Solidification/stabilization Of Phosphogypsum By Copper Smelting Slag

Phosphoric acid is usually produced by two different approaches:by pyrometallurgy or by hydrometallurgical method.Waste phosphogypsum（PG）is a by-product generated from the wet-process phosphoric acid production which is based on the chemical reaction of phosphate rock with sulfuric acid.PG mainly consists of calcium sulfate dehydrate（Ca SO₄·2H₂O）,making up over 90%,in association with various amounts of hazardous substances such as residual acid,soluble phosphate,fluoride and heavy metals.PG has the characteristics of strong acidity,major impurities and difficult to utilize as resource.In recent years,the annual production of PG in China was in the range of 70-80 million tons.At present,the stock of PG in China exceeds 500 million tons,and it is increasing gradually.Large amount of PG storage not only generates land waste,but also results in serious environmental contamination of soil,water,and atmosphere.Therefore,finding an economic and efficient way to achieve large-scale consumption of phosphogypsum has great application value.Copper smelting slags（CSS）are solid waste produced from the process of pyrometallurgical copper production.In 2018,the production of CSS was 18 million tons in China.Generally,the content of SiO₂in CSS is more than 30%.As CaO content increases or under the activation of NaOH,CSS can exhibit cementitious properties and can be used as partial or full replacement for Portland cement.In fact,only part of CSS was reused in cement production or for valuable metal recovery,while other parts are dumped in storage piles.Due to the heavy metal contents such as copper,lead,and zinc,it could easily cause heavy metal pollution to the surrounding soil and water environment showing environmental risk.Through the study of physical and chemical properties,we found the synergistic effect between CSS and PG.In this study,a small amount of NaOH and CaO were added to the mixed slag of PG and CSS as activator and additional calcium sources.After mixing with 25～35%（Wt.%）water,it was pressed to produce cemented blocks（named PG-S）with certain compressive strength,which realized the efficient synergistic solidification/stabilization（S/S）of PG by CSS.The results showed that:1.According to the standard method,the leaching toxicity of PG and CSS were assessed.The concentrations of PO₄^3-and F^-in the leaching solution of fresh PG were 831.13 mg/L and 196.08 mg/L,respectively;the leaching concentrations of Pb,Zn and As of CSS were 165.19 mg/L,28.76 mg/L and 6.95 mg/L,respectively,which exceeded the limits of Identification standard of hazardous waste-Identification of leaching toxicity（GB 5085.3-2007）,and the leaching concentrations of Cr,Cu and Cd exceeded the limits of Integrated wastewater discharge standard（GB8978-1996）.2.The addition of NaOH and CaO could enhance the S/S behavior of PO₄^3-and F^-.The addition of CaO reduced the leaching concentration of PO₄^3-and F^-from 27.78 mg/L and 48.95 mg/L to 0.07 mg/L and 1.70mg/L,respectively;the addition of NaOH reduced the leaching concentration of PO₄^3-and F^-to 1.04 mg/L and 35.27 mg/L,respectively.3.Phosphogypsum was treated by CSS under different PG/CSS mass ratio.By adding NaOH,the mole ratio of SiO₂/Na₂O in the PG/CSS system was equal to 1.89.CaO was added according to the content of soluble PO₄^3-and F^-in the system.When the mass ratio of PG/CSS was5:1,the 28 d UCS of PG-S was 18.9 MPa;when the mass ratio of PG/CSS was 20:1,10:1 and 5:1,the leaching concentration of heavy metals such as Zn,Pb,As and the leaching concentration of PO₄^3-and F^-meet the first grade of Integrated wastewater discharge standard（GB8978–1996）.When the ratio increased to 2:1 and 1:1,the leaching risk of heavy metals in PG-S also increased due to the increase of CSS and NaOH addition,and the leaching concentrations of Pb and Zn failed to meet the requirements of emission standard.4.XRD,SEM,XPS and FTIR analysis showed that the formation of C-S-H and Ettringite in the PG/CSS system provided strength for the PG-S;the co-precipitation reaction of PO₄^3-and F^-with Pb and Zn stabilized PO₄^3-,F^-,Pb and Zn.The geochemical modeling was consistent with the characterization results,and the simulation showed that with the increase of p H,the fixation effect of Pb and Zn may decrease.