The mineral processing facility at Lisheen operates 24 hours a day, seven days a week, at a rate of 4,500 tonnes per working day.
The first stage of processing is comminution, which involves the reduction in size of the ore particles to separate valuable minerals contained in the ore from the non-valuable host rock.
From the covered surface stockpile, the ore is conveyed via 6 vibrating feeders onto a conveyor, which delivers it into a semi-autogenous grinding (SAG) mill. The SAG mill is a rotating cylindrical vessel containing 8-12% steel balls, in which the particle size of the ore is reduced. Substantially oversize discharge is screened out and conveyed back into the SAG mill for further grinding and the remaining slurry is directed to a cyclone for sizing.
The course overflow materials from the cycloning process are directed to a ball mill containing 40% steel balls, where further grinding takes place. The discharge from the ball mill is fed to the same cyclone as the finer materials from the SAG process.
The second stage of processing is floatation, a process used to separate the zinc and lead minerals from the ore. Air is pumped into the slurry, forming bubbles to which the minerals attach themselves. The mineralised bubbles form froth on the surface of the slurry, which overflows into launders and is pumped to the next stage in the process. The material that does not float is called "tailings".
Lisheen uses floatation lines for lead floatation and zinc floatation, each of which has 3 sections-roughers, column cleaners and cleaner scavengers. Slurry first passes through the lead rougher where, aided by the addition of specific reagents, the lead content is separated from the rest of the slurry or tailings.
The tailings then pass into the zinc floatation line where different reagents are added. After roughing, the zinc concentrates undergo a further comminution process known as regrinding, during which very fine particles of sphalerite are released before production of the final concentrate.
The lead floatation line upgrades the concentrate to 64% lead, while the zinc floatation line produces a concentrate of 54.5 - 55% zinc.
After floatation, the zinc concentrate may undergo an acid leach quality control process, which helps to reduce magnesium levels in the concentrate if required.
At this point the concentrates contain too much water for storage and shipping and must therefore be de-watered.
The first stage of de-watering is thickening. The slurry is fed into an un-agitated tank, where chemicals are added to assist the settling process by amassing individual particles into larger clumps. These solid particles of the slurry are then allowed to settle to the bottom before being raked gently to a central underflow pumping point.
The underflow of thickened concentrates is directed to the respective filtration stage for final de-watering. Using pressure filters (1 for lead, 2 for zinc), the moisture concentrate of the thickened concentrates is reduced to approximately 6% for lead concentrate and 8% for zinc concentrate.