Reprocessing the waste liquid that contains fluorine, produced by manufacturers of photovoltaic products.

In the process of photovoltaic production, hydrofluoric acid (or a mixture of nitric acid and hydrofluoric acid) is utilized to treat silicon wafers, resulting in the generation of a waste acid solution containing fluorine. Table 1 provides information on the composition of this waste acid solution.

The waste acid solution containing fluorine produced in the process of photovoltaic production is a strong corrosive acid and classified as hazardous waste. Acid-base neutralization is a common method used to treat the waste solution, which converts the fluorine into calcium fluoride sludge and achieves the goal of "harmless" treatment.

As an example, we will explain our recycling technology for waste acid that contains both nitric acid and fluorine.

The process begins by mixing the fluorine-containing waste acid with concentrated sulfuric acid, which decomposes the fluosilicic acid into hydrofluoric acid and silicon tetrafluoride under the appropriate temperature and pressure. HNO3, HF, and SiF4 then volatilize as steam. HNO3 is collected into the nitric acid temporary storage tank after passing through the first-stage cooler and becomes a 68% nitric acid product after refining. HF is collected into a hydrofluoric acid temporary storage tank after passing through the second-stage condenser and becomes a 99.96% hydrogen fluoride product after refining.

SiF4 gas is introduced into water, and it becomes fluosilicic acid and silica precipitate after three-stage absorption. Wet silica product is obtained for separating silica and cleaning. The fluosilicic acid is mixed with the waste photovoltaic acid containing fluorine, and is circularly decomposed again.

After treatment, concentrated sulfuric acid becomes a mixture of dilute sulfuric acid and sulfate. The dilute sulfuric acid is concentrated, and the sulfate is crystallized, then separated as a solid-liquid, and the dilute sulfuric acid is further concentrated until the concentration is ≥92% and is returned to the decomposition reactor kettle. The condensed water vapor is treated as waste water and treated in a sewage treatment station (H2SO4≤1.0%, HF≤0.1% in wastewater).

Using this technology, the recovery rate of fluorine resources is about 93 ~ 97%, nitric acid is about 95 ~ 99%, and sulfuric acid is about 92 ~ 96%.