Evaporation and crystallisation

Not every metal leaves solution by plating or precipitation. A large family of products — sulfates, carbonates, chlorides, hydroxides as crystals — is recovered by exploiting solubility directly: concentrate the solution until the salt can no longer stay dissolved, and it crystallises out. This is how lithium carbonate, sodium sulfate, alumina trihydrate and a long list of bulk and specialty salts are won, and it is the recovery route whenever the product is a crystal rather than a cathode.

Concentrate, then crystallise

Two levers bring a salt out of solution. Evaporation removes water, raising the dissolved concentration until it passes saturation and the salt begins to deposit; it is the lever when the solution is dilute or the salt’s solubility barely moves with temperature. Cooling lowers the solubility of most salts, so a hot, near-saturated solution drops crystals as it cools; it is the lever when solubility falls steeply with temperature. Many circuits use both — evaporate to approach saturation, then cool to crystallise — and recycle the mother liquor to recover the salt left dissolved. The crystalliser is designed around which lever dominates, and around growing crystals large and pure enough to filter and sell.

Retrograde solubility

The usual rule is that a salt is more soluble hot than cold — which is why cooling crystallises. A few important salts break it: their solubility falls as temperature rises, so they are less soluble hot than cold. This is retrograde solubility, and it inverts the design. For such a salt you crystallise by heating, not cooling — the solution drops its load as it warms. Anhydrous sodium sulfate above its transition, calcium sulfate and lithium carbonate all show it, and it is the property that makes hot-precipitation and the lithium bicarbonation loop of the previous module work. Recognising which way a salt’s solubility runs with temperature is the difference between a crystalliser that works and one that fights its own chemistry.

The evaporator duty is a heat load

Evaporation is bought with energy: every tonne of water driven off carries its latent heat away, and that heat is the evaporator duty, a thermal load the plant must supply and then often reject again when the concentrate is cooled. It is sized like any other heat duty — a mass flow, a specific heat or latent heat, and a temperature or phase change — which is why a crystallisation circuit is also a heat-transfer problem, with multiple-effect evaporators and vapour recompression deployed precisely to cut that standing energy cost. The worked thread lands the framing on the heat-duty calculator: the sensible-heat duty to raise a stream to its evaporation temperature is the same Q̇ = ṁ × Cp × ΔT that sizes any heater, and the sodium- and ammonium-sulfate hubs hold the property data for two salts recovered exactly this way.