Dearsenization of caustic solution by synthetic hydrocalumite

Highlights

• Hydrocalumite, Ca₂Al(OH)₆Cl·2H₂O, was synthesized to remove arsenic from caustic solution for the first time.

• Various parameters associated with the dearsenization process were investigated and up to 91% arsenic was removed.

• The dearsenization kinetics was studied and the apparent activation energy was estimated to be 12.43 kJ/mol.

Abstract

Arsenic is enriched in liquor during the processing of copper anode slime by alkali fusion-leaching. The conventional method of removing arsenic from caustic liquors requires the neutralization followed by the introduction of calcium oxide or calcium hydroxide. In this work, the hydrocalumite, Ca₂Al(OH)₆Cl·2H₂O, are proved for the first time to remove up to 91% arsenic from caustic solution. Hydrocalumite is a mineral anion exchanger belonging to the layered double hydroxides (LDHs) which was synthesized by co-precipitation of calcium chloride and aluminum chloride in sodium hydroxide solution. It was characterized by XRD, FESEM and TG-DSC. Experimental parameters affecting the dearsenization process, such as temperature, NaOH concentration, initial arsenic concentration and hydrocalumite dosage were investigated in detail. The main phases of dearsenization products formed from hydrocalumite at different temperature were identified as Ca₆Al₃(OH)₁₈(H₂O)₆–AsO₄ (flat hexagonal), Ca₃Al₂(OH)₆–(AsO₄)₂ (cubic) and Ca(OH)₂ (Octahedron-single). The apparent activation energy in dearsenization by hydrocalumite was estimated to be 12.43 kJ/mol over the range of 30–80 °C.

Introduction

Caustic solution containing arsenic (As) is produced in the process of alkali-fusion leaching of copper anode slime (Li et al., 2015). It is of high alkalinity and high toxicity, which can't be directly drained without proper treatment. Before the recycling of alkali from caustic solutions, consideration has been given to the removal of arsenic firstly.

There are two typical ways of removing arsenic from caustic solution. The first way consists of the precipitation of arsenate by oxidizing As(III) to As(V), based on lower solubility of arsenate than arsenite in caustic solution (Guo et al., 2015, Tongamp et al., 2010, Li et al., 2011). This produces a decrease of arsenic concentration in the solution, but it is unavailable for processing of low arsenic content (e.g., < 2 g/L) solution. The other way is to neutralize the caustic solution and then convert arsenic into some insoluble compounds by introducing chemicals, such as calcium oxide or calcium hydroxide into the solution (Cui et al., 2014, De Klerk et al., 2012, James and Paul, 1999). It is characterized with high efficiency (99%) and good applicability, but huge amount of acidic reagents consumption and salt production.

Layered double hydroxides (LDHs), known as anion-exchange clay materials, are widely used as promising absorbents for both inorganic (arsenate, chromate, selenite, bromide, phosphate, etc.) and organic anions (methyl orange, sodium dodecylsulphate, etc.) (Theiss et al., 2014, Qian et al., 2012, Zhang et al., 2012a, Zhang et al., 2012b, Liu et al., 2010, Grover et al., 2009, Yang et al., 2005, Rousselot et al., 2002). Among them, there is an interesting group of LDHs named as hydrocalumite (Ca₂Al(OH)₆Cl·2H₂O). It is formed in cements rich in tricalcium aluminate (Suryavanshi et al., 1996, Birnin-Yauri and Glasser, 1998). A net positive charge on the sheets originates from the partial replacement of Ca^2 + with Al^3 + ions, forming [Ca₂Al(OH)₆⁺] layers. The distorted “brucite-like” layers were separated by interlayers of water molecules and Cl⁻ (Zhang et al., 2012b).

Hydrocalumite is a cost-efficient absorbent and has been reported to remove arsenic from different liquors including drinking water and industrial wastewater, but only investigated on acidic ones and a limited scale (< 150 mg/L) (Grover et al., 2009, Grover et al., 2010). In this study, hydrocalumite was prepared by the reaction of mixed chlorides with sodium hydroxide and examined for its arsenate anion exchange capacity, using as a dearsenization agent in caustic solution for the first time. The experimental parameters affecting the dearsenization, such as temperature, NaOH concentration, the initial concentration of arsenic and hydrocalumite dosage, were investigated systematically.