Copper use and extraction has been in existence since 6000 BC. However, as “pure” copper (80-100%) became scarcer as in-ground pure deposits were used up, smelting and leaching of copper ore became the methods of copper production. Due to the concentrations of copper in the mined ores and the pollution concerns surrounding smelting, copper heap leaching has evolved to the point where today, using the “Electrowinning” process, mined ore concentrations of 0,3 to 2,0% can be converted into pure copper directly. The heap leach process covers a very large expanse of land and the continuous production time is months and years.
In the heap leach mining procedure there is a maxim, “We don’t get it if we don’t wet it”, and this is exactly what the mine operation focuses on as detailed below.
The ore is carefully extracted, crushed and spread – to a depth of 10 metres – over an environmentally contained area of some 100 000 square metres. The crushed ore is then evenly irrigated with a sulphuric acid solution to dissolve the copper from the ore for eventual collection at cathodes located within the field. The distribution or percolation of the acid through the ore is done through 300 mm PVC feeder pipes that are laid the length of the field, with smaller distribution lines branching off of the main line. The distribution lines are laid parallel and equally spaced to ensure proper dissemination of the acid throughout the field. This is where measuring the acid’s flow rate becomes critical. Too high a flow and the acid will form a “tunnel” through the ore instead of distributing evenly and “wetting” all the ore. Too low a flow, and the process becomes excessively time consuming and uneconomical. Therefore, it is important to select a flow metering technology that can meet or exceed the metering requirement, and be economical.
Cobre del Mayo
In this operation, the flow measuring devices chosen were Alia battery operated electro-magnetic flowmeters (magmeters). A battery operated magmeter was selected because of its high accuracy, wide turn-down ratio, low maintenance life cycle, long field life and ease of installation (no external power supply line required).
The sensor housing and process connections are constructed of 316 stainless steel for corrosion resistance. The inside of the sensor is PTFE lined with Hastelloy ‘C’ measuring electrodes. Due to the fact that the actual pipe is non-conductive CPVC, and the sensor requires grounding contact with the process, the sensor included an integrally mounted earthing electrode to ensure faultless measurement continuity. (FYI – 90% of magmeter problems can usually be traced to grounding/earthing problems.)
The transmitter, similarly having no corrosive carbon steel components, is remotely mounted and battery powered. It can be configured for power saving by only transmitting the flow rate when it changes by a predetermined operational margin. The transmitter’s internal RS-485 output was then connected to a solar powered IEEE 802.11 local wireless network transmitter, which broadcasts the flow signal to a central control room where it is monitored.
In summary, the Cobre del Mayo mine is a successful operation producing 99,9% pure copper. It is expanding the measurement and control network throughout its operation, because as the instrumentation maxim goes: “One cannot effectively control or manage a process which is not accurately and reliably measured”.
For more information contact Martin McLeman, Blanes Pressure Solutions, +27 (0)11 422 1749, [email protected], www.pressuresolutions.co.za
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