Effect of reaction conditions on agglomeration of aluminum hydroxide in the recovery of waste aluminum-catalyst

Abstract

Water-soluble waste salts, especially those contain harmful ions, are a great threat to soil, water, air, and human health. NaAlCl4 waste salt, containing harmful Al3+ ion, is massively by-produced in the manufacture of dichlorophenylphosphine (DCPP) and remains a big issue to both industry and environment. In this work, we intensively investigated the aluminum hydroxide precipitation process during the recovery of NaAlCl4 waste salt via acid-base reaction. The aluminum hydroxide crystallized under different conditions was monitored and characterized using XRD, TG, SEM, and Zeta potential. The effect of precipitation pH and temperature on crystal form, gel content, agglomerate size distribution, and moisture content of filter cake were studied. Precipitation pH was found to influence both gel-precipitation conversion and agglomerate state, but its effect varied under different temperature, indicating complex gelation and agglomeration processes. Temperature, however, has similar effect at different pH conditions that low gel content and hardened agglomerate can be achieved with increasing temperature due to intensified particle movement and collision. In addition, NaCl has little effect on the crystal form of aluminum hydroxide even though some initial NaCl exists in the solution. Then, the precipitation mechanism was proposed, in which agglomerate void filling process was discovered and verified via porosity analysis. Furthermore, the precipitation process was optimized and a drop of 25.16% in filter cake moisture content was achieved which can lead to a better separation of Al3+ and Na+ and a higher efficient recovery of the waste salts.