2017年11月20日星期一

Ten Factors Affecting the Ammonium Paratungstate Crystallization(2)

Ammonium paratungstate (APT) is an important tungsten intermediate product, not only in the purity and process requirements, as well as crystal grain size and crystal distribution requirements.

Seemingly negligible some of the objective factors often play a key role in the final product often For these reasons the difference between the least, lost thousands of miles.

The production workshop of para-ammonium tungstate image

6. The impurities in the solution: As the solution P, As, Mo, Si and other impurities easily adsorbed on the crystal growth of the active point, hinder the crystal growth, but also makes the ammonium paratungstate grain thinning, make the quality of deterioration, and easy to enter the product during the evaporation crystallization process, affecting product quality, and even lead to product failure, so in the crystallization process, should take the control of crystallization rate, improve the concentration of NH4Cl in ammonium tungstate solution, stirring and washing agents and other methods, to achieve the goal of removing impurity.

7. pH value: The nucleation rate and crystal growth rate affected by the pH value, since raw crystal nucleus of pH control in 7.5-8.0 range, is beneficial to reduce the number of spontaneous nucleation and the formation rate, small amount of crystal nucleus can grow smoothly into larger particles; And made a narrow pH range is beneficial to uniform distribution of coarse grain of ammonium paratungstate, only the uniform distribution of particles can be evenly Karl fischer granularity, this performance is particularly obvious in neutralization.

8. Ammonium tungstate solution added: The end of the nucleation period after the addition of ammonium tungstate solution, can not change the particle size distribution conditions, to improve the crystallization of ammonium tungstate ammonium particle size and bulk density, but also to overcome the initial high tungsten concentration , the product size of the smaller defects in the higher equipment capacity, produce the good fluidity of the crude ammonium paraffin.

9. The end of crystallization of the liquor density: Evaporation of the end of the crystallization of the liquor density of ammonium paraquat have a certain impact. Under normal circumstances, the lower end of the crystallization of the liquor density, the higher the crystallization rate, the easier the formation of coarse particles, because the crystallization of a long time can lead to crystal nuclei have sufficient time to grow up; the other hand, the crystallization of the end of the liquor density are larger, the easier the formation of smaller particles. Therefore, on the premise of guarantee the quality of products, can reduce the liquid evaporation at the end of the corresponding density, increase the crystallization rate accordingly.

10. Vacuum degree: With the speed up of the evaporation rate and vacuum degree is reduced, and the increase of the crystal elutriation rate, resulting in a slight increase in the number of microcrystalline precipitation, and the particle size decreases.

In conclusion, the preparation of ammonium paratungstate (APT) is a systematic project with a lot of side details. Each factor has different effects on the preparation of ammonium paratungstate crystals. In most cases, in order to produce a certain physical properties of ammonium paratungstate products, it needs comprehensive control for the various factors.


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Ten Factors Affecting the Ammonium Paratungstate Crystallization(1)

Ammonium paratungstate (APT) is an important tungsten intermediate product, mainly white crystals, there are two kinds of flakes or needle-like, used in the manufacture of tungsten trioxide or blue tungsten oxide tungsten powder.

The quality of tungsten products depends largely on the chemical purity of APT, particle size and particle size distribution.

Crystallization of Ammonium Paratungstate image

APT quality standards from the domestic manufacturers, the process and chemical purity are almost the same, but the production of APT products vary widely, this is because APT is a system engineering, not only the purity and process requirements, as well as crystal grain size and crystal distribution requirements, seemingly insignificant on some of the objective factors often play a key role in the final product is often because of these reasons the difference between the least, lost thousands of miles.

There are lots of influence factors of ammonium tungstate, mainly include: the temperature of the solution, the degree of supersaturation, stirring speed, add seed, impurities, pH value, adding ammonium tungstate solution, crystallization time and crystallization liquor density, vacuum degree, etc.

1. The influence of temperature: The temperature is important factors influencing the particle size of ammonium paratungstate.When the solution too full degrees must be under low temperature, rate of crystal nucleus grow up slowly, and nucleation rate faster, so generally get fine grain;And crystallization under high temperature, due to the crystal nucleus to grow up quickly, and the crystal nucleus formation showed relatively slow, so generally get a coarse crystals.

2. The degree of supersaturation: Degree of supersaturation is the most important factors influencing the APT granularity.Because of the formation of crystal nucleus needs a minimum energy, the activation energy, in a certain temperature, the greater the degree of supersaturation, the smaller activation energy is, the faster speed of the formation of crystal nucleus. However, in the production, the production capacity is too small, the production capacity is too small; the degree of supersaturation is too large and the grain size is small, and the crystal quality is poor. In addition, due to the concentration of ammonium tungstate solution and other solution components Of the content are different, the supersaturation of the solution also has a direct impact, therefore, in order to determine the appropriate supersaturation is very important.

3. Evaporation rate and neutralization speed: Adopt evaporation crystallization or neutralize the legal APT, evaporation rate and had a great influence on the granularity of APT and speed, the greater the evaporation speed and neutralization, at the beginning of the crystal form of the greater the degree of supersaturation, the number of crystal nucleus formation, the resulting APT particle size is fine, apparent density is small.


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Ammonium Paratungstate Crystallization Mother Liquor Treatment--Conversion Method

From dressing industrial progress of history suggests that mineral processing reagents application significance is self-evident. 

Since its discovery in 1924 xanthate has the function of collecting sulfide ore, the development of the flotation industry started by leaps and bounds, the inundated mineral processing reagents has been invented and created with a steady stream.

Ammonium Paratungstate Crystallization Mother Liquor Treatment image

The crystallization crystallization method and the neutralization method produce the crystallization mother liquor in the preparation of APT. These mother liquor contain not only impurities but also some WO3.

In order to improve the total recovery of tungsten, the mother liquor is often recovered once again in the industry treatment, as to which method should be based on the previous process and the composition of the mother liquor to use, usually available alternative methods are conversion method, solution extraction and ion exchange method.

Conversion method is commonly used in the crystallization process of ammonium paratungstate crystallization mother liquor treatment, it is added to the crystallization of the mother liquor NaOH, turn tungsten into Na2WO4 solution, and then according to the mainstream process to determine the craft.

(NH4)2WO4+2NaOH==Na2WO4+2H2O+2NH3

The presence of tungsten in the mother liquor with polyacid and heteropoly acid can be converted to sodium tungstate, the solution after heating need to boil, volatilize the ammonia.

The main process for acid decomposition--ammonia crystallization process, the crystallization of the mother liquor by alkali conversion, should be concentrated to WO3 content of 150g / L, and then by the classical chemical method for precipitation, precipitation of artificial white tungsten, and then the artificial white tungsten and scheelite mixed into the mainstream craft for processing.

Mainstream program for classic precipitation purification craft or extraction craft, the crystal mother liquor after alkali into the crude sodium tungstate solution, can be added directly after purification in the recovery of tungsten.

Main process of ion exchange craft, and its crystallization mother liquor after transformation, no longer suitable for the original ion exchange craft, it needs to be deposited artificial scheelite classic precipitation or extraction.

In APT crystal mother liquor processing technology is more in our country at present, have their own characteristics, but cannot leave the essence of removing impurity and recycling. Reforming process is suitable for the classic craft, but with the improvement of tungsten smelting technology, classic technology has been gradually withdraw from the historical stage, so the process is more like a role of relay station.


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Wolframite and Scheelite Mixed Tungsten Ore Flotation (2)

Wolframite and scheelite mixed tungsten ore flotation is a worldwide difficult problem, because of the black and white tungsten composition structure is very different, conventional flotation method at room temperature and heating method is not ideal. Would there have a more effective way to realize whether black and white tungsten mixed flotation?

black and white mixed tungsten ore flotation image

Chairman MAO once said: “There is a way, it just hasn’t been found.” He also said: “Paths are made by men.” And actually it does. After many years of industry practice, our country's scholars found that the focus on black and white tungsten study is mixed flotation collector, the flotation is strong while the collector is strong, scholars use the collector as the breakthrough point, make many light avenues.

In 70s and 90s, Chinese scholars use 731 + 733 combination of black and white mixed tungsten ore flotation collector, 731 and 733 to mix ratio of 10:1, the collector has the advantage of source is easy to obtain, the disadvantage is that not good flotation wolframite, scheelite flotation recovery of 0.010 below also is not high, metal erosion is more serious, and the production of rough concentrate grade is low.

In the 90s, mining and metallurgy research institute of Beijing through a lot of experimental research, found a natural pH pulp black tungsten method under the condition of flotation, this term is known as the CF method. Reagent regime of the method is: use a small amount of sodium silicate do adjusting agent, lead nitrate as activator, with CF (the nitrobenzene hydroxylamine ammonium salt) as collector, a tiny amount of emulsified oil and oleic acid as foaming agent, flotation pulp pH value of 7-9 black and white tungsten mixed pulp.

To CF with lime and caustic soda method of small test comparison, the results showed that the index of CF method obviously is excellent, concentrate grade slightly higher, the recovery is 10% higher than the other two methods.

In recent years, Guangzhou non-ferrous metal research institute in the same points under the guidance of the principle of heterogeneous select the high efficient tungsten sequestration collector benzene armour hydroxyl oxime acid, it is an effective collector of fine wolframite. Use the benzene armour hydroxyl oxime acid and fatty acid such as collector, oxidized paraffin soap oxidized ore collector combination, in both strong selectivity of benzene armour hydroxyl oxime acid at the same time, also improve the collecting ability of the agent, this kind of combination is called the GY series reagents. The arrival of GY combination reagent, makes the fine grained recycling effectively, and from now on, black and white mixed tungsten ore flotation no longer have to struggle, it really reduced the cost, to achieve the efficiency of mining. Fujian one tungsten mines with GY series reagents recycle mixed black and white tungsten ore, eventually made the mines getting out of the woods, made up the deficits and get surpluses, became the pillar of the economy.


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Wolframite and Scheelite Mixed Tungsten Ore Flotation (1)

Wolframite and scheelite are different with the same name, there are all “w” but their body structure are not the same, so their beneficiation method is different, black tungsten gravity commonly used the separation method, scheelite commonly used flotation process.

Black and White Mixed Tungsten Ore Flotation image

After years of development, our country each has a solid industrial foundation of beneficiation technology in wolframite and scheelite tungsten ore flotation. From a single scheelite recovery of scheelite in bed or recovery of wolframite from single black tungsten deposit is relatively easy. But why there is a certain type of skarn deposits made mistake like, they try to make the black and white tungsten together under the same roof, so the deposit is called "black and white mixed tungsten deposit.

Mixed black and white tungsten ore flotation is a worldwide difficult problem, in this type of ore deposits, the black and white tungsten are in fine grained disseminated separately symbiosis in the strata, although there are all tungsten, they have always not buying it from one another, they go their own way. The difficulty of black and white mixed tungsten ore mixed is that the inclusion of black tungsten, In Jiangxi quartzite mines, black tungsten can be solved with gravity separation, magnetic separation, but when it comes to in the skarn deposits, it also learn pee-a-boo from scheelite, escapes from the gravity separation method. Of course the most embarrassing thing is that the suspect relationship, in the process of flotation of wolframite and scheelite natural flotability difference is bigger, often the flotability of scheelite is superior to the black tungsten, tungsten oxide ore collector line of collecting ability is stronger, collecting of wolframite ability is weaker; In addition, wolframite and scheelite the best flotation pH interval does not agree, the best flotation pH interval of scheelite is relatively narrow, between 7-9.

Wolframite the best flotation pH interval often swings between 5-10. And wolframite it brittle than flotation technology, only after a minor in the coarse grinding, into the coarse grain of spiral classifier wolframite precipitation will be priority major, with the sand return to the mill grinding again, until the grain will overflow into the sorting operation. It caused most of the classification system of wolframite granularity are belong to fine grain or even micro-fine particle, general flotation reagents are difficult to catch.

In the past, wolframite and scheelite separation method of mixed flotation concentrate with room temperature and heating method of two kinds: normal temperature method is in a large amount of sodium silicate for rough concentrate (60 ~ 90 k young) under normal temperature stirring, to curb the fluorite and calcite, quartz and other gangue minerals, and most of wolframite is restrained, white tungsten concentrate were obtained through the multiple selection, room temperature method is more difficult to obtain high grade tungsten concentrate containing WO365 % above, and the dosage of sodium silicate is more large consumption. Heating method only need to add a relatively small amounts of sodium silicate, and then heated to 90 ℃ or so, thermal agitation for an hour, this process makes the gangue minerals and wolframite surface adsorption of collector stripping, and strong inhibition by sodium silicate, and scheelite collector can still on the surface of the solid adsorption, thus containing WO365 % above the high grade can be obtained by flotation of scheelite concentrate, cleaner tailings need to recycle of black tungsten, the traditional process is: the black tungsten content is less, usually with magnetic separation enrichment,and shaking table selection for black tungsten concentrate, the black tungsten content is higher, can be directly with the table selected black tungsten concentration, The process of the disadvantage is the microgranular wolframite erosion, recycle like go through the motions, is better than nothing, only so many of the early black and white tungsten mixed mining waste is serious, the efficiency is low. There would have a more effective way to realize whether black and white tungsten mixed flotation?


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Combination Collector Mechanism--Chelating Effect

In the flotation system, two or more collectors are used in combination, and by combining the collector and producing synergistic effects, such tungsten ore flotation collector is known as combination collector in practice. It has been proved that many combination collector has better and more effective flotation results than the use of a single collector, and there are divergent views on the causes.

For the study of the mechanism of the combination collector, there are scholars think that the combination collector also has a significant chelating effect, it refers to the combination collector and mineral surface of the metal between the combination collector metal's special affinity caused by the real chemical reaction. The result of this reaction is the formation of insoluble, stable metal chelate. 



The scholars of South China University used the quantum chemistry ab initio algorithm to calculate the molecular model of benzohydroxamic acid and benzyloxamic acid. The benzohydroxamic acid-based mixed collector BH was used to study the tungsten ore in Shizhuyuan polymetallic ore Test and in the wolframite and scheelite tungsten mixed ore flotation research made a breakthrough. The experimental results show that the BH combination collector has a good catching effect on scheelite and wolframite. The main reason is the chelating effect of BH combination because the benzohydroxamic acid molecule is a planar molecule and benzene Methionine is a nonplanar molecule, both of which can coexist and that the methacryloxamic acid is more stable than benzohydroxamic acid. When the benzohydroxamic acid and benzyloxamic acid and mineral surface of the metal ions chelate, and metal ions easily formed O, O five ring chelate, rather than O, N quaternary ring chelate. Compared with acetohydroxamic acid (hydroxamic acid), benzohydroxamic acid (benzyloxamic acid) and mineral electrostatic effect becomes smaller, the ability to match the key to reduce the ability to accept the formation of electronic feedback bond, Hydroxamic acid (benzyloxamic acid) selectivity increased.


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