In Australia back in the 1880s as the miners moved from alluvial riverbed mining of pallio channels where one could use a pan to sift the gold out to hardrock underground mining where the rock had to be finely ground in stamp mills the capital expense of the mill became too great for independent miners to mine.
The States and Shires got involved in supplying the stamp mills where for a fee , mines could take their rock to get it pulverised and the gold extracted ,
Same benefits apply now days for those using toll treatment , they get almost immeditate use of a mill to crush and process their ore, without the upfront CAPEX
Problems are
1. mill is not designed for individual ores being sent to it so recoveries may be lower
2. batch processing of ore occurs so may have to wait months to get your particular ore processed.
3. There is normally a lower limit in tonnage that can be processed as each time processing occurs for a different miners ore the plant has to be cleaned out of the resident ore there which reduces mill availability
4Credits for other elements apart from the main element being processed may be heavily discounted
5. You are relying on the honesty of the mill owner for ore processed.
6, Contractual obligations may not be met and result in court case ie NME with Bright star mill
