Research On Chromium-contaminated Soil As Siliceous Raw Materials In Cement Production

In recent years, mining, metallurgy, chemical and other industries brings a large number of solid waste containing Cr(VI) and its compounds, as well as a lot of chrome contaminants, such as chrome-contaminated soil (CCS) and chromium water. These CCS distributes widely, with large amount and strong toxicity, but have not been properly and effectively treated. Cement kiln combines waste disposal and clinker production via high-temperature calcination, which is the most effective way for almost all kinds of solid waste, is a good reference for this issue.To explore the feasibility of co-processing disposal of CCS in cement kilns, here CrO3was taken as simulation of high-chrome-content waste, and the effects of CrO3on the performances of cement products were researched. Results show that:(1) When Cr adding quantity is less than0.5%, raw meal’s burnability is better, and the formation of silicate minerals is promoted, hydration reaction of clinker begins earlier and the heat of hydration increases, thus compressive strength of cement products is slightly improved; when Cr is more than1%, the performances of cement products mentioned above will be on the contrary. With the increase of CrO3dosage, leaching concentration of Cr(VI) and∑Cr of clinker hydrate increases; with an extended curing age, Cr(VI) leaching concentration of each sample decreases, and the stronger leaching toxicity in3d is, the weaker it will be in90d.On the base of the study above. CCS was taken as a partial substitution of siliceous raw material to produce cement clinker, through which the effect of co-process disposal of some specific chrome pollutant with cement kiln could be evaluated, and results show that:(2) Until CCS is up to15%(in raw meal), the burnability of raw meal keeps fine, and mechanical properties of cement products can be slightly improved. The larger adding quantity of CCS is, the higher leaching concentration of Cr(VI) and∑Cr of clinker hydrate will be, while it drops off with an extended curing age. Until CCS increases to8%, leaching toxicity of cement clinker can meet the relevant standard (0.05mg/L and0.15mg/L), no matter how it was used (in weak acid, weak alkaline environment or just in cement mortar).Now clinker that made from15%CCS was taken as an example, some common mineral admixtures and appropriate chemical reductants were added into it to make subsequent reduction treatment, and results show that:(3) Slag, steel slag, fly ash and other industrial waste can all effectively reduce the leaching toxicity of cement products, and the more industrial waste is added, the better the effect will be. Reduction effect on Cr(VI) under the same dosage:steel slag>slag>fly ash; further decrease of leaching concentration of each sample comes with an extended curing age. With the increase of reductants’dosage, leaching concentration of cement products significantly reduces; notably, along with an extended curing age, leaching concentration of Cr(VI) increases. When Slag, steel slag and reductants are used together, reduction treatment of cement products can be most effective.(4) The decrease of Cr(VI)/∑Cr concentration should be attributed to solid solution of Cr in clinker mineral and its hydrate, then hydrate (C-S-H mainly) can wrap some Cr ions and prevent them from leaching. Beacause there are always some Fe(Ⅱ) or Mn(VI) in slag and steel slag, they can work as reductant in cement paste, and with the prolongation of cement paste’s curing time, the hardened paste blocks the leaching path of Cr(VI). All in all, innocuous treatment and utilization of CCS with cement kilns is available, and relevant research will be appreciated.