There are four items for evaluating the filtration work performance: filter cake moisture, filtrate cleanliness, filter productivity and power consumption. These four indicators are technical and economic indicators for filtering operations. The technical and economic indicators for improving the filtration operation are mainly related to the area of ​​the filter, the pressure difference between the two sides of the filter medium, the nature of the filtered slurry and the nature of the filter medium itself. The impact of these factors on the filtration process is now analyzed as follows:
(1) Filtration properties of the slurry The properties of the slurry include the concentration of the slurry, the temperature of the slurry, the particle size composition of the solid material in the slurry, and the influence of the flotation agent contained in the slurry.
The large concentration of the slurry is advantageous for filtration, and the productivity of the filter increases within a certain range as the concentration of the slurry increases. Tests have shown that when the slurry concentration is increased from 20% to 60%, the productivity of the filter will increase by 20 times and the moisture content of the filter cake will decrease by about 1/2. Therefore, in the production, the concentration of the filtered slurry should be increased as much as possible to increase the productivity of the filter. The concentration of the filtered slurry should be increased as much as possible in order to increase the productivity of the filter and reduce the moisture of the filter cake. The pulp concentration is generally required to be greater than 40%.
As the temperature of the slurry rises, the viscosity decreases, which speeds up the filtration and increases the productivity of the filter. In order to improve the filtration of fine materials, in recent years, some foreign beneficiation plant has adopted the method of high-temperature steam filtration.
The particle size of the solid material in the slurry has a great influence on the filtration. The coarse-grained material has large porosity due to its formation of the filter cake, and the filter cake has small resistance, which is favorable for the passage of the filtrate, the productivity of the filter is high, and the moisture of the filter cake is low. When the fine-grain slurry is filtered, it tends to clog the pores, which makes filtration difficult and reduces the productivity of the filter.
The particle size composition of the slurry has a great influence on the filtration. If the fine particle size is large, the filterability is poor. In order to improve the filterability, it is possible to increase the productivity of the filter by adding a coarse material to change the particle size characteristics under possible conditions.
In production practice, in order to solve the difficulty of filtering fine particles, a filter aid or a coagulant may be added to improve the filtration efficiency. The addition of a coagulant can increase the thickness of the filter cake and increase productivity, but does not reduce the moisture of the filter cake.
As for the effect of flotation agent filtration, the flotation agent tends to reduce the filterability of the slurry, but it also improves the filterability. Therefore, the effect of the agent on the filtration should be judged according to the specific situation.
(2) Properties of the filter cake The greater the porosity of the filter cake, the lower the moisture content of the filter cake. The porosity of the filter cake is related to the particle size and particle size composition of the filter material.
The resistance of the filter cake affects the filtration rate. The porosity, thickness and pulp viscosity of the filter cake determine the resistance of the filter cake. The filtration rate of the filter is inversely proportional to the resistance of the filter cake. Therefore, in the filtration operation, it is necessary to keep the thickness of each part of the filter cake layer uniform, so that the filter cake resistance distribution is uniform, which is beneficial to the filtration process.
In addition, the filter cake is prevented from cracking during filtration, which reduces the vacuum of the filter chamber and reduces the pressure difference between the two sides of the filter medium, thereby reducing the productivity of the filter. The cracking phenomenon is caused by wrinkles when the filter cake shrinks. There are many solutions, such as speeding up the filter, so that the resulting filter cake is thinner and has a lower moisture content.
The thickness of the filter cake is determined by the concentration of the slurry, the time during which the cylindrical filtration zone is submerged in the slurry (related to the rotational speed), and the degree of vacuum of the filter.
In the filtration operation, in order to achieve higher productivity, the concentration of the filtered slurry should be made larger. The longer the total filtration time (filtration cycle), the greater the immersion depth, the longer the filter cake is produced, and the thicker the filter cake, but the slower the filter cake increases. Therefore, excessively prolonging the filtration time will reduce productivity. In the production, the filtration cycle should be appropriate, and the proportion of time for each filtration stage, that is, the generation of the filter cake, the suction of the filter cake, the removal of the filter cake, and the cleaning of the filter cloth, should be reasonable.
(3) Pressure difference and vacuum Generally speaking, the finer the particle size of the material in the slurry, the greater the vacuum. However, when the degree of vacuum is too high, it will increase the consumption of electric energy and filter cloth, and even reduce the filtering effect. In production practice, refinery concentrates are filtered for fines. The degree of vacuum by the filter is 33320.5 Pa (250 mm mercury column, 1 mm Hg = 1 mm Hg = 133.322 Pa, the same below) starts. For flotation concentrates with higher mud content, the vacuum can sometimes be as high as 700 mm mercury column.
In general, the filtration rate is proportional to the pressure difference, but if plasticity containing colloidal substances (such as iron hydroxide), an increase in pressure differential will reduce the porosity of the cake, but slow down the filter. For filtration with fine pulp, increasing the pressure difference tends to block the fiber pores of the filter cloth and also reduce the filtration speed. Therefore, for the filtration of fine mud slurry, it is better to use a smaller degree of vacuum, as long as this vacuum can ensure the required speed of the filtrate and contribute to a certain water content, and can also be filtered with a thin layer of filter cake. Properly extend the filtration time.
(4) Filter media Filter media is one of the important factors affecting the filtration effect. In order to improve the productivity of the filter, the filter medium should meet the following requirements: 1 to ensure a high recovery rate of the material to be filtered; 2 less resistance to filtration; 3 meet the requirements of a certain mechanical strength, durable; 4 not easy to block, Easy to clean; 5 easy to install and disassemble, and when filtering, the filter cake is thickened and can be firmly attached, and the filter cake is easy to be removed when it is removed; 6 the cost is low.
The filter medium may be a horizontally loose bed made of a granular solid material, or a naturally occurring porous plate such as diatomaceous earth or a porous plate made of a man-made material. Generally used in the concentrator are cotton and man-made fiber cloth.
During the filtration process, clogging of the filter cloth affects productivity. The reason why the filter cloth is clogged is on the one hand that the fine solid particles mechanically block the pores of the filter cloth; on the other hand, the precipitates formed by the chemical reaction (usually carbonate compounds and sulfuric acid compounds) are accumulated on the pile of the filter cloth. When the filter cloth is severely clogged and affects productivity, clean or replace the filter cloth. The precipitation of the carbonic acid compound can be carried out by using an acid solution, usually a hydrochloric acid solution having a concentration of 1% to 2%, to dissolve the carbonate. For sulfates and metal fine particles clogging the filter cloth obtained, general cleaning by mechanical means.
It can slow down the reaction, maintain chemical balance, reduce surface tension, prevent light, thermal decomposition or oxidative decomposition. it mainly includes lead salts, metal soaps, organic tin, organic antimony, organic rare earths, pure organic compounds. Thermal stabilizers commonly used in industry mainly include lead salts, metal soaps, organic tin, organic antimony, organic rare earths, pure organic compounds, etc.
1.1 Organotin
(1) Excellent transparency The biggest advantage of organotin stabilizers is that they have excellent transparency. The use of organotin stabilizers in PVC formulations can produce crystalline products. Because of this, organic tin can be used in bottles, containers, corrugated boards, various types of rigid packaging containers, hoses, profiles, films, etc.
(2) Extraordinary thermal stability In terms of thermal stability, there is no other type of thermal stabilizer that can surpass it. Therefore, it is the preferred stabilizer for rigid PVC, and some varieties have better performance in soft products. Suitable for all PVC homopolymers, such as emulsion, suspension and bulk PVC, as well as vinyl chloride copolymers, graft polymers and polymer blends.
(3) The product is non-toxic. Most organotin stabilizers are non-toxic, and the migration of organotin stabilizers in rigid PVC is minimal. Therefore, organotin stabilizers are the preferred heat stabilizers for PVC for food contact.
(4) Good compatibility. Organotin stabilizers have good compatibility with PVC, so there is generally no precipitation on the metal surface, such as lead salt stabilizers and metal soap stabilizer systems.
(5) Poor lubricity Sulfur-containing tin stabilizers have poor self-lubricity. Therefore, many commercially available sulfur-containing organotins are equipped with lubricants to prevent hot melt from adhering to processing equipment during processing.
(6) High cost Compared with other types of stabilizers, the overall performance of organotin stabilizers is closer to the ideal stabilizer. However, all organotin stabilizers, regardless of their structure, have the main disadvantage that their manufacturing cost is much higher than that of lead stabilizers or metal soap compounds. In recent years, by adopting new synthetic technology, or reducing its usage in formulas, its formula cost has been reduced. In the 1970s, foreign countries developed low-priced tin products, which reduced the tin content, which also reduced the price to a certain extent.
1.2 Lead salt
(1) l Excellent stability experiments proved that among the commonly used basic lead salts, the heat resistance of sulfite is better than that of sulfate, and the heat resistance of sulfate is better than that of phosphite. The tribasic lead sulfate, which is widely used in the PVC industry, has a higher effective lead content and shows better thermal stability than other products.
(2) Excellent insulation. Because lead salt is non-ionic and non-conductive, it is inert, which makes lead salt stabilizers widely used in wire and cable industries.
(3) Excellent weather resistance. Many salt compounds can act as white pigments and can show strong covering power, so they have strong weather resistance.
(4) Poor transparency Transparency is a problem related to weather resistance. When used in wires, cables and record materials, there is no need to care about transparency, because most of these products are white or very dark black.
(5) Inexpensive lead salt stabilizer is the lowest price among all stabilizers. Therefore, despite the continuous introduction of new stabilizers, lead salt stabilizers still dominate the stabilizer market after half a century. The composite lead salt introduced to solve the dust and dispersion problem has increased in price, but still maintains a competitive advantage with other types of stabilizers.
(6) The toxicity of the toxic lead salt stabilizer limits its application in many occasions with strict hygiene requirements. For example, many countries have revised the lead content standards in drinking water, and it is no longer possible to use lead salt in PVC water pipes.
(7) Poor dispersibility The dispersibility of salt lead is poor, but the newly launched one-package product is equipped with lubricants, which solves the dispersibility problem to a certain extent. Because of the above characteristics of lead salt stabilizers, they are especially suitable for high-temperature processing. They are widely used in various opaque hard and soft products and cable materials, such as various pipes, plates, indoor and outdoor profiles, foam plastics, artificial leather, and wires. Cables, records, welding rods, etc. The most important lead salt stabilizers are tribasic lead sulfate, dibasic lead phosphite, and dibasic lead stearate.
1.3 Organic antimony
(1) Better stability. The organic antimony stabilizer has basically the same color stability and lower melt viscosity as organotin at the same temperature. In the twin-screw extrusion process, the effect is particularly outstanding when used in combination with calcium stearate. .
(2) The price of organic antimony stabilizer is much lower than that of methyl tin or butyl tin. In addition, the amount of organic antimony used is relatively low, so the use of organic antimony can achieve a better performance/price balance.
(3) The product is non-toxic. In the United States, when a twin-screw extruder is used to manufacture PVC pipes, the PVC water supply pipe manufactured with a formula composed of antimony stabilizers, calcium stearate and other lubricants conforms to the U.S. NSF (National Sanitation Foundation) )Regulation.
(4) Poor transparency and light stability. The transparency of organic antimony compounds is not as good as organotin stabilizers, and it is also lower than the barium/fu and calcium/zinc metal soap system. It is close to lead salts and has poor light stability, so antimony is stable. The agent is mostly used for indoor products without color requirements. The antimony stabilizer itself is also required to be stored in an opaque container.
(5) Poor lubricity. Antimony stabilizers have poor lubricity. Therefore, its use must be combined with a large amount of lubricant without exception.
1.4 Metal soap
(1) The tin soap stabilizer Fu soap is the best type of metal soap. Its advantages are also reflected in the absence of initial coloring, and can produce colorless and transparent products; excellent light stability; it has the effect of preventing precipitation and adhesion. . However, because tin salt is toxic, there are strict regulations on its manufacture and use in the Labor Safety and Health Law. In recent years, the use of Fuzao has shown a downward trend.
(2) Zinc soap stabilizerThe thermal stability of zinc-based stabilizer to PVC is extremely poor. The sample with zinc soap suddenly turns black when heated, that is, the so-called "Zine burning" phenomenon occurs, but it has the following advantages: The initial coloring is excellent; the effect of preventing fouling is good; it can improve the weather resistance; many zinc soaps are recognized as non-toxic stabilizers, so they can be used in non-toxic formulations with calcium soaps.
(3) Barium soap stabilizer Barium compounds have good thermal stability and good lubricity, but the red initial coloration occurs during processing, and it is easy to cause sticking to the roller.
(4) The stability of calcium soap stabilizer is poor, but it is recognized as a non-toxic additive in the world and has excellent lubricity.
(5) Other metal soap stabilizers used in the industry include magnesium stearate, file stearate, aluminum stearate, potassium stearate, etc., among which magnesium stearate is similar to calcium stearate and can be used Materials that are in contact with food; aluminum stearate is similar to zinc stearate, and is approved by the U.S. FDA and the Japanese Vinyl Chloride Food Hygiene Association for food packaging; Stearic Acid files and potassium stearate are also non-toxic products and belong to lead salts , Tin soap and barium soap substitute.
(6) The performance requirements of the composite metal soap stabilizer processing industry for stabilizers are various, and a single metal soap often cannot meet the use requirements, so the use of composite stabilizers has become a trend. A single metal soap compound is rarely used in the PVC industry, and it is usually a compound of several metal soaps. This compound is not a simple addition of properties, but takes advantage of the synergy between the components. The composite metal soap stabilizer generally includes the stabilizer main body (i.e. metal soap), solvent (organic solvent, plasticizer, liquid non-metal stabilizer, etc.), functional additives (auxiliary stabilizer, transparency modifier, light stabilizer, Lubricants, etc.). According to the form, it is divided into solid compound and liquid compound. According to the main component, it can be divided into calcium/zinc compound stabilizer, barium/fu compound stabilizer, barium/zinc compound stabilizer, etc. Among them, the calcium/zinc composite stabilizer has a pivotal role in replacing toxic metals because it is non-toxic.
1.5 Rare earth stabilizer
(1) Excellent thermal stability The thermal stability of rare earth stabilizers is better than traditional lead salt series and barium/zinc, barium/ho/zinc stabilizers. In some applications, rare earth stabilizers can partially or completely replace organotin.
(2) Good transparency. The refractive index of rare earth stabilizers is very close to that of PVC Resin, which can replace traditional organotin and be used in the field of products with higher transparency requirements.
(3) Excellent weather resistance. Rare earth elements can absorb 230-320nm ultraviolet light. Therefore, rare earth stabilizers have anti-photoaging effects and are suitable for outdoor products such as PVC corrugated boards and window materials.
(4) Excellent electrical insulation properties. Some rare earth multifunctional stabilizers can be used to replace lead salt stabilizers in cable material formulations, and their electrical insulation properties are comparable to lead salts.
(5) Non-toxic, safe and hygienic. Rare earth elements are low-toxic elements and have no toxic hazards to the human body in their production, processing, transportation and storage. Rare earth stabilizers are non-toxic products and can be used in food packaging and medical packaging products.
(6) The processing performance is slightly worse. In the case of a large amount of rare earth stabilizer, the roll release of the material is not ideal, and there is a tendency to press out. Generally, better results can be achieved by using stearic acid or calcium stearate in combination.
In summary, rare earth stabilizers can be used for water pipes, injection pipe fittings, window frame profiles, door panels, wire ducts, foam products, artificial leather, cable materials, soft and hard transparent products, food packaging materials, etc.
1.6 Auxiliary stabilizers Auxiliary stabilizers include phosphite, epoxy soybean oil, hindered phenols, etc., which mainly rely on the synergistic effect between metal stabilizers to improve the stabilizing effect, and are generally called co-stabilizers. In addition to compounds such as mesaminocrotonic acid ester, 2-phenyl saccharin, vein derivatives, and diketone can be used in combination with metal stabilizers to improve the effect of metal stabilizers, they also have a certain stabilizing effect. Such compounds are usually called pure organic stabilizers. With the development of PVC stabilizers today, the progress of metal stabilizers is relatively slow, and the research and development of auxiliary stabilizers are unprecedentedly active, which has constituted a major trend in the field of PvC stabilizers. Auxiliary stabilizers are rarely used alone, and are often used in conjunction with primary stabilizers to improve initial coloration or improve long-term stability.
Lead Salt Stabilizer, Compound Stabilizer,Pvc Stabilizer,Titanium Dioxide Anatase
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