With the promulgation and implementation of the regulations on the limit and measurement method of water-depth chromium in cement in 2016, the standard requirements for the content of water-soluble chromium in cement have been strictly limited, which has greatly increased the quality control requirements of major cement manufacturers. The treatment of deep chromium in the product has further improved the technical requirements. The most low-cost treatment for the removal of chromium in cement is reduction and removal with ferrous sulfate heptahydrate.
Method for removing hexavalent chromium in cement by ferrous sulfate
Ferrous sulfate removal of cement chromium
The reducing agent method uses a reducing agent to reduce the soluble Cr (Ⅵ) to low-toxic trivalent chromium Cr (Ⅲ). Combining the characteristics of cement, consult relevant domestic and foreign documents, commonly used reducing agents include ferrous sulfate, Sn2+ salt, Mn2+ salt, organic aldehyde reducing agent, and sodium borohydride. Among them, Sn2+ salt has hygroscopicity and high cost of use, so it is not suitable for large-scale use; manganese sulfate, most aldehydes, and sodium borohydride are all toxic, which can cause secondary pollution in cement applications. Ferrous sulfate is more suitable as a cement soluble hexavalent chromium reducing agent. It has the advantages of low price, low toxicity (normal protective measures), and good reducibility, but its shortcomings such as easy oxidation and caking restrict its use.
After adding 0.08% ferrous sulfate heptahydrate, the water-soluble hexavalent chromium in the cement can be reduced from 10 mg/kg to 4～2 mg/kg; it is verified that the effect of adding after grinding is better than adding before grinding, which can avoid it It is oxidized at high temperature; the water-soluble hexavalent chromium 7d and 28d in the cement after addition has increased compared with the detection result of 1d, but the increase is not high and still meets the national standard limit requirements; the addition of reducing agents before grinding affects the cement production process Basically no impact; no major impact on the standard consistency of cement water demand and setting time; the impact on cement strength is different, but the impact is not large; the impact on the adaptability of cement and admixtures depends on the performance of different cements and water-reducing agents There are differences, which need to be demonstrated through experiments.
The main bottleneck of ferrous sulfate as a cement hexavalent chromium reducing agent is that FeSO4·7H2O has high water content, is easy to cohesive, and has poor fluidity, which increases the difficulty of storage and measurement. The dried ferrous sulfate sold in the market loses part of its crystal water, and the price is 4-6 times that of FeSO4·7H2O. Considering the cost factor, the feasibility of adding a carrier to FeSO4·7H2O was demonstrated through experiments. The ultrafine stone powder, fly ash, and mineral powder commonly used in cement plants are selected and mixed with FeSO4·7H2O in a ratio of 1:1, which has fluidity and difficulty in agglomeration and rolling.
According to the physical properties of FeSO4·7H2O mixed with each carrier, it is more appropriate to select fly ash as the FeSO4·7H2O carrier. Fly ash contains a large amount of glass. The fly ash particles have a porous honeycomb structure with a large specific surface area and high adsorption activity. The particle size ranges from 0.5 to 300 μm. In addition, the bead wall has a porous structure with a porosity as high as 50% to 80%, and has strong water absorption. In terms of phase, fly ash is a mixture of crystalline minerals and amorphous minerals. Its mineral composition varies widely. The general crystalline minerals are quartz, mullite, iron oxide, magnesium oxide, quicklime and anhydrous gypsum, etc. The amorphous minerals are glass, amorphous carbon and secondary limonite, of which the glass content accounts for more than 50%.