Page 511 - Industrial Wastewater Treatment, Recycling and Reuse

Simulation, Control, and Optimization of Water Systems in Industrial Plants 481

Evaluating impact of flow upgrades/cuts on boiler feed water tempera-
ture and availability.
Estimating penalty in delaying cleaning of boiler feed water heaters.

12.3.2.3 Caustic Loss Reduction
Reducing dilution by 30% by transferring some of the heat load in slurry
heating from direct to indirect heating; balancing with capital cost required
for additional heat exchanger/area.
Optimizing evaporation throughput.

12.3.2.4 Product Quality
Guaranteeing fill temperature for precipitation.
Guaranteeing required temperature after slurry heating for better
desilication.

12.3.2.5 Water Recovery
Minimizing cooling water in cooling of green liquor to precipitation by
improving heat recovery in heat interchange.
Reducing solids and liquor carryover and piping wear in flash tanks.

12.3.3 Water Use in the Bayer Process
It can be seen that the alumina industry is fairly water intensive and also gen-
erates a large amount of waste. Typically, an alumina refinery consumes
2–2.5 tons of raw water per ton of alumina produced (Martin and Howard,
2011). There are opportunities, therefore, for water reuse, depending on the
(caustic) contamination level in the water. Zero wastewater discharge can be
the ultimate goal. A refinery water balance is presented in Figure 12.6.
About 10% of the total water intake is accounted for by free moisture in
the bauxite feed and in the 50% caustic soda solution added as makeup.

12.3.3.1 Water Demand
Water is required for mud and hydrate washing. Cooling water is required,
notably in the barometric condensers in evaporation and interstage coolers
in precipitation, which is an exothermic reaction. In addition, water is used
in pumps as packing and purge water, in other auxiliary equipment, for
cleaning, and of course as potable water. There is also considerable water
loss around the plant.