Stabilization and Solidification of Waste Phosphate Sludge Using Portland Cement and Fly Ash as Cement Substitute

Abstract

Stabilization and solidification of the waste phosphate sludge (WPS) using Portland cement (PC) and fly ash (FA) were studied in the present work. The WPS content in the cement mortars varied from 5% to 15%. Setting times were measured, and unconfined compressive strengths (UCS) were determined for the mortars cured in water for 3, 7, 28, 56, and 90 days. Zinc and nickel leaching of the solidified products were measured according to the Toxicity Characteristic Leaching Procedure. Setting times were extended as the WPS content in the paste samples increased. The UCS values of the mortar containing 5% WPS solidified by using 95% PC were similar to the reference sample. Use of 10% FA as cement substitute increased the UCS values by 10% at the end of curing period of 56 days. The WPS contained initially 130.2 mg L(-1) of zinc and 22.7 mg L(-1) of nickel. The zinc and nickel leached from the 5% WPS solidified by using 95% PC were measured as 3.8 mg L(-1) and 0.4 mg L(-1), respectively. These metal concentrations were below the limits given by the U.S. Environmental Protection Agency for landfilling the solidified wastes.

A safer disposal of hazardous phosphate coating sludge by formation of an amorphous calcium phosphate matrix

“…1500-1400 cm In connection with the toxic metals amounts, Cu, Mn, Ni, Cr and specially Zn were the most abundant hazardous metals found in the samples. These results are in good agreement with the results reported in previous works for similar wastes (Dogan and Karpuzcu, 2010;Ucaroglu and Talinli, 2012;Caponero and Tenório, 2000;Pinarli et al, 2005), as PS are mainly composed of zinc phosphates coming from decanter baths of phosphate, very commonly used in the industry responsible for this kind of wastes. Both sludges can be classified as hazardous wastes, due to their reactive and toxic characteristics according to the regulations and cannot be landfilled without a previous S/S process (Ucaroglu and Talinli, 2012).…”

“…In the reference sample, pure sodium hexametaphosphate (acidic compound) added quickly reacted with the CAC (basic compound) yielding a strong and low porosity matrix (Swift at al., 2013a;Fernández et al, 2014). Given that wastes are composed of lower percentages of phosphates and present heavy metals and organic matter, the strength reduction was an expected result, in good agreement with previous works on S/S processes of phosphate sludge into cement matrices (Dogan and Karpuzcu, 2010;Ucaroglu and Talinli, 2012;Pinarli et al, 2005). Comparatively, the BS induced a stronger reduction, which is in line with the more intense interference with the setting process observed for these samples.…”

“…However, it must be noticed that the hardening of the mortars always took place below 100 min and in the majority of the samples below 60 min, so that the handling of these matrices was not difficult. For comparison purposes, literature has reported, when Portland cement was used as solidifying matrix, setting times well above 400 min for samples with just 10 wt.% of sludge (Pinarli et al, 2005) or extremely long solidification periods higher than 28 days (Ucaroglu and Talinli, 2012).…”

“…Phosphates, for instance, have been reported to act as inhibitors of the setting process (Pinarli et al, 2005). As detrimental consequences of this interference can be mentioned the inability of the system to set and to effectively encapsulate the hazardous materials, the increase in porosity and a parallel reduction in compressive strength that, at short and medium term, leads to an increased release of the toxic compounds and a clear decrease of the integrity and stability of the binding matrix.…”

“…Regarding the S/S treatment of PS, scarce studies have been reported in the literature. In all of them, authors have used ordinary Portland cement (OPC) as binder (Dogan and Karpuzcu, 2010;Ucaroglu and Talinli, 2012;Pinarli et al, 2005).…”