But what was the smelter “smelting”, and why would that lead to a thallium spill?
The Shaoguan smelter (behind the pollution) has an annual production of 200,000 tons of zinc and 100,000 tons of lead, and that explains the source, I should say, as thallium occurs (in trace amounts) in the chief ore of zinc, sphalerite (also known as zincblende - (Zn,Fe)S - zinc-iron sulphide), and some other ores with iron. When the zinc ore is leached, the leach product may contain things like: cadmium, copper, arsenic, antimony, cobalt, germanium, nickel, and indeed thallium. (A nice cocktail of stuff that nobody would like to consume!). Even trace amounts add up to appreciable levels at production plants of this size.
The reasonable thing to do is of course to extract arsenic and thallium as by-products, and sell them with a profit (yes these elements are commercially interesting!), which I would also guess that the owners of the Shaoguan smelter normally do.
The trace elements in zinc and lead ores are one of my worries about the probable opening of a zinc mine in the northernmost part of Greenland - see my post on the Most Northerly Zinc Mine. The smelting will of course take place somewhere else, but how much of the pollutant elements will be released to the local environment during the mining? 0%?