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The introduction of dyeing sludge in the fired brick production is an economic and practical disposal approach. Wet dyeing sludge with high moisture usually produces block mass during mixing process of raw materials, not only reduces the production efficiency but negatively impacted products engineering properties. This study proposed a highly efficient method, wet sludge firstly being pelleted and then introduced in the production of fired bricks, to reduce the negative impacts on mixing process and the products quality. It was proved that the introduction of dyeing sludge pellets remarkably improved physical and mechanical properties of fired bricks. Especially, water absorption declined by 12% and compressive strength increased by 35% comparing with dyeing sludge without pelleting. The engineering properties of fired bricks containing 25 wt% sludge pellets are similar with that containing 15 wt% sludge without pelleting. The decrease in water absorption and porosity is mainly attributed to the reduction in number and volume of pores with diameter below 100 nm. It was considered that pellets with greater density and higher strength might play a role as aggregate to improve compressive strength. However, the introduction of sludge pellets with particle size over 8 mm is adverse to improving properties of products, because a large amount of macropore with diameter over 100 μm is produced, and destroys the homogeneous and compact structure of products. The optimal sludge pellet size is in the range of 2–6 mm based on products performance. The leaching toxicity of heavy metals from products is far lower than regulatory limitation. This study provides an advanced method to promote the disposal efficiency of solid waste and improve the engineering quality of products. • The introduction amount of dyeing sludge is promoted remarkably. • Physical and mechanical properties are improved significantly. • The optimal particle size of sludge pellets is 2–6 mm. • Pore volume below 100 nm in fired brick are reduced. [ABSTRACT FROM AUTHOR]

Incineration is an alternative approach to disposal of tannery sludge due to the significant volume reduction. But the Cr(III) oxidation caused by Ca hydrates or carbonates in the incineration process could lead to incineration residue exceeding the required standard in the leaching toxicity of Cr(VI) and being listed in hazardous waste. A novel method of suppressing Cr(III) oxidation by introducing sulfates during the incineration of tannery sludge is proposed in this study. Cr(III) oxidation can be effectively inhibited with the introduction of NH 2 SO 3 H, NH 4 HSO 4 and NaHSO 4 at n (S)/ n (Ca) over 1.2. The combination reaction of sulfates and CaO prevents the Cr(III) oxidation. The inhibition of Cr(III) oxidation by sulfates is the most effective at 700–900 °C because almost all Ca is incorporated into CaSO 4 phase. NH 2 SO 3 H and NH 4 HSO 4 perform better in inhibiting Cr(III) oxidation than NaHSO 4. The intermediate product of Cr 2 (SO 4) 3 , which is formed below 500 °C, would decompose into Cr 2 O 3 with the temperature of over 600 °C. However, part of Cr 2 O 3 is still oxidized into CaCrO 4 aroused by CaSO 4 at a temperature of over 900 °C. Kinetic study showed Cr(III) oxidation could be effectively suppressed even if the heating time extends to 2 h at 800 °C. Toxicity Characteristic Leaching Procedure showed that the leaching concentrations of Cr(VI) from incineration residues with the introduction of NH 2 SO 3 H and NH 4 HSO 4 at n (S)/ n (Ca) of over 1.2 satisfied the requirement, and resultant residues could not be classified as hazardous waste. Due to inhibiting Cr(III) oxidation with high efficiency at 700–900 °C, the introduction of sulfates can be considered as a potential approach to prevent Cr(VI) formation during the incineration of tannery sludge by fluidized bed incinerator. • Cr(III) oxidation are suppressed by sulfates during heating of tannery sludge. • Inhibiting Cr(III) oxidation is strongly determined by temperature. • NH 2 SO 3 H and NH 4 HSO 4 are more effective in hindering Cr(III) oxidation than NaHSO 4. • Cr(III) oxidation is suppressed with sintering time being extended to 2 h. • Incineration residues cannot be listed as hazardous waste with adding of sulfates. [ABSTRACT FROM AUTHOR]