Electrowinning Cell

Overview

An electrowinning cell uses electric current to plate dissolved metal out of solution as solid sheets. It is the last step in a solvent extraction and electrowinning, or SX-EW, plant. Leaching dissolves copper from ore into a pregnant solution, solvent extraction concentrates and purifies that copper into a clean strong electrolyte, and electrowinning passes direct current through the electrolyte so copper deposits on cathodes while oxygen evolves at anodes. The result is cathode copper of 99.99 percent purity, ready to sell, won directly from ore without smelting. The same cell design wins zinc, nickel and cobalt with changed chemistry and operating points.

A cell is a long lined tank holding rows of alternating cathodes and anodes hanging in acidic electrolyte. Current enters each anode, drives the electrode reactions, and leaves through each cathode where metal grows. Cells are wired in series into a tankhouse with hundreds of cells, all fed from one transformer-rectifier. Over a six to seven day cycle copper builds to a thick sheet on each cathode, then the cathodes are pulled and the copper stripped off, and the cycle repeats.

The cell tank

The Cell Tank holds everything. Its Tank Body is polymer concrete, which takes hot sulfuric electrolyte without the corrosion plain concrete would suffer, protected further by an Acid Liner. Along each top edge an insulating Capping Board carries and spaces the electrode hanger bars and isolates them from the tank, while an Overflow Weir weir sets the electrolyte level and spills spent solution back to recycle. The geometry fixes the gap between every electrode, because uneven gaps make uneven deposits and short circuits.

Cathodes and anodes

Metal grows on the Cathode Set. Modern cells use a permanent Cathode Blank of 316L stainless that copper plates onto and is later mechanically peeled from as a sheet, replacing the old practice of starter sheets. Each blank hangs from a copper-cored Hanger Bar that also carries its current, with an Edge Strip down each side and an Bottom Strip that stop the deposit wrapping the edges so it peels cleanly. Facing them are the Anode Set, rolled Anode Plate sheets of lead-calcium-tin alloy where oxygen evolves; the alloy resists corroding into the copper, helped by a Cobalt Feeder that doses cobalt to slow lead attack. Each anode hangs on a Anode Hanger and is held parallel by Anode Guide spacers.

Power and busbars

The cell runs on low-voltage high-current DC from the DC Rectifier. A Step-down Transformer drops line voltage to a few volts per cell, a Thyristor Stack rectifies and regulates the current, and the output runs through a DC Busbar measured by a Current Shunt, with Rectifier Cooling carrying off the rectifier losses. Inside the tankhouse the Busbar System system distributes that current: an Intercell Busbar links the cathodes of one cell to the anodes of the next, hanger bars seat on a triangular Contact Rail rail for low resistance, an Busbar Insulator keeps the busbar off the structure, and an Current Equalizer evens current sharing across electrodes. At 10-30 kA per cell, even a poor contact wastes power and heats up.

Electrolyte and acid mist

A Electrolyte Circulation system keeps the cell fed and warm. A Feed Pump driven by a Servo Motor on Ball Bearing supports pushes rich electrolyte through a Distribution Header along the cell bottom, metered by a Flow Meter, with a Heating Element holding the bath at 40-50 C where deposition runs well. Oxygen evolving at the anodes lifts a fine sulfuric mist off the surface, so the Acid Mist Control system caps the cell with a floating Bead Blanket that breaks rising bubbles, doses a surfactant from a Suppressant Doser, and draws any escaping mist into an Extraction Hood through a Mist Fan to a scrubber. Acid mist control protects both workers and the tankhouse steel.

Control and operation

The Control Panel panel runs the deposition. Its PLC Controller sets rectifier current to hold the target current density, and a Cell Voltage Scanner reads each cell voltage so a short circuit, where a nodular deposit bridges to an anode, shows up at once as a low cell voltage and is cleared before it ruins the sheet. An LCD Panel shows the tankhouse, Relay interlocks guard the rectifier, and a Power Supply feeds the loop. Current density sets the trade between speed and quality: push it too hard and the deposit grows rough and nodular and traps electrolyte, lowering purity. A typical copper cell runs six to seven days, after which a crane lifts the cathodes, a stripping machine peels two copper sheets off each blank, and the blanks go straight back into the cell for the next harvest.