THE TIN MILLING PROCESS
The tin metal is locked into ore which is now generally mined deep down in the depths of the earth. In the early days this tin bearing material may have been found in streams and also in waste material inadvertently left by earlier processing. To ‘win’ the metal tin from the material within which it has been locked by nature there are generally three stages - Mining, Milling and Smelting. To release the tin it is first necessary to process the material to a fairly high degree of purity (black tin) and this was the function performed by Tolgus Mill. From Tolgus the black tin was sold to smelters who turned it into pure (white) tin.
The ore of tin oxide cassiterite is heavier than most of the minerals that accompany it. It is weight that was used by the tinners to separate the valuable tin from the waste material. Washed and concentrated a number of times the ore containing perhaps 0.3% tin was worked until the final product contained up to 70% tin. This concentrate was then sent to the smelters who would turn the black sand into the “white tin” metal.
Material treated at Tolgus came from two sources, rock from mine dumps which was crushed in the Cornish stamps to the size of coarse beach sand and the sands and slimes which came from mine processing plants. This was mixed with water and churned into a slurry in the screen washer. All the material was then lightly ground in the ball mill.
Following the grinding the material was split into two sizes known as “sands” about the size of ordinary beach sand and “slimes” that are about the consistency of face powder. In modern times this material was graded by first passing it through a hydroclone (three examples of this machinery can be seen by the entrance to Tolgus) to remove the fine slimes which passed with most of the water to the slime pits where the solid particles were settled out. The sands passed over the curved screens to be sized further before passing over the sand tables which concentrated the tin oxide. Some material from the sand tables which contained tin but needed further grinding known as “middlings” was reground and then passed over more sand tables after having been deslimed in a rake classifier.
The slimes, being so fine, had to be treated carefully. A controlled amount of water was first added to wash the slimes from the slime pits, an operation known as “beating”, the material then passed over Cornish round frames. From the round frame the slime concentrates containing about 8% tin passed over Holman slime tables to produce a final concentrate ready for the smelters containing about 50-60% tin metal.