Page 510 - Industrial Wastewater Treatment, Recycling and Reuse

480 Industrial Wastewater Treatment, Recycling, and Reuse

Alumina trihydrate (gibbsite) from the green liquor crystallizes in the
precipitation area, with the fine crystals acting as seed and the coarse material
removed as product hydrate.
This is washed and filtered, and its water of hydration is removed by
calcination, resulting in the final primary product, smelter grade alumina.
The liquor leaving the precipitation process, now depleted of alumina
and therefore called spent liquor, continues in the liquor circuit and returns
to digestion via the heaters in the heat interchange building and evaporation
to remove excess liquid due to dilution (added for washing in clarification).
Steam is used to heat up the liquor in evaporation, which is then let down in
a series of flash tanks and heat exchangers, producing condensate.

12.3.2 Application of Simulation in the Bayer Process

In order to reduce capital and operating costs, increase production, and
improve product quality, an organization draws up a portfolio of opportu-
nities to be implemented, depending on business and process conditions at a
particular site. Process models assist in prioritizing and refining these oppor-
tunities by resolving the tradeoffs between capital, energy, and raw materials
using the levers of process topology, operating conditions, and equipment
design, accounting for the interactions between various proposed solutions.
Here, we mention some of the opportunities in the Bayer process that have
benefited from process modeling.

12.3.2.1 Heat Recovery
Reducing boiler energy usage by 2% by designing an appropriate boiler feed
water heater network, resulting in hotter boiler feed water and balancing
with capital cost.
Absorbing 50–80% of excess energy in digester slurry by installing an
additional heater in the last stage of the flash train and balancing with
capital cost.
Improving the approach temperature in the first heater in the digester
train through more frequent cleaning, 1 C translating into an energy sav-

ings of $100,000 per annum and balancing with maintenance cost.

12.3.2.2 Optimizing Spent Liquor Return Temperature
Optimizing boiler-house/plant steam balance; improving energy utilization
by substituting low-grade vapor in place of high-grade vapor where possible,
thus increasing availability of high-grade vapor for power generation.