This report details the processing route and recovery methods for the Gaocheng Ag-Zn-Pb project, based on design, testwork, and operating data.
Report context
This technical report update on the Gaocheng Ag-Zn-Pb Project in Guangdong Province, People’s Republic of China, for Silvercorp Metals Inc., is dated 2012. The report describes recovery methods for the Ying Property, incorporating metallurgical testwork outcomes, trial operations that commenced in 2013, and commercial production that began in 2015 at the Gaocheng mill.
Processing route
Introduction to recovery methods
Prior to operations, the silver mineralogy indicated an opportunity to increase silver recovery from all species, including sphalerite and pyrite, to the lead concentrate, within constraints of minimum % Pb specifications, with implications for lead cleaner circuit and filtration capacity. No comminution testwork was available to serve as a basis for the crushing and grinding circuit design. The flotation testwork culminating in the closed-circuit test provided an adequate basis for the flotation process design.
Circuit options investigated included copper-lead separation and tin recovery. Although both were included in the GMADI Design Instructions, neither had been included in financial modelling. AMC considered the copper-lead separation not to be viable and of limited materiality. AMC believed that a tin recovery circuit had potential merit and was considered an opportunity and a material circuit option, although the base case for operations did not include it.
Since the start of trial operations in 2013 and commercial production in 2015, lead and zinc concentrates have been produced in commercial quantities at the Gaocheng mill. Some small amounts of tin concentrate and sulphur have also been produced, but these quantities have not been material to mine economics.
Process flowsheet
The process flowsheet is shown schematically in Figure 17.1 of the report, being very similar to the process adopted in the closed-circuit flotation tests. No significant alterations have been made to the plant since completion of commissioning, and it has processed approximately the same amount of ore each year (around 260 ktpa).
Process description summary
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.
Crushing
Two-stage crushing is carried out, with the second stage in closed circuit. Run of mine ore at -350 mm is reduced to crusher product at -10 mm. 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 jaw crusher product is screened on a vibrating screen, with -10 mm fines conveyed forwards to the fine ore bin while +10 mm material feeds the secondary cone crusher via a buffer storage bin to maintain choke feeding. The fine ore bin has a capacity of 1,600 t.
Grinding
Two-stage grinding in ball mills achieves a product size of 80% passing 75 µm (P80 of 75 µm). The two-stage grinding circuit is sized for 1,600 tpd. Given that 1,600 tpd could be the ultimate throughput, four mills with 400 kW motors have been installed. Typical of Chinese practice and conforming to the design used at Silvercorp's Ying mine, the grinding 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.
Flotation
The flotation process consists of a standard flotation of lead, with three-stage cleaning of the lead concentrate, then flotation of zinc concentrate with three-stage cleaning, leaving pyrite tailings as sulphur concentrate. Following on from the grinding circuit, the flotation circuit is similarly configured in two parallel trains. 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 general layout makes use of gravity and the sloping site terrain.
Concentrate handling
The respective concentrates are thickened and then filtered on ceramic disc filters. The filters are sized at 9 m², 15 m², and 30 m² for the lead, zinc, and pyrite concentrates respectively. The filters are positioned above the concentrate storage shed for direct discharge, from which 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 an initial pre-concentration stage on eight sets of four-start spiral concentrators, the stream is cycloned to split at 75 µm. The +75 µm size fraction is fed over 25 coarse shaking tables, and the -75 µm material is fed over 51 fine shaking tables. The final step is a batch flotation stage to remove residual sulphides, which takes place in a small unit in the main flotation building. Tin quantities produced to date are not material to overall mine economics.
Process control and automation
Process control and automation consists of 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 system flows, and measurement and control of key parameters including ball mill feed tonnage, critical bin and tank levels, critical densities, flotation cell pulp levels, and reagent dosage. Automatic sampling of key metallurgical accounting streams is also employed.
Key reported parameters
| Parameter | Unit | Design/Proposed | Historical Operating | Testwork Basis |
|---|---|---|---|---|
| Overall throughput base case | tpd | 1,000 | approximately 800 | , |
| Potential expansion throughput | tpd | 1,600 | , | , |
| Annual throughput | tpa | , | 264,000 | , |
| Operating days per year | days | 330 | 330 | , |
| Crushing operating hours | hrs/day | 18 | , | , |
| Grinding-flotation operating hours | hrs/day | 24 | , | , |
| Feed rate | tph | , | 42 | , |
| Daily utilization of time | % | , | 80 | , |
| Crushing product size | mm | -10 | , | , |
| Grinding product size (P80) | µm | 75 | 75 | , |
| Flotation rougher residence time | mins | 15 minimum | , | 15 minimum |
| Flotation scavenger residence time | mins | 15 | , | 15 |
| Conditioning time | mins | approximately 5 | , | approximately 5 |
| Fine ore bin capacity | t | 1,600 | , | , |
| Installed power (total) | kW | 5,043 | , | , |
| Actual power drawn | kW | , | approximately 3,657 | , |
| Annual power consumption | kWh | , | 28,963,000 | , |
| Total water demand (including recycle) | L/day | , | approximately 3,200,000 | , |
| Water demand per tonne ore | m³/t | , | 4 | , |
| Fresh water requirement | m³/t feed | , | approximately 0.4 | , |
| Water demand at 1,600 tpd | L/day | 6,000,000 | , | , |
| Lime consumption | kg/t | , | 8 | , |
| Lead concentrate filter area | m² | 9 | , | , |
| Zinc concentrate filter area | m² | 15 | , | , |
| Pyrite concentrate filter area | m² | 30 | , | , |
| Number of primary mill trains | , | 2 | 2 | , |
| Capacity per train | tpd | 800 | , | , |
| Mill motor power (each) | kW | 400 | , | , |
Project website: https://silvercorpmetals.com/ying-mining-district/
Technical qualifications
The following specific limitations apply to the recovery methods information in this report:
- There was no comminution testwork to serve as a basis for the crushing and grinding circuit design.
- The flotation testwork culminating in the closed-circuit test provided an adequate basis for the flotation process design.
- Copper-lead separation was considered not viable by AMC, with only limited copper resource data to support any copper recovery process.
- The tin recovery circuit is experimental, and tin quantities produced to date are not material to overall mine economics.
- The report states that the recovery methods used are appropriate for the ore characteristics, and that the flowsheet has demonstrated ability to achieve targeted recoveries and concentrate grades.
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., 2012, Section 17 Recovery Methods.
