Study on high purity sand washing plant

1.Application Status of high purity silica sand 

High purity silica sand refers to silica sand with SiO 2 content higher than 99.9%, which is chemically stable and insoluble except HF.Other acids, slightly soluble in KOH solution.It is difficult to purify silica sand because of its various impurities, complex occurrence and different distribution. The earliest raw material for producing high purity silica sand is natural crystal, but with the gradual depletion of natural crystal resources, it has become an inevitable trend to extract high purity silica from ordinary silica sand. High purity silica sand is widely used in high-tech industry. In the semiconductor industry, wafers with excellent thermal properties at high temperatures can be made from molten silica o produce high-tech heat pipes with good thermal stability at high temperatures. In the communications industry, silica glass made from silica sand is the basic material for optical fibers and other photoelectric devices. In addition, it can also be used to produce silicone resin, solar cell substrates, and has greater demand in abrasives, fillers, coatings and silicone rubber industries .

2.Pretreatment

2.1.Physical methods

Physical methods are mainly to remove impurities in silica sand by water washing-grading desliming, scrubbing( attrition scrubber as usual), magnetic separation and flotation ( flotation machine as usual).

Several processes can be combined to further remove impurities.

2.1.1 Washing-Grading Desliming and Scrubbing

In silica sand beneficiation process, the fine grain size less than 0.1 mm is usually called slime. For the silica sand with a large amount of clay and slime, the grade of silica decreases gradually with the grain size becoming finer, while the impurities such as iron and aluminium increase. The quality of the product can be effectively improved by water washing and grading desliming.

For the thin film iron on the surface of quartz sand and the adhesiveness impurities, the effect of water washing-grading desliming is poor, which requires the aid of machine.

The abrasive force between the mechanical force and the sand grains is used to remove the film iron and adhesion impurities adapted to the sand surface, and the better effect of removing impurities can be achieved by grading desliming.

2.1.2 Reconcentration, Magnetic Separation and Flotation

Gravity separation is based on the difference of relative density, particle size and shape between mineral particles and their velocity in water, air or heavy liquid.

Beneficiation methods for separating different minerals due to different rates and directions.

Magnetic separation can remove the weakness of hematite, limonite and biotite in quartz sand grains to the greatest extent.

Magnetic impurities and strong magnetic minerals mainly consisting of magnetite, for weak magnetic impurities minerals more than 10000Oe of strong magnetic machine, for strong magnetic impurities minerals commonly used weak magnetic machine or medium magnetic machine.

2.2 Chemical Method

The main chemical methods for purifying silica sand are acid leaching or alkali leaching, because of its high leaching efficiency, it is also the most commonly used method. Relative

The cost of chemical method is higher than that of physical method, but the purification effect is better, especially when the purity of silica sand is higher. Sulfuric acid, hydrochloric acid, nitric acid and hydrofluoric acid are commonly used in acid leaching. Dilute acid has good removal effect on iron, aluminium, magnesium and other impurities, while concentrated sulfuric acid and hydrofluoric acid have good removal effect on titanium and chromium impurities. Therefore, in actual mineral processing, mixed acid has the best purification effect on quartz sand. Alkali leaching mainly uses alkaline solutions such as sodium hydroxide or sodium carbonate to dissolve aluminosilicate impurities, so as to achieve the purpose of purification.

2.3 Other methods

Ultrasound iron removal method. The surface of natural silica sand particles is easily polluted by solution containing iron impurities, which forms iron on the surface.The thin film causes the problem that the iron content of natural quartz sand is too high to remove.Ultrasound is a kind of high frequency vibration, the general frequency is more than 20KHz. When it is emitted in solution, it will produce a lot of compression and expansion areas, forming and breaking numerous microbubbles，That is the phenomenon of cavitation.In the process of cavitation, the pressure inside the liquid changes abruptly and produces shock waves. The pressure can reach tens of thousands or even tens of thousands of atmospheric pressure. Under this force, the iron film on the surface of the particles will fall off, thus achieving the purpose of removing impurities such as iron.

Oxalic acid leaching.Oxalic acid belongs to weak acid, which has less environmental pollution, less corrosion to equipment and obvious iron removal effect.

The reaction mechanism is that oxalic acid reacts with iron and other impurities on the surface of silica sand particles to form several complexes, thus separating from the surface of particles to achieve the purpose of separation and purification.

3. Joint process for preparation of high purity silica sand

For high purity silica sand, the content of iron impurity should not be higher than 0.03% and that of aluminium impurity should not be higher than 0.03% except that the content of SiO 2 should not be lower than 99.9%. Because silica ores in different areas have different properties, impurities and inlay sizes, it is necessary to adopt different combined processes according to the ore properties of silica sand in order to remove impurities and obtain high purity silica sand in further purification and preparation of pretreated silica sand.

In addition, conventional pickling methods such as HF acid or acid pickling under heating conditions can remove most of the impurities on the surface of silica  while it is difficult to remove the impurities encapsulated in the interior of silica sand.The high temperature chlorination calcination method is to heat silica to 1000-1300 。C, and inject the mixture of chlorine and hydrogen chloride, which can effectively remove alkali metals and alkaline soil in gas-liquid inclusions.The purity of silica sand chlorinated at high temperature can reach over 99.99% for impurities such as metals and transition metals. The chemical reaction formula is: MexOy+0.5xzCl2_xMeCl2+0.5yO2.