Characterization of magnesium hydroxide flocs resulting from the treatment of high turbidity kaolin clay suspensions

Abstract

This study evaluated the effect of operating conditions on magnesium hydroxide floc formation and characteristics, mainly strength and size, besides total suspended solids (TSS) and turbidity removal efficiencies. The parameters investigated included pH, temperature, initial turbidity of suspension, rapid mixing speed and time, in addition to slow mixing speed and time. Highly turbid kaolin clay suspensions were treated with liquid bittern (LB) as a coagulant to exploit its excessive Mg²⁺ ions concentration. After alkalization of the kaolin clay suspensions of turbidities 513±12, 1208±21, and 1988±4 NTU with sodium hydroxide (NaOH) to pH values of 10.52±0.03, 11.00±0.03, and 11.50±0.02, temperature was varied between 10.1±0.4, 21.1±0.7, and 29.7±0.5 °C. Floc aggregation was continuously passed through a dynamic optical monitoring apparatus, photometric dispersion analyzer (PDA2000) which enabled the evaluation of flocculation rate, floc strength and relative floc size. At 21.1±0.7 °C the strongest flocs were formed, while floc growth rate was fastest at pH=11.50. Lower turbidity suspensions (513±12 NTU) were least sensitive to low temperatures (10.1±0.4 °C) at any pH value. Then, optimum conditions of 20°C at pH=10.50 for the 1220 NTU kaolin clay suspension were used to study the impact of both fast and slow mixing speeds (velocity gradient) and time on magnesium hydroxide flocs. Stronger flocs were formed after enough fast mixing time (60 seconds) and speed (120 rpm). For the slow mixing, 20 and 30 minute time were both found to be optimum for turbidity and TSS removal efficiencies, as well as floc sizes. Slow mixing of 40 and 50 rpm ensured the formation of large settleable flocs. In general, each of the previously mentioned parameters showed an impact on the coagulation-flocculation kinetics; nevertheless an interdependent effect of operating conditions was evident.