This report updates the processing and recovery methods for the Gaocheng silver-zinc-lead project, describing the current flowsheet, operating parameters, and testwork basis.
Report context
The NI 43-101 Technical Report Update on the Gaocheng Ag-Zn-Pb Project in Guangdong Province, People's Republic of China, was prepared for Silvercorp Metals Inc. The report includes information on processing operations from trial operations starting in 2013, commercial production from 2015, and a process flowsheet optimization in 2019. The report presents design parameters, historical operating data, and metallurgical testwork results.
Processing route
Process overview
The overall process consists of crushing, grinding, sequential flotation of lead, zinc, and pyrite concentrates, and concentrate dewatering by disc filtration. An experimental tin recovery gravity separation circuit is installed on pyrite flotation tails.
The process flowsheet is very similar to the process adopted in the closed-circuit flotation tests described in Section 13.4.3 of the report. The tin recovery circuit is also shown in the flowsheet.
In 2019, the process was optimized by changing from zinc-sulphur priority flotation to zinc-sulphur mixed flotation followed by zinc-sulphur separation flotation. The quantity of ore processed has increased to around 300 ktpa.
Crushing
The crushing circuit consists of a run-of-mine ore bin from which ore is drawn by a vibratory feeder into the primary jaw crusher. The crusher product is screened on a vibrating screen, with -10 mm material conveyed to the fine ore bin and +10 mm material feeding a secondary cone crusher via a buffer storage bin. The fine ore bin has a capacity of 1,600 t.
Run of mine ore at -350 mm is reduced to crusher product at -10 mm.
Grinding
Two-stage grinding in ball mills achieves a product size of 80% passing 75 µm. The grinding circuit is sized for 1,600 tpd, with four mills with 400 kW motors installed. The circuit consists of a grate-discharge ball mill in closed circuit with screw classifier, followed by an overflow ball mill in closed circuit with hydrocyclones. The circuit is configured in two parallel trains, each of 800 tpd capacity.
The primary mills have weightometers fitted to feed conveyors linked to variable speed belt motors for mill feed control.
Flotation
The flotation circuit is configured in two parallel trains, following the grinding circuit configuration. Flotation cell sizing is adequate for 1,600 tpd, with rougher residence time designed to be a minimum of 15 minutes plus scavenger time of 15 minutes. Conditioning times of approximately five minutes apply.
The flotation process is a standard flotation of lead with three-stage cleaning, followed by zinc flotation with three-stage cleaning, leaving pyrite tailings as sulphur concentrate.
Concentrate handling
Concentrates are thickened and filtered on ceramic disk filters sized at 9 m², 15 m², and 30 m² for lead, zinc, and pyrite concentrates respectively. Filters are positioned above the concentrate storage shed for direct discharge. Concentrates are loaded by front-end loader into trucks for transport to smelter customers.
Tin recovery circuit
The experimental tin recovery circuit treats pyrite flotation tailings. After pre-concentration on eight sets of four-start spiral concentrators, the stream is cycloned to split at 75 µm. The +75 µm fraction is fed over 25 coarse shaking tables, and the -75 µm material is fed over 51 fine shaking tables. A final batch flotation stage removes residual sulphides.
Tin quantities produced to date are not material to overall mine economics.
Reagents
Reagent storage and mixing is located adjacent to the grinding and flotation plant. Reagents include:
- Depressants and modifiers: sodium sulphide, zinc sulphate, sodium sulphite, copper sulphate
- Collectors: di-ethyl dithiocarbamate, ammonium dibutyl dithiophosphate, butyl xanthate
- Frother: no. 2 oil (added directly)
Lime usage is large at 8 kg/t, with separate lime storage and milk-of-lime mixing area. Sulphuric acid tank and dosing pumps are separately located for safety reasons.
Process control and automation
Process control includes a central control room in the grinding-flotation building with TV imaging of key operating points, centralized monitoring of equipment run status, on/off interlocking of main crushing and grinding flows, and measurement and control of key parameters including ball mill feed tonnage, bin and tank levels, densities, flotation cell pulp levels, and reagent dosage. Automatic sampling covers key metallurgical accounting streams including flotation feed, concentrates, and tailings.
Ancillary facilities
The laboratory is equipped with sample preparation, wet chemistry, and basic photometric analytical equipment, as well as crushing, grinding, flotation, and gravity-separation metallurgical testing equipment. Routine analyses of ores and concentrates are conducted, along with water quality and environmental testing.
Maintenance workshops are equipped with craneage, welding, and basic machine-shop capabilities.
Key reported parameters
| Parameter | Unit | Value | Basis |
|---|---|---|---|
| Ore processed | ktpa | ~300–310 | Actual, from 2019 |
| Daily throughput | tpd | 900–950 | Actual |
| Design feed rate | tpd | 1,000 base / 1,600 expansion | Design |
| Grinding product size | µm | P80 75 | Design |
| Crusher product | mm | -10 | Design |
| ROM feed top size | mm | -350 | Design |
| Operating days | days/year | 330 | Design |
| Total installed power | kW | 5,043 | Design |
| Actual power drawn | kW | ~3,657 | Actual |
| Annual power consumption | kWh | 28,963,000 | Actual |
| Fresh water requirement | m³/t feed | ~0.4 | Design |
| Total water demand | L/day | ~3,200,000 | Design |
| Water demand at 1,600 tpd | L/day | 6,000,000 | Projected |
| Water recycle | % | ~90 | Actual |
| Lime consumption | kg/t | 8 | Actual |
| Feed head grade, Ag | g/t | 88–98 (LOM average 94) | Planned |
| Feed head grade, Pb | % | 1.26–1.69 (LOM average 1.48) | Planned |
| Feed head grade, Zn | % | 2.95–3.72 (LOM average 3.22) | Planned |
| Flotation rougher residence time | minutes | 15 min minimum | Design |
| Flotation scavenger time | minutes | 15 min | Design |
| Conditioning time | minutes | ~5 | Design |
Project website: https://silvercorpmetals.com/silvercorp-issues-updated-technical-report-for-the-gc-mine/
Technical qualifications
The report identifies several limitations and qualifications regarding processing information:
- There was no comminution testwork to serve as a basis for the crushing and grinding circuit design.
- Flotation testwork culminating in the closed-circuit test provided an adequate basis for flotation process design.
- Copper-lead separation was investigated but considered not viable by the qualified person, with limited Cu resource data available.
- A tin recovery circuit was considered to have potential merit but was not included in the base case for operations.
- The tin recovery circuit is described as experimental, and tin quantities produced to date are not material to overall mine economics.
- The qualified person considered necessary availability and utilization factors for 1,600 tpd throughput to be reasonable and in line with normal mining industry practice.
Source: NI 43-101 Technical Report Update on the Gaocheng Ag-Zn-Pb Project in Guangdong Province, People's Republic of China, Silvercorp Metals Inc., sections 17.1–17.8.
