full use of the existing resource advantages, improve existing backward processes, research and development of new processes, and vigorously develop a variety of types of hydroxide Magnesium products, and improve the quality and added value of products, reduce production costs, to meet the growing demand for environmentally-friendly and other industries, the increasing demand for magnesium hydroxide quality and increasing demand, to promote healthy and rapid economic development.

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