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A flexible approach manages effluent water volumes and excess salt for a steel mill.

Columbus stainless steel factory

THE CHALLENGE

PROXA was tasked with managing both the effluent water volumes and excess salt on a major steel mill, while meeting budget constraints. What’s more, the client required a Zero Liquid Discharge (ZLD) plant in order to avoid any future liabilities and meet environmental legislative requirements.

OUR SOLUTION

PROXA committed to working with the commissioning client to value-engineer a project execution philosophy that would meet Columbus Steel’s long term objectives, while being mindful of their short term budget constraints.

As such, PROXA executed the project in 3 phases:

  1. The first phase included the design, supply, installation and commissioning of a forced circulation evaporator designed with 2 heating legs. This enabled Columbus to initially install 50% of the heating capacity, while the vapour body was designed for the full future load.
  2. The second leg was installed a few years later once capacity on site increased and additional funds became available. To maintain performance of the evaporator a small purge was required, which was temporarily stored on site.
  3. The third phase included the design and installation of a small purge crystalliser, treating the remaining purge and meeting the client’s objectives of achieving a ZLD plant.

HOW PROXA ADDED VALUE

Our flexible approach to design and value engineering enabled us to meet both long term objectives and short term site budgetary constraints. We also achieved energy efficiency through the use of 2 sets of 2-stage mechanical vapour recompression fans. And, before the client committed to the full scale plant, the process for the purge crystalliser was developed on bench scale and piloted. What’s more, we implemented a unique process generating 2 solid and 2 liquid products, all of which offered value to the customer. In short, a true ZLD plant. We also attained plant longevity through the use of high grade materials such as stainless steel 904L and titanium grade 2 and 12. The purge crystalliser was manufactured in a variety of materials such as Avesta 254SMO, FRP, titanium and carbon steel.

FEED WATER

  • Pickle liquors from a stainless steel production process containing HF, HNO3, SO4 and some Cl
  • The main cation is Fe, with some H and Na

PRODUCT WATER

  • The primary effluent treatment yields iron hydroxide precipitates, containing calcium fluoride.
  • The products of the ZLD crystalliser include:
  • Calcium sulphate cake – for resale to local farmers
  • Sodium nitrate crystals – for resale to the explosives industry
  • Calcium nitrate solution – for resale to the fertiliser market
  • Clean process condensate – to be recycled within the mill.

PERFORMANCE TARGET

96 000 kg/hr water evaporation after expansion to 4 fans.

PROCESS

  • Solution pre-treatment
  • Forced circulation evaporator
  • 2-Stage mechanical vapour recompression fans
  • Double effect forced circulation crystalliser
  • Filter press for removal of calcium sulphate
  • Cooling crystalliser for sodium nitrate
  • Centrifuge to remove sodium nitrate
  • Maturation tanks for storage and post precipitation of calcium sulphate

PROCESS RELIABILITY

The plant has been in continuous operation since 2004 and is meeting all client’s key performance objectives.


Client: Columbus Stainless Steel

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