The Pumping Cost Problem in Leaching Circuits
Hydrometallurgical leaching circuits move aggressive acid solutions through kilometers of piping, reactors, and heat exchangers. Pumping costs represent a major operating expense. Every pressure drop across a heat exchanger increases the energy bill.
Metallic heat exchangers develop internal surface roughness as corrosion progresses. What begins as a smooth tube interior degrades into a pitted, scaled surface with friction factors multiples higher than design values. Pump discharge pressure rises to maintain flow. Energy consumption climbs. Eventually the circuit cannot achieve design flow rates without pump upgrades.
The problem compounds over time. Corrosion roughens surfaces. Rough surfaces increase friction. Higher friction demands more pumping energy. The feedback loop accelerates until maintenance intervention replaces the exchanger.
Surface Roughness and Friction Factor
Pressure drop in heat exchanger tubes follows the Darcy-Weisbach relationship. For turbulent flow, friction factor depends on surface roughness relative to tube diameter. A new stainless steel tube with 0.0015mm roughness has a Darcy friction factor of approximately 0.019. After two years in acid leaching service, surface pitting increases roughness to 0.05mm, nearly doubling the friction factor.
The downstream consequence is increased pump head requirement. A circuit designed for 3 bar pressure drop through the heat exchanger at installation may require 5 bar after corrosion roughens the surface. The pump must work harder, consuming 60-70% more energy for the same flow.
Table 1: Surface Roughness and Pressure Drop Comparison in Leach Circuit Heat Exchangers
| Tube Material | Initial Roughness (mm) | Roughness After 2 Years (mm) | Relative Pressure Drop Increase |
|---|---|---|---|
| SS 316L in HCl leach | 0.0015 | 0.05 - 0.10 | 60 - 90% |
| Titanium in H₂SO₄ leach | 0.0015 | 0.02 - 0.04 | 30 - 50% |
| PTFE (virgin) | 0.0013 | 0.0013 | 0% |
Data from published pipe friction tables and field measurements in copper and nickel leaching operations.
The PTFE Smooth-Bore Advantage
PTFE tubing has an initial surface roughness of approximately 0.0013mm, comparable to drawn metal tubing. The critical difference is that this roughness never increases. PTFE does not corrode, pit, or scale. The internal surface after five years of acid service is identical to the surface on day one.
The low surface energy of PTFE provides an additional operational benefit. Scale-forming compounds that adhere to metal surfaces tend to slough off PTFE. Calcium sulfate, metal hydroxides, and other precipitates find fewer nucleation sites on the fluoropolymer surface.
Pressure drop across a PTFE heat exchanger remains stable throughout its service life. Pump sizing calculations made at installation remain valid years later. Energy consumption for pumping does not drift upward. Circuit flow rates stay at design values without incremental pump speed increases.
Layout Flexibility Reduces System Pressure Drop
Long-run leaching circuits often require heat exchangers in locations constrained by existing structures, tank arrangements, and piping runs. Metallic exchangers with fixed shell-and-tube configurations force piping compromises that add bends, elbows, and extra length.
Flexible PTFE immersion coils can be configured to fit directly into existing tanks or channels, eliminating external shell-and-tube exchangers and their associated piping pressure drops entirely. The coil simply immerses in the leach solution. The only pressure drop is the internal steam or hot water circuit, which is minimal.
When external PTFE heat exchangers are required, lightweight fluoropolymer bundles simplify support structures and reduce the need for heavy pipe stands and expansion loops common with metal exchangers in long runs.
Field Observations from a Copper Leach Operation
A copper heap leach solvent extraction plant replaced two shell-and-tube stainless steel heat exchangers with PTFE immersion coils placed directly in the pregnant leach solution tanks. The stainless exchangers had developed internal roughness over three years, increasing circuit pressure drop from 2.8 bar to 4.6 bar.
Pump energy consumption had risen 55% over the same period. Post-replacement, circuit pressure drop returned to 2.7 bar with PTFE coils. Pump energy consumption dropped back to near-original levels. Annual pumping electricity savings alone recovered 40% of the PTFE coil capital cost in the first year.
Summary
Corroded metallic heat exchangers increase pressure drop in leaching circuits through progressive surface roughening. Higher friction demands more pumping energy, raising operating costs continuously until replacement.
PTFE heat exchangers maintain their initial smooth surface indefinitely. Pressure drop stays constant. Pump energy consumption does not drift. The flexibility of immersion coil configurations can eliminate external exchanger pressure drops entirely by placing heat transfer surface directly in process tanks.
For leaching operations evaluating heat exchanger replacement, engineering analysis is available upon submission of current circuit pressure drop data, flow rates, leach solution chemistry, and available heating medium specifications.
