2018年4月27日星期五

Preferential Flotation Process and Mixed Flotation Process

Deal with mineral stones containing two or more useful minerals, the single concentrate is sequentially selected from useful  minerals. This process is called preferential flotation.

Deal with mineral stones containing two or more useful minerals, first, mix up and emerge useful minerals, then, gradually separated them. This process is called mixed flotation.

The preferential flotation process has high flexibility. It can adapt to the change of mineral stone grade. But it requires a higher content of the main mineral stones. There should not be too many associated mixed minerals. 

The advantage of the mixed flotation process is that a large amount of gangue can be discarded after flotation. It reduces the amount of minerals that goes into follow-up procedure and cut down the subsequent cost of the action. But mixed flotation also has a painful problem, after flotation, the concentrate surface is sticky with collecting agent. Surplus chemicals are also retained in the mineral pulp. How to solve the problem of chemicals removal is the key to the success of mixed flotation.

flotation process image

Which one should we pick? Preferential flotation process or mixed flotation process? Please see the following strategies:

The preferential flotation process suits for the wolframite. Because in most of the wolframite stones, disseminated wolframite is the largest, the mineral content is the highest. 

The mixed flotation process suits for scheelite. In nature, scheelite not only accompanies with minerals such as calcite, fluorite and molybdenum, but also mixes with phosphorus, arsenic and fluorine, etc. Therefore, the most common way is to make flotation of useful mineral stones, and then prepare a special renovation.

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【Know Tungsten】Ammonium Metatungstate Applications




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Blue Tungsten Oxide Preparation -- APT Autoreduction Method

Blue tungsten oxide is one of the important raw materials for making tungsten powder. As an important intermediate of tungsten in modern industry, it has been popular for many years. 

The commom methods of preparing blue tungsten oxide are APT closed calcination method and hydrogen mild reduction method etc. Although they have been popularized for many years, they have more or less some environmental safety factors. As a result, people have never abandoned the quest for a more effective and environmentally friendly method for producing blue tungsten oxide.

APT autoreduction method is an upgraded version of the APT hydrogen mild reduction method. What's different between them is that the APT autoreduction method does not add hydrogen or ammonia into the reacting furnace, instead, the heating temperature is increased to 600°C -800°C. APT decomposes hydrogen under high temperature. Hydrogen processes autoreduction with crackate of ammonium paratungstate under high temperature, and gets blue tungsten oxide that meets the requirement. 

blue tungsten oxide preparing image

Some scholars used rotary calciner to test APT hydrogen autoreduction. Through the improvement of process technology and the optimization of process parameters, the following conclusions are obtained, with the increase of temperature, the degree of crystalline water and ammonia out of ammonium paratungstate intensifies. Hydrogen decomposed by ammonia also intensifies under high temperature. The hydrogen atmosphere is increased, the reduction degree of hydrogen to ammonium metatungstate was strengthened. When the temperature reaches 600°C~800°C, rotary furnace speeds at 0.3~2 r/min, delivery rate at 150~300kg/h, and then gets blue tungsten oxide that meets the requirement. 

Compared with the traditional methods for preparing blue tungsten oxide, APT autoreduction has three advantages. First, It can reduce ammonia emissions and reduce environmental pollution factors. Second, ammonia and hydrogen are dangerous gases, and there are potential safety problems in the process of production, the APT autoreduction method is less risky. Third, ammonia and hydrogen as auxiliary materials need cost. Autoreduction can eliminate the expenses. Therefore, APT autoreduction method not only has good environmental benefits, but also achieves better economic benefits.

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Tungsten Powder

Tungsten powder is the main raw material for processing powder metallurgy tungsten and tungsten alloy. 

In the extraction and smelting of tungsten, the production of metal tungsten powder is a key step in the production of tungsten and tungsten alloy because the properties of tungsten powder largely affect the subsequent operation of tungsten products.

Pure tungsten powder can be made into wires, bars, tubes, plates and other processed materials and products of a certain shape. Tungsten powder and other metal powder, can be made into all kinds of tungsten alloy, such as tungsten molybdenum alloys, tungsten rhenium alloy, tungsten copper alloy and high density tungsten alloy. Another important application of tungsten powder is to make tungsten carbide powder, and then prepare carbide tools, such as turning tools, milling cutters, drills and dies, etc.

tungsten powder image

Hydrogen tungsten reduction and carbon reduction are the main methods for the production of tungsten powder. The main production method of tungsten metal powder is the reduction of high purity tungsten oxide, blue tungsten oxide and purple tungsten oxide hydrogen, mainly used in the production of tungsten carbide, In addition, tungsten acid is also used for hydrogen reduction.

The equipment for producing tungsten powder is usually a multiple tube furnace, rotary furnace. Reduction furnace usually has several heating zone, with total hydrogen or ammonia flow in a tube furnace tube, the temperature is generally between 600 and 1200 degrees. In order to ensure the full weight of tungsten powder, the gas in the furnace tube will be excessive, and the excess gas will be recycled by the hydrogen purification system.

Tungsten powder not only has the requirement of chemical purity, but also has the requirement of physical property and technological performance. In physics, tungsten powder has the requirements of particle size, particle size distribution, particle shape and particle accumulation state, while flowability, compaction density and sintering properties are the requirements of tungsten powder.

The quality of tungsten powder is a guarantee of the superior properties of tungsten and tungsten alloys. According to the statistics, about 80% of the tungsten products in the world are produced by powder metallurgy. Therefore, the quality of most tungsten products depends on the quality of tungsten powder.

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Ultrafine Tungsten Powder

Tungsten powder is the main raw material for processing powder metallurgy tungsten products and tungsten alloys.

With the rapid development of science and technology and extensive and deep research on nanometer technology, ultrafine tungsten powder has been developed and applied widely.

Ultrafine tungsten powder in the field of materials, through nanoparticles and a variety of ultra-fine structural modules, led to the emergence of many new materials with excellent properties and new applications. Ultrafine tungsten powder is superior to common coarse grain material in mechanical, physical and chemical properties because of its ultrafine grain and large amount of atoms on the grain boundary.

Ultrafine tungsten powder can be divided into three grades according to particle size: submicron tungsten powder (<1 um), ultrafine tungsten powder (<0.5 um) and nano tungsten powder (<100 nm). The most widely used tungsten powder is nanometer grade tungsten powder.

ultrafine tungsten powder image

Ultrafine tungsten powder can be used for high proportion alloy, green bullets, alloy steel, drill, hammer and other products; high activity nanometer tungsten powder can be used as the additives of powder with high performance and high proportion alloy; also can be used for preparing nano cemented carbide. Tungsten is a kind of semiconductor material with excellent gas-sensing characteristics, sensitive to a variety of gases. Nano tungsten powder has a larger specific surface area, surface effect, have strong capability to absorb electromagnetic wave, can be used as excellent solar absorption materials and stealth materials. In addition there is a catalytic performance of nano tungsten powder special.

As ultrafine tungsten powder can be used more widely, from the beginning of 1950s, both home and abroad on the tungsten smelting industry carried out a lot of research work on the preparation of ultrafine tungsten powder for many years of systematic research, some effective methods have been developed, such as hydrogen reduction-carbonization, high energy ball milling, gas evaporation, plasma, self propagating high temperature reduction, spray drying and fluidized bed method.

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Blue Tungsten Oxide Preparation -- APT Closed Calcination Method

Blue Tungsten Oxide is one of the important raw materials for making tungsten powder.

Compared with yellow tungsten, blue tungsten oxide is considered to be an upgraded version of yellow tungsten oxide because of its good activity, large surface area, easy to dope and maintain original crystal shape. And it’s widely used.

There are three main methods for making blue tungsten, APT closed calcination method, APT hydrogen light reduction method, intrinsic reduction method. APT closed calcination is one of the most widely used methods.

blue tungsten oxide preparing image

APT closed calcination method is the most used to produce blue tungsten oxide production. The characteristics of APT closed calcination method is: use thermal decomposition of ammonia or a small amount of ammonia into the reducing agent, thermal decomposition temperature fluctuations in the range of 550 °C - 700 °C,  high yield, stable quality, which is composed of 70%W20O58 blue tungsten oxide, 30WO3%.

APT closed calcination method its essence is in the heating process, always keep the furnace system with positive pressure, so that the air can not enter the furnace, APT produced by the decomposition of ammonia can be split into hydrogen and nitrogen, the furnace has weak reductive atmosphere, the final product is blue tungsten.

APT transformed into TBO by closed calcination has roughly four stages.

1. When 220 °C, generating amorphous or amorphous bronze APT.

2. When 300 °C, generating ammonium tungsten bronze.

3. When 420 °C, generating yellow tungsten.

4. When 480 °C - 600 °C, generating blue tungsten.

The main equipment of rotary calcination furnace APT closed calcination method, the size of the rotary kiln was decided according to production requirements, general furnace length 6 - 8 meters, 300 - 450 mm in diameter, made of stainless steel welded.

In calcination, the factors affecting the final quality of blue tungsten are temperature, time, specific surface area, etc. The most important factor would be temperature.

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