The Cerro Lindo processing plant employs conventional crushing, grinding, and flotation to produce separate copper, lead, and zinc concentrates, with tailings thickened and filtered for backfill or dry stacking.
Report context
This article is based on the Nexa Resources S.A. Cerro Lindo Mine Technical Report NI 43-101 dated January 29, 2021 (Project #3246). The processing plant is located on a ridge adjacent to the mine at an altitude of 2,100 MASL to 2,200 MASL. The plant commenced operations in 2007 with a processing capacity of 5,000 tpd and has since been expanded to a name-plate capacity of 21,000 tpd.
Processing route
Crushing
Crushing is carried out in three stages, with the first stage occurring underground. A primary jaw crusher, fed via a stationary grizzly, crushes ore to less than 100 mm. Crushed ore is conveyed to coarse ore stockpiles located at the processing plant. When the primary crusher requires maintenance, ore is trucked directly to surface, stockpiled, and then crushed in a mobile primary crusher that discharges onto the coarse ore stockpile feed conveyor.
Coarse ore is reclaimed from the stockpile and fed to two parallel crushing circuits, each consisting of secondary and tertiary crushing. Reclaimed ore is screened, with oversize reporting to the secondary crushers and undersize directed to the fine ore bins. Secondary crusher product is screened, with screen oversize reporting to the tertiary crushers and undersize directed to the fine ore bins. The tertiary crushers are in closed circuit with the tertiary screens, with screen undersize directed to the fine ore bins. Ore is crushed to 80% passing (P80) 4 mm, and the two fine ore bins provide approximately 16 hours of storage capacity.
Grinding
Fine ore is fed from the fine ore bins to two parallel ball mill circuits, each in closed circuit with high frequency classifying screens, at a rate of approximately 900 tph. Each grinding circuit also includes flash flotation, producing bulk (copper and lead) concentrate. The first grinding circuit is the original single-stage ball mill circuit installed in 2007 and capable of processing 5,000 tpd. The second circuit consists of two identically sized ball mills in series and can process approximately 15,000 tpd. Grind size for the circuits is P80 150 µm to 170 µm.
Flotation
Flotation consists of bulk rougher and scavenger flotation to produce a copper-lead concentrate, which is then cleaned and combined with the flash flotation bulk concentrate prior to being separated into copper and lead concentrates. The bulk flotation tails form the feed to zinc rougher and scavenger flotation to produce a zinc concentrate, which is then cleaned. The three concentrates are thickened and filtered, then deposited into dedicated concrete storage bunkers.
Concentrate is reclaimed by front-end loader, and each bucket is sampled before being loaded into trucks. The trucks are weighed on a weigh bridge adjacent to the concentrate handling area before being despatched to the Port of Callao (copper and lead concentrates) or Nexa’s Cajamarquilla refinery (zinc concentrate) near Lima, Peru. Concentrate is despatched during the night shift with an average of 32 trucks per night, each carrying approximately 35 wet tonnes of concentrate.
Tailings
Final tailings consist of zinc scavenger tails. Tailings are directed to the tailings thickener, and the thickened underflow is divided, with a portion going to the two paste backfill plants and the remainder going to the dry stack tailings filtration plant. The ratio between tailings to paste backfill and dry stack tailings is approximately 50:50.
The tailings filtration plant and paste backfill plants are located between the dry stack TSFs to the south of the concentrator. Horizontal belt filters are used to reduce the moisture of the tailings to approximately 12%. Dry stack tailings are discharged onto a stockpile, which is reclaimed by front-end loader and trucks for subsequent placement, grading, and compaction on one of the two dry stack TSFs. Cement and fly-ash are added to the paste backfill tailings filter cake, producing a paste of approximately 79% solids. This is pumped underground by high-pressure positive displacement pumps.
A concrete spillage containment pond with a capacity of 10,000 m³ is located below the plant platform and tailings thickeners, providing emergency containment if needed.
As mining has progressed, the pumping distance of the paste has increased, leading to limitations in the amount of paste that can be pumped to the furthest points in the mine, in addition to placing strain on the pumping and piping system. This in turn means that dry stack tailings filtration is fully utilized. Any breakdowns in the paste backfill system therefore result in throughput reductions in the processing plant, as there is no tailings surge capacity other than limited space in the tailings thickener. Mitigating steps will be necessary to ensure that paste backfill system limitations do not become significant processing rate constraints in the future. Nexa plans to install an additional belt filter for dry stack tailings filtration, which will help to minimize the effect on processing plant throughput in the case where the paste backfill system is down.
Concentrate quality
The Cerro Lindo concentrates contain low concentrations of deleterious elements and higher than average concentrations of the primary metals. The lead and zinc concentrates are clean and do not currently incur penalty charges. Due to the combined lead and zinc content of the copper concentrate (approximately 4.8% to 5.6%), however, the copper concentrate attracts a small penalty of approximately US$2.00/t.
Process water
Water is supplied from a reverse osmosis desalination plant located on the coast capable of producing 60 L/s and is pumped 60 km to the mine site. This is sufficient to supply the requirements for make-up water and potable water (treated at the mine site). Most of the process water requirement is recovered from tailings thickening and filtration and is returned to the three 3,600 m³ water storage tanks. Approximately 90% of total tailings water is recovered and recycled to the Cerro Lindo processing plant as process water.
Process consumables and power
The main process consumables include steel grinding balls, sodium cyanide, lime, and various flotation reagents. No significant changes to the feed ore, process, or capacity of the processing plant are anticipated, and therefore unit consumption of these materials is expected to remain similar to historical consumption rates. The same applies to electrical energy consumption, which is supplied from the national grid.
Manpower
The processing plant personnel comprises management and supervisory staff, including metallurgical personnel, and operators, totalling 108, as well as contractors amounting to 42. Maintenance personnel and contractors number 101 and 399, respectively. These numbers are not anticipated to change significantly in the foreseeable future.
Key reported parameters
| Parameter | Value | Basis |
|---|---|---|
| Plant location altitude | 2,100 MASL to 2,200 MASL | Design/actual |
| Original processing capacity (2007) | 5,000 tpd | Historical operating data |
| Name-plate capacity | 21,000 tpd | Design |
| Crushing product size | P80 4 mm | Design/actual |
| Fine ore bin storage capacity | Approximately 16 hours | Design |
| Grinding circuit feed rate | Approximately 900 tph | Historical operating data |
| Grind size | P80 150 µm to 170 µm | Design/actual |
| First ball mill circuit capacity | 5,000 tpd | Design (installed 2007) |
| Second ball mill circuit capacity | Approximately 15,000 tpd | Design |
| Tailings to paste backfill vs dry stack ratio | Approximately 50:50 | Historical operating data |
| Dry stack tailings moisture after filtration | Approximately 12% | Design/actual |
| Paste backfill solids content | Approximately 79% | Design/actual |
| Spillage containment pond capacity | 10,000 m³ | Design |
| Copper concentrate penalty | Approximately US$2.00/t | Historical operating data |
| Copper concentrate combined lead+zinc content | Approximately 4.8% to 5.6% | Historical operating data |
| Reverse osmosis plant capacity | 60 L/s | Design |
| Water pipeline distance | 60 km | Design |
| Water storage tank capacity (each) | 3,600 m³ | Design |
| Tailings water recovery | Approximately 90% | Historical operating data |
| Concentrate trucks per night | Average 32 | Historical operating data |
| Concentrate truck load | Approximately 35 wet tonnes | Historical operating data |
| Processing plant personnel | 108 | Historical operating data |
| Processing plant contractors | 42 | Historical operating data |
| Maintenance personnel | 101 | Historical operating data |
| Maintenance contractors | 399 | Historical operating data |
Project website: https://ri.nexaresources.com/operations/cerro-lindo/
Technical qualifications
The report notes that as mining has progressed, the pumping distance of the paste has increased, leading to limitations in the amount of paste that can be pumped to the furthest points in the mine. This places strain on the pumping and piping system. The report states that any breakdowns in the paste backfill system result in throughput reductions in the processing plant, as there is no tailings surge capacity other than limited space in the tailings thickener. The report indicates that mitigating steps will be necessary to ensure that paste backfill system limitations do not become significant processing rate constraints in the future. Nexa plans to install an additional belt filter for dry stack tailings filtration, which will help to minimize the effect on processing plant throughput in the case where the paste backfill system is down.
Source: Nexa Resources S.A., Cerro Lindo Mine, Technical Report NI 43-101, January 29, 2021, Section 17 Recovery Methods.
