The historic Cauquenes and Colihues tailings impoundments are man-made deposits that were generated as sites for the containment of tailings produced during the processing of copper-molybdenum ore at the El Teniente mine owned by Codelco.

MVC currently receives approximately 130,000 tonnes per day of Fresh Tailings from El Teniente’s Colon concentrator. The Fresh Tailings are transported via gravity to MVC via a 36 km long concrete open channel launder.

MVC’s contract with El Teniente provides for the supply of Fresh Tailings up to the year 2037. Details are provided in Section 19.0. Every year, MVC receives an updated life of mine plan from DET and uses this plan to estimate the quantity and quality of Fresh Tailings available to be processed at MVC. Due to volumetric transport constraints in the tailings channel launder to the Caren impoundment, there is insufficient capacity to simultaneously process both Fresh Tailings and Cauquenes tailings at their maximum rates. Surplus Fresh Tailings material is planned to bypass MVC’s mill and be deposited into the voids created in the Colihues and Cauquenes impoundments.

A total of 796 million tonnes of Fresh Tailings at a grade of 0.111% Cu and 0.005% Mo is planned to be delivered to MVC’s processing facility.

Extraction of material from the Colihues deposit ceased in 2015 and is planned to recommence in 2031 when the Cauquenes deposit is depleted. Tailings in the Colihues deposit were extracted at a nominal rate of 35,000 tonnes per day via a hydraulic monitoring system using high pressure water guns operating at 30 bar. The flow of water at high pressure water is directed onto the surface of the tailings in a sweeping motion in such a way as to create a ditch on the surface. The monitors work horizontally and vertically, operated by remote control using a hydraulic system of electronic valves that allow them to rotate and advance. A single monitoring unit is capable of sustaining a production rate of 8,000 to 10,000 tonnes per day. Mining bench heights are 10m high and access ramps are created to relocate the hydraulic monitors. The depth of the Colihues tailings deposit is approximately 50 m requiring four mining benches. The slurry drains to the center of the sump by gravity and is channelled by the ditches made by the monitor. The slurry has a density of approximately 45% solids and is pumped to the processing plant. by means of vertical 150 kW sump pumps and 350 kW horizontal booster pumps, via 10” high density polyethylene (HDPE) pipelines. The operation of four hydraulic monitors and two sumps is necessary In order to sustain a production rate of 35,000 tonnes per day.

The first phase of the Cauquenes expansion project was completed in 2015 enabling extraction of material from the Cauquenes deposit into the existing MVC process facility. The processing rate from 2017 onward is planned to be 62,500 tpd and extraction of the Cauquenes deposit is anticipated to be complete at the end of 2030. Tailings are extracted from the Cauquenes deposit via a hydraulic monitoring system similar to Colihues. The general method of operation is as follows:
-Construct a sump in the tailings deposit and install vertical slurry pumps
-Commence extraction down to 10m using hydraulic monitors and allow slurry to drain to sump
-Pump tailings to the plant at a density of 43% solids
-Reposition the monitors sequentially away from the sump to advance the extraction

This procedure is repeated until the bottom of the tailings deposit is reached, approximately 50m deep. Safety zones are maintained near the walls to ensure the integrity of the walls.

The existing process plant at MVC recovers copper and molybdenum from Fresh Tailings and from tailings extracted from the Cauquenes deposit. The tailings slurry is processed in ball mills and flotation cells to produce a bulk copper concentrate. The copper concentrate is treated in the molybdenum separation facility to produce a molybdenum concentrate. The final copper and molybdenum concentrates are filtered, dried and bagged for shipment to smelters.

Processing of Fresh Tailings at a rate of 130,000 tonnes per day commences with a primary classification stage. Four cyclone clusters each containing five 500 mm diameter cyclones are used and the cyclone underflow reports to ball mills. The Fresh Tailings slurry has a particle size distribution of 80% passing 250-300 microns and approximately 25% of the Fresh Tailings cyclone feed material reports to the ball mills. The underflow from the primary classification stage reports to the primary mill feed box where flotation collector reagent is added and the pH is controlled at 9.5 via the addition of lime. There are four ball mills (14´ x 28´, 2,500 HP) using 1.5” steel grinding media. The discharge from each mill is pumped to four 500 mm diameter cyclones and the cyclone underflow is returned to grinding. The cyclone overflow from each mill (P80=120µ) reports to rougher flotation.

The primary cyclone overflow slurry contains lower grade fine material and oxide copper and this stream is processed in a scavenger cascade flotation circuit. This static, self-aerating, gravity assisted process consists of five lines of concrete cascade flotation cells each 1.80 m x 0.70 m providing 185 cascades in total. Low grade flotation concentrate from the cascade cells at approximately 0.4% Cu is recovered and pumped to the conventional flotation circuit. Feed to the cascade flotation circuit consists of Fresh Tailings primary cyclone overflow, Cauquenes tailings primary cyclone overflow and rougher tailings.

A separate grinding circuit is used to process the Cauquenes tailings, currently being processed at a rate of approximately 60,000 tonnes per day. The slurry with a particle size of 80% passing 210 microns is pumped to four cyclone clusters each containing five 500 mm diameter cyclones and the cyclone underflow reports to three ball mills. Approximately 65% of the Cauquenes tailings cyclone feed material reports to the ball mills for further attrition and particle size reduction. The primary cyclone overflow slurry reports to the secondary cyclones where the fine material is removed and sent to the cascade flotation circuit. The underflow from the secondary cyclones reports to a single ball mill for further attrition. The Cauquenes grinding circuit consists of four ball mills (two 16.5 x 18’, 3,250 HP, one 17’ x 28’ 3,500 HP and one 16.5’ x 22’ 3,500 HP) using 1.5” steel grinding media. In the grinding circuit, flotation collector reagent is added and the pH is controlled at 9.5 via the addition of lime. The discharge from each mill is pumped to four 400 mm diameter cyclones and the cyclone underflow is returned to grinding. The cyclone overflow from each ball mill (P80=100µ) reports to rougher flotation.

The product from the Fresh tailings grinding circuit is combined with the tailings from the first cleaner flotation and the concentrate from the cascade circuit and fed to rougher flotation. The Fresh tailings rougher flotation circuit consists of two rows of 4 x 127m3 conventional cells. The product from the Cauquenes tailings grinding circuit is fed to the Cauquenes rougher flotation circuit, consisting of two rows of 4 x 127m3 conventional cells. The combined rougher tailings are fed to the cascade scavenger flotation circuit. The combined rougher concentrate is fed to the regrind ball mill pump box where it is mixed with the second cleaner tailings and the mill circulating load. The concentrate is pumped to the closed regrind circuit consisting of a 12’ x 16’ mill with a 1,250 HP motor and a cyclone cluster with seven 400 mm polyurethane cyclones. The overflow (P80= 98µ) feeds the first cleaner circuit, composed of 26 x 41m3 conventional cells. The first cleaner concentrate reports to three stages of cleaner column cells to produce a final concentrate. A scavenger flotation circuit of 6 x 27m3 conventional cells processes the column tails and the scavenger tailings are fed back to the first cleaner circuit.

The final concentrates, containing 30% Cu and 1% Mo, are pumped to the molybdenum separation plant. The pulp pH is conditioned to an approximate value of 8.5 with sulphuric acid and a copper depressant reagent (primarily a phosphorous compound). Molybdenum concentrate is extracted from the bulk concentrate via a countercurrent selective flotation process. The molybdenum concentrate is leached with sulphuric acid to lower the phosphorus content remaining from the use of the copper depressant, in order to obtain a product of commercial quality. The final molybdenum concentrate containing 40% Mo is filtered, dried and packaged for dispatch in bulk bags. The final copper concentrate containing 28 to 30% Cu is filtered to a moisture content of 9% and stored in a warehouse before being sent to the Las Ventanas smelter via truck.

Final tailings from the cascade flotation circuit are delivered to three 100m diameter thickeners. The tailings are thickened to a density of 53% solids and are returned to the El Teniente tailings channel for delivery to the Carén impoundment. MVC is authorized to discharge up to 185,000 tonnes per day of tailings to the Carén tailings channel.