Manufacturing Calcium Fluoride Products Utilizing Sludge Containing CaF2.

Introduction of technology for producing calcium fluoride products from fluorine-containing sludge

 Sludge containing calcium fluoride is produced in the process of manufacturing inorganic and organic fluorine chemical products, integrated circuits and electronic products, photovoltaic panels, display panel glass thinning and other fluorine-related industries. The main pollutants are fluoride ions and suspended substances, which are hazardous wastes since these industries will produce fluoride-containing waste acid and fluoride-containing wastewater in the production of products.

There are more and more fluorine-containing wastewater (F-≤1.0%) and waste acid (F->1.0%) with the rapid development of the fluorine chemical industry and the continuous expansion of the application field of fluorine products. Currently, most enterprises adopt harmless treatments to turn hazardous waste into general waste. The specific treatment method is as follows: calcium-containing materials such as calcium chloride, calcium hydroxide or calcium carbonate are added after neutralization by adding alkali (sodium hydroxide, calcium hydroxide, etc.), and fluoride ions in wastewater react with calcium to produce calcium fluoride precipitate. Then, the calcium fluoride sludge is separated by a centrifuge or a plate-and-frame filter press by adopting the treatment mode of "coagulation → flocculation → precipitation → sludge separation". The main content of sludge is calcium fluoride, silica, calcium hydroxide or calcium carbonate, etc. The water content of sludge is between 30% and 75%.

The water content of calcium fluoride sludge produced by different factories is different. In general, the water content of calcium fluoride sludge produced by waste fluoride acid or wastewater containing no organic matter is 35%~60%, while the sludge produced by the organic fluorine chemical industry is sticky and has many impurities, so it is more difficult to filter press, and its water content is 45%~75%. The content of CaF2 in calcium fluoride sludge is above 60% in the dry state, and the rest is calcium hydroxide, some inorganic particles and organic precipitates.

Calcium fluoride has low toxicity, but it is easily absorbed by plants. The direct landfill is harmful to the soil, and then poses a threat to crop growth and human health. In case it is not properly disposed, it is easy to cause secondary pollution of fluorine since calcium fluoride is slightly soluble in water (16mg/L), which will increase the fluorine concentration in surface water, and cause the fluorine content in groundwater to exceed the standard or aggravate the already exceeded standard. Hence, how to properly dispose of sludge and realize the reduction, stabilization, harmless and resource disposal of sludge has always been the goal pursued by experts and engineers. Many people in China are making efforts to make use of calcium fluoride sludge as a resource. Nevertheless, the actual situation is not optimistic. More than 80% of calcium fluoride sludge in China has been landfilled, which not only pollutes the environment, but also wastes fluorine resources.

The weight of solids is about 65~50% in calcium fluoride sludge, and the moisture is about 35 ~ 50%. The content of CaF2, SiO2, Ca(OH)2 and other hydroxides is 65-72%, 3-15%, 5-15% and 3-10% respectively after the solid is dried. Only calcium hydroxide Ca(OH)2, "other hydroxides" can be neglected in case it is calcium fluoride sludge from photovoltaic enterprises.

Components of calcium fluoride sludge from a glass thinning plant are shown in Table 1:

Components of calcium fluoride sludge from a solar company are shown in Table 2:

Only by making rational use of the effective components in waste containing calcium fluoride can we achieve the purpose of reducing environmental pollution, recycling resources and developing a circular economy.

Calcium fluoride sludge was used as raw material to produce calcium fluoride products.

Ca(OH)2 + 2HF → CaF2 + 2H2O ⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯(1)

SiO2 + 6HF →H2SiF6 + 2H2O ⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯⋯(2)

H2SiF6+H2SO4 (concentrated) →SiF4+ 2HF+ H2SO4 (diluted) (3)

Flow Chart:

Flow Chart:

Calcium sludge enters the stirred reactor, and hydrofluoric acid solution also enters the stirred reactor, which is stirred into the slurry and fully reacted. The slurry enters a plate-and-frame filter press for solid-liquid separation after the reaction. Cleaning the solid to obtain wet calcium fluoride, and drying to obtain finished calcium fluoride powder. The separated liquid enters the decomposition reaction kettle.

It generates silicon tetrafluoride and hydrogen fluoride gas after adding concentrated sulfuric acid into a decomposition reaction kettle. It is condensed to obtain hydrogen fluoride liquid, and returns to step 1. It is concentrated and sulfuric acid becomes dilute sulfuric acid.

Silicon tetrafluoride gas is absorbed into fluorosilicic acid and silica by water, and the silica is separated by a plate-and-frame filter press and cleaned to obtain a wet silica product, which can be sold out. The fluosilicic acid returns to Step 2.

Dilute sulfuric acid is concentrated to obtain concentrated sulfuric acid and wastewater. The concentrated sulfuric acid returns to Step 2. Wastewater will be taken to the treatment station. The treatment of wastewater can achieve zero discharge.