ABSTRACT
A new method in which Pb/Zn smelter waste containing arsenic and heavy metals (arsenic sludge), red mud and lime are utilized to prepare red mud-based cementitious material (RCM) is proposed in this study. XRD, SEM, FTIR and unconfined compressive strength (UCS) tests were employed to assess the physicochemical properties of RCM. In addition, ettringite and iron oxide-containing ettringite were used to study the hydration mechanism of RCM. The results show that the UCS of the RCM (red mud+arsenic sludge+lime) was higher than that of the binder (red mud+arsenic sludge). When the mass ratio of m (binder): m (lime) was 94:6 and then maintained 28days at ambient temperature, the UCS reached 12.05MPa. The red mud has potential cementitious characteristics, and the major source of those characteristics was the aluminium oxide. In the red mud-arsenic sludge-lime system, aluminium oxide was effectively activated by lime and gypsum to form complex hydration products. Some of the aluminium in ettringite was replaced by iron to form calcium sulfoferrite hydrate. The BCR and leaching toxicity results show that the leaching concentration was strongly dependent on the chemical speciation of arsenic and the hydration products. Therefore, the investigated red mud and arsenic sludge can be successfully utilized in cement composites to create a red mud-based cementitious material.
ABSTRACT
Wastewater treatment sludge from a primary lead-zinc smelter is characterized as hazardous waste and requires treatment prior to disposal due to its significant arsenic and heavy metals contents. This study presents a method for the stabilization of arsenic sludge that uses a slag based curing agent composed of smelting slag, cement clinker and limestone. The Unconfined Compressive Strength (UCS) test, the China Standard Leaching Test (CSLT), and the Toxicity Characteristic Leaching Procedures (TCLP) were used to physically and chemically characterize the solidified sludge. The binder ratio was determined according to the UCS and optimal experiments, and the optimal mass ratio of m (smelting slag): m (cement clinker): m (gypsum sludge): m (limestone) was 70:13:12:5. When the binder was mixed with arsenic sludge using a mass ratio of 1:1 and then maintained at 25 °C for 28 d, the UCS reached 9.30 MPa. The results indicated that the leached arsenic content was always less than 5 mg/L, which is a safe level, and does not contribute to recontamination of the environment. The arsenic sludge from the Zn/Pb metallurgy plant can be blended with cement clinker and smelting slag materials for manufacturing bricks and can be recycled as construction materials.