Optimization of parameters in planetary ball milling for titanium recovery from automotive paint sludge using the acid digestion process

 Given the high amounts of metals in the paint sludge, the recovery of metals can be seen as an effective and economic method for reducing its pollution. The combination of mechanochemical process and acid digestion may enhance the recovery yield of metals. In this process, milling parameters play a pivotal role in determining the chemical structure of the final products.

 Paint sludge samples were milled at three speeds of 150, 250, and 350 rpm for 2, 4 and 6 hours. Two weight ratios of ball-to-sample, i.e., 10 and 20 also was worked out. The milled samples were then subjected to acid digestion under the same conditions, and the amount of titanium was measured. The data were analyzed using the Taguchi method with Minitab software. Ethical considerations were taken into account at all stages of the study implementation.

 Among the milling parameters, ball milling speed showed the greatest impact on increasing titanium recovery, with a contribution of 49%, followed by time and ball-to-sample ratio, with contributions of 34% and 17%, respectively. The optimal milling conditions for achieving the highest recovery were determined to be a speed of 350 rpm, a time of 4 hours, and a ball-to-sample ratio of 20.

 Generally, the recovery yield of metals built up by increasing the amount and intensity of milling parameters. However, excessive increases in these parameters may result in the opposite effect and lead to the loss of cost, time, and energy. Therefore, the operating parameters of milling should be optimized, so that the optimal values of speed, time, and the weight ratio of ball-to-sample were obtained here.