Heating and cooling duties in hydromet

Hydrometallurgy runs warm. Leaching goes faster hot, evaporators boil water off, electrowinning and crystallisation reject heat, and autoclaves run at temperatures and pressures that demand heat in and heat out. Across a circuit these are the heating and cooling duties, and they are sized with the same handful of relations whatever the metal. This closing topic gathers them, because thermal load is a running cost and a piece of equipment wherever it appears.

Where the loads arise

Heat enters and leaves a circuit at recognisable places.

• •Leach heating: most leaching is faster hot, so the leach feed is heated and the tanks held at temperature — a sensible-heat duty on a large slurry flow.
• •Evaporator duty: driving water off to concentrate or crystallise a salt is a large latent-heat load, as the evaporation topic showed.
• •Electrolyte cooling: a tankhouse generates ohmic heat, so the electrolyte is cooled to hold its temperature in the control window.
• •Crystalliser cooling: cooling crystallisation removes heat to bring a salt out of solution as the solubility falls.
• •Autoclave heat: pressure leaching adds heat to reach temperature and removes it from the exothermic reactions, often recovering it by flashing.

Sizing a duty

A sensible-heat duty — heating or cooling a stream without a phase change — is sized by the same relation everywhere: the heat rate is the mass flow times the specific heat times the temperature change, Q̇ = ṁ × Cp × ΔT. Supply the stream’s flow, its specific heat and the temperature rise or fall, and the duty falls out in kilowatts. A phase-change duty — evaporating or condensing — uses the latent heat instead of Cp × ΔT, and a real heater often carries both: a sensible pre-heat to the boiling point, then a latent load to evaporate. Sizing the duty is the first step; it is what tells you how big a heater, cooler or exchanger the stream needs.

The heat-exchanger tie

A duty is met by a heat exchanger, and once the duty is known the exchanger is sized from it: the duty, the temperature driving force between the two streams, and an overall heat-transfer coefficient give the area required. This topic lands on the heat-duty calculator for the sensible duty itself, on the air-cooled heat-exchanger sizing calculator for the case where a stream is cooled against ambient air — common for electrolyte and crystalliser cooling — and on the subcooling calculator for the degrees of cooling below a saturation point that a condensing or refrigeration-side duty is specified by. The worked thread runs the heat-duty relation exactly, on the calculator’s committed example.