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Technology

White Sugar Mill Technology

Tongaat Hulett Sugar South Africa Ltd. (THSSA) in partnership with S A Bioproducts Ltd. (SABP) have developed and patented a technology known as WSM. WSM stands for White Sugar Mill and the patented technology is owned by UFION Pty. Ltd. which has a 50% equal shareholding by THS and SABP. The technology was developed to produce white sugar to EEC2 standard together with a high quality fermentation feedstock directly in a cane sugar mill, without the intermediate production step of crystallizing raw sugar.

To demonstrate the technology a WSM slipstream plant to handle 15% of the Felixton mill throughput, and produce white sugar was commissioned in 2006. The White Sugar Mill process as installed at FX is shown in the figure below

The White Sugar Mill process as installed at FX

The processing steps that differentiate the WSM process from a standard sugar mill are shown in green in the preceding flow diagram. The final products are refined sugar, fermentation feedstock (WSM molasses) and ultra filtration Retentate.

Ultra-filtration

The ultra-filtration stage removes suspended solids and high molecular weight compounds from the juice to prevent excessive fouling of the resin columns. Clear juice is evaporated to 25 Brix before being fed to the ultra-filtration plant, this concentration giving the best combination of flux rate, volumetric flow rate and pressure drop across the membranes. Both ceramic and stainless steel membranes are installed.

Heat Recovery & Juice Refrigeration

To minimise sucrose inversion by the cation resin, the juice is cooled to below 15C, which is accomplished in the refrigeration unit. A heat exchanger is used to heat up the cool juice exiting the decolourising ion exchange stage using the hot juice before refrigeration in order to improve thermal efficiency. Felixton uses a steam ejector set for this purpose.

De-mineralisation

De-mineralisation is carried out in two Simulated Moving Bed ion-exchange units, one containing a strong acid cation exchange resin and the other a weak base anion exchange resin. Each unit uses a multi-port distribution valve developed by THS which distributes the various process flows to 36 stationery resin vessels. A three pass system is used with cation exchange followed by anion exchange, the liquor flow crossing backwards and forwards between the cation unit and the anion unit.

The cation resin is regenerated with either Hydrochloric or Nitric acid at 7-9% concentration and the anion resin with Ammonia at 4% concentration. Apart from removing inorganic ionic species, the two de-mineralisation resins also remove at least 80% of the colour in the juice by a combination of ion-exchange and adsorption.

Final de-colourisation to produce refined quality sugar is done in an ion-exchange unit similar in design to those used for de-mineralisation. The resin used is a strong base resin in the chloride form and regeneration is with a caustic brine solution.

The typical colours of juices obtained from the WSM plant at Felixton are:

ProductColour ICUMSA
Clear Juice20 000
Permeate Juice18 000
Deashed Juice2 500
Decolourised Juice400

Benefits of WSM Technology

  • Sugar Recovery is improved.
  • The crystallisation rate from massecuites after WSM treatment is considerably higher as a result of higher purities and lower viscosities.
  • Cooling crystallisers are not needed for the white strikes.
  • There is significantly reduced Scaling in evaporators and pans due to the demineralisation of the juice.
  • A high quality molasses is produced.
  • The spent regenerants from the demineralisation can be used as fertilizer -Potassium is recovered from the juice and Nitrogen from the regeneration chemicals.

WSM Slipstream Plant

  • Capacity: 200 tons white sugar / day
  • Capital Cost: ~$10m