This report describes the gold recovery methods at the San Juan mill, including crushing, grinding, flotation, cyanidation, and a carbon-in-pulp circuit, with design capacities and historical operating data reported for 2010.
Report context
The gold mill at San Juan was established in 1970 and has processed San Juan ore since that time. This technical report documents the recovery methods and mill performance as of 2010. The mill is predominantly a flotation mill but also has the ability to process ore through a carbon-in-pulp (CIP) circuit. The report includes data for the 2010 calendar year.
Processing route
Crushing and Grinding
Ore is transported from the various veins to the coarse ore bin that feeds a 10-inch by 24-inch Blake jaw crusher that reduces the ore to 2.5 inches, followed by screening. The coarse material is then fed to a 3-foot Symons standard cone crusher that reduces the ore to 80% minus 0.5 inch. The crushed ore is conveyed to a fine ore bin with a capacity of 120 tonnes, from which the material moves to the grinding circuit. The number 1 mill circuit contains two 6-foot by 6-foot ball mills and a 5-foot by 6-foot COMESA mill in closed circuit with a duplex 16-inch by 24-inch jig and a 42-inch by 22-inch helical classifier. This circuit had a capacity of 500 tonnes per day (tpd) as reported.
The ground ore is passed through the 6-foot by 6-foot mills with sufficient water to produce a pulp density of 1,650 grams per litre. The ground ore is then passed through the jig before going to the classifier. The jig product is unloaded manually, reground and amalgamated. The jig overflow goes to the classifier, with the classifier overflow going to the flotation circuit and the underflow going to the 5-foot by 6-foot mill for further grinding.
Flotation
The ground ore from the classifier overflow has a particle size ranging from 45% to 50% minus 200 mesh. The overflow, which has a density of 1,280 grams per litre, passes for flotation in 16 Denver flotation cells arranged as rougher, scavenger and cleaner cells, followed by three DR30 cells before the tailings are pumped to the tailings pond. Xantate Z-6 and copper sulfide are used in the flotation process, with Pluriol F-203 used as a frother. The report states that the flotation process recovers 90% of the gold.
The sulphide concentrates and jig concentrates are stored in a cement lined pond before being reground in a 16-inch by 4.5-foot conical regrind mill to 70% to 80% minus 200 mesh. The concentrates are ground in cyanide solution, and the regrind mill discharges to a 36-inch Wheeler pan amalgamator. The discharge is then passed through a duplex 3-foot by 17-foot rake classifier, with the overflow transferred to the Pachuca tanks.
Cyanidation and Gold Recovery
There are 13 Pachuca tanks, and each tank is loaded with 5 to 6 tonnes of concentrate in cyanide solution. Air is passed through the mixture for 6 hours. A flocculant is then added to cause the concentrates to settle over a period of 7 to 24 hours. The pregnant solution (rich in gold) is then sent to the Merrill Crowe circuit to recover the gold. The process is repeated until the grade of the concentrate falls to 2 to 3 grams per tonne, and it is then discharged to the tailings pond. The report states that a total of 93% of the gold is recovered from the concentrates.
The pregnant solution is transferred to the clarification circuit, made up of two pre-clarifier tanks with the last tank having a filter system. The clarified solution goes into the vacuum tower for the extraction of dissolved oxygen. Metallic zinc powder, lead acetate and sodium cyanide are added to the solution, and the resulting precipitate is filtered out in 52 canvas sleeves. The barren solution is then recycled to the plant.
Doré gold is produced by using a retort to evaporate the mercury from the amalgam, adding the sponge gold that is produced to the cyanide zinc precipitate, and fusing the mixture in a diesel powered gold electric furnace to produce a doré gold bar containing approximately 83% gold and 12% silver. The doré bars are transported by armored truck to Lima for further processing.
Carbon-in-Pulp Circuit
The CIP circuit uses the same crushing facilities as the flotation circuit; however, the crushed ore is stored in a 35-tonne hopper. The 5-foot by 6-foot FUNCAL mill, used at the time as a high-grade circuit, is fed at a rate of 2.5 tonnes per hour, and the feed is ground in a cyanide solution. The material is classified by D-4 cyclones, with the oversize returned to a small 4-foot by 8-foot ball mill and the undersize discharged to a box for transfer to the CIP circuit. The ore is ground to 85% minus 200 mesh.
The circuit has five 20-foot by 20-foot agitated tanks and an estimated total capacity of 160 cubic metres of pulp. Only 4 tanks are used, and one of those is used as a conditioning tank. The tanks have a retention time of 35 hours, and the density of the activated charcoal used is 12 grams per litre of pulp. The report states that recoveries of up to 88% are achieved by this circuit. The plant was designed for the treatment of old tailings with an installed capacity of 100 tpd. At the time of the report it was used to treat oxidized ore at a rate of 40 tpd and operated only 16 hours per day.
The carbon is removed from the pulp by conventional screening, and desorption of the gold is carried out in a one-tonne capacity reactor. The gold is stripped from the carbon by a hot solution of 14% ethyl alcohol, 1% sodium hydroxide and 0.3% sodium cyanide, and it is then recovered from this solution by electrolysis. The process takes 26 hours and recovers 95% of the gold.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Mill processing rate (at time of report) | Up to 350 tpd | Actual operating data |
| Mill capacity (with refurbished ball mill, five active mills) | 500 tpd | Proposed design |
| Mill capacity (with additional flotation cells and upgrades) | 700 tpd | Proposed design |
| Flotation circuit gold recovery | 90% | Reported testwork result |
| Cyanidation gold recovery from concentrates | 93% | Reported testwork result |
| Flotation feed particle size | 45–50% minus 200 mesh | Actual operating data |
| Flotation feed pulp density | 1,280 g/l | Actual operating data |
| CIP circuit recovery | Up to 88% | Reported testwork result |
| CIP circuit installed capacity | 100 tpd | Original design |
| CIP circuit operating rate (at time of report) | 40 tpd | Actual operating data |
| CIP circuit retention time | 35 hours | Design parameter |
| Carbon density in CIP circuit | 12 g/l of pulp | Operating setpoint |
| CIP gold desorption recovery | 95% | Reported testwork result |
| Crushed ore product size (cone crusher) | 80% minus 0.5 inch | Design specification |
| Regrind mill product size | 70–80% minus 200 mesh | Operating setpoint |
| Doré gold content | Approximately 83% gold, 12% silver | Historical production data |
| 2010 total ore processed | 111,804 tonnes | Historical operating data |
| 2010 average ore grade | 6.45 g/t | Historical operating data |
| 2010 total gold produced | 19,447.75 ounces | Historical operating data |
| 2010 average mill efficiency | 83.88% | Historical operating data |
Project website: https://www.osmre.gov/news/2025/san-juan-mine-mod-approval
Technical qualifications
The report states that the mill currently processes up to 350 tpd, but with its refurbished ball mill bringing the active number to five, it has the capacity to expand to 500 tpd with refurbishment and replacement of its gravity equipment. To help achieve this increase, the classifier will probably need to be replaced with hydro cyclones. The report further states that the mill can be expanded to 700 tpd with the addition of a second bank of flotation cells, miscellaneous upgrades and a high-grade mill circuit. The company also planned to reconfigure the mill system so that the CIP system would replace the aging Pachuca tanks. No subsequent expansion, construction, or operational status beyond these proposed changes is reported in this document.
Source: San Juan Mine , 2010 Technical Report, sections 17.0 through 17.6.
