Silica Fume is often used as cement or concrete admixture in the construction industry, which can fill the pores between cement particles and effectively improve its compressive and water retention properties. The main component of Silica Fume is silicon dioxide, but also contains iron, aluminum and other oxide impurities, in order to increase the content of silicon dioxide in Silica Fume, it is necessary to purify it, so what are the Silica Fume purification methods?
Acid leaching method (acid purification)
Acid purification refers to the use of silica insoluble in acid characteristics, the use of acid leaching or acid washing and other methods to dissolve the metal oxide impurities in the Silica Fume to achieve the purpose of purification.
Acid leaching method mainly removes metal oxide impurities in Micro Silica Fume, specifically by using acid to react with the metal oxide impurities in Micro Silica Fume under certain reaction conditions. In the acid leaching process, the main chemical equation that occurs is as follows.
Fe2O3+6HCl→2FeCl3+3H2O
Al2O3+6HCl→2AlCl3+3H2O
MgO+2HCl→MgCl2+H2O
CaO+2HCl→CaCl2+H2O
Na2O+2HCl→2NaCl+H2O
K2O+2HCl→2KCl+H2O
The orthogonal experiments done on the conditions of purifying Micro Silica Fume by hydrochloric acid showed that the purity of the obtained sample silica reached 92.79% with a particle size of 96.6 nm and a specific surface area of 23.81 m2/g at a solid-liquid ratio (g/mL) of 3:200, adding hydrochloric acid with a mass fraction of 20% and stirring the reaction at 300 r/min for 3 hours, and the particle size of the purified sample was more uniform. The study on the removal of metal oxide impurities in Micro Silica Fume by acid leaching process showed that the removal rate of calcium metal impurity by nitric acid was as high as 93.82%, and the removal rate of magnesium metal impurity by sulfuric acid was 70.38%, but the removal effect of nitric acid and sulfuric acid on potassium and calcium was poor, while the removal effect of hydrochloric acid on potassium, calcium and magnesium was in the middle, so it was more suitable to choose hydrochloric acid as the acid leaching solvent.
The process of acid purification is highly operational and can improve the quality of Silica Fume to a large extent, but the large amount of acid used will cause serious pollution to the environment, and the method will also improve the acid resistance of the process equipment, corrosion resistance and other performance requirements, which will indirectly increase the operating costs of enterprises and reduce economic efficiency.
High-temperature calcination method
High-temperature calcination method is the Silica Fume for high-temperature calcination, so that carbon, sulfur, ash, volatile matter and other non-metallic oxide impurities in the form of gaseous state for removal, where the ash and volatile matter are mostly Silica Fume in the collection or transportation process adsorption of fibers or fine particles in the air, which will also be removed with the calcination of Silica Fume at high temperatures.
Wet purification
Wet purification refers to the preparation of Silica Fume with water into a mineral slurry solution, adjusting the pH of the mineral slurry solution with hydrochloric acid to change the surface charge of Silica Fume, increase the expansion coefficient and water absorption of Silica Fume, reduce the agglomeration phenomenon of Silica Fume, and make the impurities easy to separate, the process flow is as follows.
Micro Silica Fume → slurry → modification (impurity precipitation) → collection → drying → purified product
Experiments show that if the two-step method is used to purify and remove impurities, when the temperature is 80℃, the settling time is 4 hours, and the stirring speed is 160r/min, the impurity removal rate of iron reaches 99.1% and the impurity removal rate of aluminum reaches 82.3%. The surface modification of Micro Silica Fume using hydrochloric acid, and then two-step acid leaching purification using hydrochloric acid and sulfuric acid, respectively, can prepare spherical nano-silica with 98.3% purity.
Wet purification is easy to operate, simple equipment, low production cost, and less acid, and the combination with acid purification can further improve the quality of Micro Silica Fume products.
Flocculation purification method
Flocculation method is to use the different wettability of the surface of each component particle in the pulp (i.e., the difference in the strength of the interfacial phenomenon of the interaction between the solid surface and water), adding the appropriate flocculant to separate the different components of the particles, so as to achieve the purpose of separating silica from impurities, in essence, is a flotation method, the process flow is as follows.
Silica Fume → slurry → mixing, flocculation → precipitation and separation → drying → purification products
By studying the effects of flocculant type, dosage, pH and settling time on the purification effect, the results showed that the silica mass fraction in Micro Silica Fume increased from 74% to 91% when the dispersant polyvinylpyrrolidone was added at 80g/t, the solution pH was 5.3, the flocculant sodium oleate was added at 1824g/t and the settling time was 50min.
At present, there are relatively few studies on flocculation method. As a new purification process, the flocculation method does not require as much process equipment as the acid washing method, and the demand for acid is not large. However, the method has problems such as insufficient purification effect, low utilization rate of adjusting agent and many waste by-products, which need to be further studied and solved.
In fact, in the normal production of Silica Fume, in addition to its purification process, sometimes in order to meet the requirements of the use of Silica Fume encryption process, meaning that it is to improve its density.
