Elsevier

Journal of Hazardous Materials

Volume 347, 5 April 2018, Pages 227-232
Journal of Hazardous Materials

Kinetic study of heavy metals Cu and Zn removal during sewage sludge ash calcination in air and N2 atmospheres

https://doi.org/10.1016/j.jhazmat.2017.12.061Get rights and content

Highlights

  • The in-depth study on the kinetics of heavy metals removal from SSA with MgCl2 was completed.

  • Cl-addition is beneficial to remove Cu/Zn during thermo-treatment of SSA to get fertilizer with low heavy metal toxic.

  • The reaction activation energy of both Cu and Zn in an air atmosphere was lower than in N2.

  • Sub-step chlorination reduced the reaction requirement of the activation energy.

Abstract

Heavy metal control is essential during the thermochemical recovery of phosphorus (P) from sewage sludge ash (SSA). For medium volatile heavy metals, i.e. Cu and Zn, the effect of chlorine additive was complicated and more sensitive to temperature variation. So, in the in-depth study on the removal kinetics of Cu and Zn was necessary. Thus, the studies described in this paper considered the experiments and kinetic models of Cu and Zn removal in SSA through calcination under different atmospheres and temperatures. The results showed that within 15 min, the removal of Cu and Zn was more effective at the same temperature in air than in N2. The result is consistent with kinetic analysis: Reaction activation energy of both Cu and Zn in an air atmosphere is lower than in N2. In addition, the reaction orders, energy and frequency factors of Cu and Zn removal reaction during SSA calcination at high temperature with air and N2 atmosphere were calculated.

Introduction

It is estimated that approximately 7.91 million tons of dry sewage sludge (SS) are produced annually, and this amount is likely to increase more than 10% annually in China [[1], [2], [3], [4]]. Although most SS is currently landfilled after incineration, this option results in the loss of the valuable phosphate in SS [5,6]. The amount of phosphate rock and the quality of mineral phosphate resources is constantly decreased [7]. Therefore, new methods must be developed because P is an essential element, and the resources of P are not renewable. SSA is a significant potential P-source due to its huge amount and large P content [8]. However, direct application of SS in agriculture as a P-fertilizer cannot be achieved in the presence of organic pollutants and heavy metals [9,10]. Incineration treatment for SS can degrade the organic pollutants completely [11]. However, heavy metals are also enriched in SSA [12].

In order to achieve phosphorus enrichment and removal of heavy metals, Adam et al. 2009, Vogel & Adam, 2011 and Vogel et al. 2013 and others had done a lot of research [6,10,11]. It was found that: For heavy metals that are difficult to volatilize, i.e. Cr and Ni, the effect of chlorine additive was very little; For heavy metals that are easy to volatilize, i.e. Pb and Cd, a little chlorine additive can make heavy metals evaporate almost completely; For medium volatile heavy metals, i.e. Cu and Zn, the effect of chlorine additive was complicated. So, in the in-depth study on the removal kinetics of Cu and Zn was necessary.

Therefore, further research on heavy metal removal, especially kinetic and thermodynamic mechanisms, is required [13]. Investigated a local CFD kinetic model of Cd vaporization during municipal solid waste (MSW) incineration. However, the huge difference between MSW and SSA, especially the high P content in SSA, has significant influence on the kinetics of heavy metal volatilization [14]. The extensive research on kinetics has mainly focused on the kinetics of MSW incineration or SS pyrolysis and seldom on SSA calcination [15]. Thus, an in-depth study on the kinetics of heavy metals removal is conducted during SSA calcination under different technological conditions, including temperature, atmosphere and time. The results provide a theoretical basis for the harmless and highly efficient P-resources recovery in SSA, which has not been researched previously in this related field.

Section snippets

Materials

SS used in the experiment was taken from a municipal wastewater treatment plant in Dalian City, Liaoning Province, China. The SS used was produced from the A2/O pool and the second pond in the SS dewatering system. The moisture content of the original SS was 79%, the industry analysis result of SS is shown in Table 1 [16]. After drying at 105 °C for 48 h, the dried SS was triturated and sieved with 100 mesh (150 μm) and was burned in a muffle furnace at 850 °C for 2 h with the addition of CaO at

Influence of atmosphere

Fig. 1 shows that the removal of the heavy metals Cu and Zn varies with the atmosphere: 95.6% of Cu and 91.8% of Zn can be removed from phosphorus-enriched sewage sludge ash., and two kinds of metal in nitrogen slightly higher than that of the air, only 5% Cl-addition for Cu is significant higher for N2. This is similar to the study of Christian Vogel and Christian Adam et al. The addition of a very small amount of chlorine can make the removal of copper and zinc more than 90%. However, the

Conclusions

The studies described in this paper considered the kinetic model of heavy metal removal in SSA through calcination with different atmospheres, temperatures and times. The reaction of heavy metal removal during SSA calcination at high temperatures can be represented by the equation of isothermal heterogeneous reaction kinetics: dαdt=Aexp(-ERT)(1-α)n. The reaction orders of the heavy metal Cu removal reaction during SSA calcination at high temperature with air and N2 atmospheres were 1 and 3/2,

Acknowledgement

This research was supported by the National Natural Science Foundation of China (No. 51576134).

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