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Thickeners Improve Production at Cerro Pasco: A Peruvian plant experienced a dramatic increase in capacity through the use of thickeners.

Imagine doubling plant throughput from 1,150 metric tons per day (mt/d) to almost 2,500 mt/d, and perhaps as much as 3,000 mt/d within a reasonable timeframe and at an acceptable cost, all the time working hand in hand with a trustworthy contractor. That would be a plant manager's dream. It happened recently in Peru.

The oxide plant at the Cerro Pasco operation, which is owned by Volcan Compama Minera, consists of a series of leaching and metal recovery systems. It had a designed capacity of 2,500 mt/d. Until June 2015, the throughput of the Volcan oxide plant, however, was limited to 1,150 mt/d due to its filtration process. To achieve the desired design throughput, engineers decided to upgrade the existing tailings washing stream, a counter current decantation (CCD) circuit, by adding CCD thickeners as fourth and fifth stages to improve the recovery of the dissolved metals and reduce the cyanide content in the feed to the detox stage, thus increasing metal revenue and decreasing operating costs for the plant.

A local firm, Bisa (Buenaventura Ingenieros S.A.) carried out the full design and engineering of the original Cerro Pasco oxide plant. Bisa and Volcan launched a tender for the supply of four drum filters, which was awarded to FLSmidth. However, after an initial commissioning and startup phase, the design turned out to be far from optimal as the filters were creating bottlenecks because they were undersized and the design criteria were incorrect. FLSmidth proposed a redesign and replaced the filters with two thickeners instead of the filters.

A Breakthrough Solution

FLSmidth's Process Engineer Walter Gamero described the breakthrough solution implemented at Cerro Pasco's oxide plant. "At the time, the main concerns were the length of the lead times," Gamero said. "To sort this out, pilot testing was performed using paste thickeners to replace the filters."

"The testing proved successful and FLSmidth had the gear ready to be supplied, so the decision was made to install two paste thickeners," Gamero said. Working under an engineering, procurement and construction (EPC) contract valued at $10.8 million, the project was fasttracked and completed in seven months.

"The fast track project consideration required a technical office on-site, and from the first day since day Cerro Pasco and Bisa worked simultaneously with our engineering team, by providing topographic and soil information to complete and release the first package of drawings," Gamero said. "This would allow our procurement team to start a bidding process for the steel fabrication supply, activity that was part of the critical path. In parallel, the site erection team had to be assembled, recruiting top-notched technically skilled personnel, with the necessary capabilities to work safely within a high-risk environment, that included extreme altitude and weather conditions."

The scope of contract included a complete detail engineering; high-level execution plan and technical support for the CCD circuit extension; full procurement, including tanks, agitators, piping bulk, pumps, structural streel bulks, MCC, control system, electrical and instrumentation; bulk materials handling; main plant equipment; construction and pre-commissioning/commissioning; and advisory on the actual operation for two months.

The detail engineering confirmed the outstanding accuracy of the calculations during the proposal stage, allowing FLSmidth to maintain the original specifications for the main and ancillary equipment prior to procurement.

Once mechanical and electrical installation were completed, the next two stages, i.e., pre-commissioning and commissioning, were critical to guarantee a smooth and proper plant startup and the related ramp-up stage, as well as to mitigate damage risks in the installed mechanisms and peripheral equipment during operations.

After completing the required conditions during the commissioning process to have both CCD thickeners working, the plant started operations on schedule.

Once the upgrade was implemented, the throughput improved by a staggering 60% and the plant now has a processing capacity of 2,500 mt/d, and the subsystem implemented by FLSmidth could allow it to increase to as much as 3,000 mt/d.

Taking It to the Next Level

The standard cyanide leach process consists of grinding the ore, then mixing the ground ore-water slurry with sodium cyanide and enough quick lime to keep the pH of the solution at about 11. At a concentration of 45% solids, the slurry is pumped to a series of agitated mixing tanks sized to provide the required residence time. The metal-bearing liquid then is separated from the leached solids in a decanter, and the tailings are washed to remove metal and cyanide before disposal. The separation and washing of these tailings takes place in a series of units by a process referred to as CCD. The liquor is progressively pumped back into the plant from the solids in a counter-current fashion.

Volcan currently processed the decanter underflow through a three-stage CCD circuit and is adding fourth and fifth stages (CCD 4 and CCD 5) to improve the recovery of dissolved metal.

"By counter current washing, the dissolved metals--cyanide compounds --are recovered to the maximum extent before the solids are pumped to the Detoxification circuit for the destruction of the residual cyanide before discharge," Gamero said. Maximizing the recovery of cyanide back into the plant reduces the consumption of cyanide for leaching and also reduces Detox chemical consumption, thus reducing the plants overall operating costs.

The flocculant dosing system consists of bulk storage of flocculant crystals, feeder, eductor, and three preparation and storage tanks. "Raw water is added at the eductor where it mixes with the flocculant crystals and feeds the three compartment preparation with storage tanks," Gamero said.

Beyond normal operating conditions, each thickener can be bypassed and operated separately. "CCD 4 can be bypassed and the feed from the existing CCD 3 can be directed to CCD 5 Mix Tank," Gamero said. "CCD 5 overflow can be sent to the Process Water Tank via a by-pass line. If CCD 5 is not in operation, CCD 4 can still operate. For this operating condition, CCD 4 underflow slurry is sent to the re-slurry tank and raw water is added directly to CCD 4 Mix Tank to provide optimum feed solids density."

Following the completion of the project, FLSmidth continues to work with Volcan in various optimization projects to help improve the productivity of the plant. Currently, a pilot upgrade project is being implemented in the plant--not at industrial scale yet--consisting of replacing raw water for acid water coming from an old tailings pond after going through CCD 4 and CCD 5, to obtain a slurry that will go to the detox stage. This upgrade was designed to avoid sulphur release caused by the contact between pyrite and air/water.

By Oscar Martinez, Latin American Editor

Caption: FLSmidth installed two CCD thickeners to replace four filters that were undersized for the mine's oxide plant design.
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Title Annotation:DEWATERING
Author:Martinez, Oscar
Publication:E&MJ - Engineering & Mining Journal
Article Type:Report
Geographic Code:3PERU
Date:Oct 1, 2017
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