Chemical fractionation of NPEs in slaker grits for environmental risks assessment

Aim: The non-process elements (NPEs) in slaker grits, which are solid residues from the causticizing process of a pulp mill, were sequentially extracted with water (H2O), ammonium-acetate (CH3COONH4) and hydrochloride acid (HCl) in order to evaluate their mobile forms and their release potential from the sample matrix under changing environmental conditions. Materials and Methods: The slaker grits investigated in this study were sampled on November 2013 from a sulfate (kraft) pulp mill located in Finland. The slaker grits were extracted as follows: (1) once in 30-50 mL ultra-pure H2O for 24 h under stirring (65 rpm), (2) 3 times in 3 mL × 25 mL 1.0 M CH3COONH4 (aq) for 3 h ×24 h under stirring, and (3) twice in 2 mL × 25 mL 1.0 M HCl (aq) for 2 h ×24 h under stirring and heating (80°C). Results: Compared with the total NPE concentrations, the efficiencies of H2O in extracting S (6020 mg/kg; dry weight [d.w.]) and Na (17200 mg/kg; d.w.) were highest, being 92.6% and 88.7%, respectively. Compared with the total NPE concentrations, the efficiencies of CH3COONH4 in extracting Mg (4010 mg/kg; d.w.) and B (5.0 mg/kg; d.w.) were highest, being 39.7% and 31.3%, respectively. Conclusion: From the environmental and utilization perspectives the greatest hazards to the environment are those metals, which leach out as a result of contact with H2O or CH3COONH4. Therefore, it is worth noting the ability of H2O and CH3COONH4 to release high concentrations of alkali metals (Na, K), alkaline earth metals (Mg, Ca), and sulfur (S) from slaker grits, and the need to apply for an environmental permit if this residue is to be utilized.