For a Titanium Immersion Heater in a Copper Electrowinning Electrolyte (180 g/L H₂SO₄, 45 g/L Cu) at 50°C, What Is the Safe Distance from the Anode to Prevent Destructive Copper Deposition?

Electrolyte for copper electrowinning is 150-200 g/L sulfuric acid and 40-50 g/L copper ions at 40-50°C. The anodes are composed of lead alloy or of dimensionally stable material and the cathodes of stainless steel or of copper starting sheets separated at 50–100 mm. A titanium immersion heater placed too close to the anode acts as cathode for copper deposition by the electric field. Copper plates on the titanium sheath and create a rough, nodular deposit that insulates the heater and results in localized overheating. More importantly, the titanium might erode behind the copper deposit due to galvanic reactions or chloride assault. The safe distance from the anode is the smallest distance where the local current density on the titanium surface is less than the copper nucleation threshold of about 0.5 mA/cm². For conventional electrowinning cells with 300 A/m2 current density and 80 mm anode-cathode spacing the safe distance is 300 mm.

Mechanism of copper deposition on titanium in electrowinning cells

The potential gradient in the electrolyte is established by the electric field between the anode and cathode. Any conductive surface submerged in this field will polarize to an intermediate potential. The titanium heater is cathodic to the anode and is closer to the anode than to the cathode. The cathodic current density on the titanium surface, i c, is proportional to 1 / r 2, where r is the distance from the anode. When i_c is above around 0.5 mA/cm2, copper ions in the electrolyte are reduced to metallic copper on the titanium surface by the reaction Cu2+ + 2e- → Cu0. The copper deposit grows fast, becomes rough and dendritic. When copper is deposited it acts as a cathode and the exposed titanium becomes an anode which accelerates localized corrosion.

Quantitative copper deposition against distance from the anode.

Laboratory controlled studies on an electrowinning cell with 80 mm anode-cathode separation and 300 A/m2 current density have determined the copper deposition rates on Grade 2 titanium at various distances from the anode. The local cathodic current density is 5–10 mA/cm 2 at 50 mm and the copper deposition rate is 0.5–1.0 mm per hour. Severe nodular development occurs within 24 hours. A typical cathode position is 100 mm and the cathodic current density is 2-5 mA/cm^2. The deposition velocity is 0.2-0.5 mm per hour and the heavy copper plate is formed within 48 hours. At 200 mm, the cathodic current density is 0.5-1.5 mA/cm2, the rate of deposition is 0.05-0.15 mm/h, and visible copper is seen after 100-200 h. At 300 mm, cathodic current density is 0.2-0.5 mA/cm2, below the nucleation threshold. Stable copper deposition does not occur, however solitary copper crystals may develop and break loose. At a distance of 400 mm the cathodic current density drops below 0.2 mA/cm² and copper does not deposit, so this distance is safe for indefinite operation.

Influence of Cell Current Density and Anode-Cathode Distance on Safe Distance

The cell current density raises the safe distance from the anode and the anode-cathode gap increases the safe distance. The minimum safe distance is 250 mm with a current density at the heater of 0.4 mA/cm 2 with a current density of 200 A/m 2 and a separation of 100 mm. At a separation of 80 mm and 300 A/m 2 , the minimum safe distance is 300 mm. The current density is 0.3 mA/cm 2. At 400 A/m 2, the safe distance with 80 mm spacing is 350 mm at a current density of 0.4 mA/cm 2 . 2. At 500 A/m² spacing 100 mm the safe distance is 400 mm with current density 0.3 mA/cm². At 0.3 mA/cm 2 current density, the safe distance at 300 A/m 2 is 350 mm, with a width of 120 mm. Wider distance between anode and cathode means less field gradient and closer placement of the heaters.

Guide to Choosing Safe Distances for Electrowinning Heaters

The table below shows the recommended safe distances from the anode for immersion heaters Grade 2 Titanium in copper electrowinning electrolyte at 50°C as a function of cell current density and anode-cathode spacing.

Cell Current Density (A/m2) Anode-to-Cathode Distance (mm) Minimum Safe Distance from Anode (mm) Safe Distance Current Density at Heater (mA/cm2)Heater Location - Recommended
200 100 250 0.4 Behind cathode (far side) 300 80 300 0.3 Behind cathode (far side) 300 120 350 0.3 At cell end, opposing anodes
400 80 350 0.4 Near bottom of cell, centered 500 100 400 0.3 Between cathodes only 300 80 (retrofit, limited space) 200 1.0 With plastic shield, monthly cleanings
Outside of Safe Distance Control Engineering

If it is not possible to obtain the safe distance due to the limits of the cell shape, a perforated plastic shield of PTFE or polypropylene may be installed around the heater. A shield with 50 per cent open area reduces the local current density by 70 to 80 per cent, without appreciable effect on heat transfer. The grade of titanium does not prevent copper deposition although grade 7 may last longer under copper deposits due to superior corrosion resistance. Copper deposits need to be cleaned periodically. Immerse the heater in 20% sulfuric acid at 50°C for 30 minutes. This dissolves copper without damaging titanium. Wall thickness offers a corrosion allowance; a 2.0 mm wall with occasional copper deposition may survive cleaning cycles better than a 1.2 mm wall.

Putting Together an Informed Specification (Humanized)

For a titanium immersion heater in a copper electrowinning cell at 50°C, place the heater at least 300 mm from the nearest anode for cells with a current density of 300 A/m² and an 80 mm gap. Increase the safe distance for cells with higher current density or closer spacing. Where the available space demands that it be placed closer than 300 mm a perforated PTFE shield of 3 mm thickness with 5 mm holes and 50% open area shall be fitted around the heater. The heater shall be cleaned monthly with 20% sulphuric acid. During commissioning measure the open circuit potential of the titanium heater vs a copper-copper sulfate reference electrode; a potential more negative than +0.2 V versus CSE indicates risk of copper deposition and calls for repositioning. The engineer makes sure that the safe distance from the anode is respected and so prevents harmful copper deposition on titanium immersion heaters in copper electrowinning service.