ball mill (ball mill)

A ball mill is an industrial grinding machine that reduces feed material to fine particles by tumbling free-moving grinding media — usually hardened steel or wear-resistant ceramic balls — inside a rotating cylindrical drum. The drum rotates below the critical speed (typically 65-75% of the speed at which centrifugal force would pin the balls to the wall), so the balls cascade inside the drum, crushing the feed by impact and shearing it by attrition.

Mechanism and operating variables: Grinding action depends on four primary variables — ball diameter (25-90 mm), ball-charge fill ratio (35-45% of mill volume), feed-charge fill ratio (15-25% of remaining volume), and rotation speed. Larger balls deliver more impact energy and break coarse feed; smaller balls produce more attrition and finer end products. Wet ball mills add water or process slurry, reducing dust and improving heat removal; dry ball mills are simpler but generate substantial heat and dust. Wet-mill product fineness is typically 80% passing 75 micron, with extended residence time reaching sub-10 micron.

Application in e-waste processing: Ball mills sit downstream of primary shredding, grinding PCB scrap to fine powder before hydrometallurgical leaching for precious-metal recovery. The metallic and non-metallic fractions of PCB respond differently — brittle solder, gold-bearing wire bonds, and silicon-die fragments shatter to fine particles, while malleable copper foil and base-metal pins deform and resist size reduction. This natural size segregation is exploited downstream: the fine metallic fraction is concentrated by air or hydraulic classification before leaching, while the coarse copper foils are recovered by physical sorting at much higher purity than would otherwise be possible.

Trade-offs: Ball mills are energy-intensive — typical specific energy is 15-40 kWh per tonne for PCB grinding to 75 micron, two to three times that of jaw or impact crushers on coarser products. Capital cost for a 1 TPH ball mill (motor, gearbox, auxiliaries) runs Rs 25-50 lakh INR. Ball wear is significant — 1-3 kg of grinding media is consumed per tonne of feed, a recurring operating cost. Indian PCB recyclers typically operate ball mills only when downstream hydromet capacity justifies the fine product; smaller plants stop at coarser shredded fractions and sell to integrated refiners.