Recovery systems
The following systems are used.
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Evaporation unit
Evaporation of the water generated in the chemical reaction between the viscose and the spin bath.
To remove the water that is generated during the spinning process in the spin bath, the diluted bath is fed to an evaporation unit.
To minimise the specific steam consumption, a direct expansion evaporator is used.In this type of evaporator, the spin bath is heated by several preheaters (tubular heat exchangers) and two heating elements fed with fresh steam, and then cooled through several expansion stages.
Most of the vapour is extracted by means of mixing and secondary condensers and operated with cooling water.
Most of the vapour produced during expansion is however used to preheat the spin bath in the preheater.
The concentrated spin bath is transferred to a thick bath feed tank from where it is fed back into the spin bath circulation system for chemical treatment and subsequent use in the spinning process.
The excess of concentrated spin bath solution is processed in the crystallizer to be converted into sodium sulphate.
The design of this avaporation process ensures low specific steam and cooling water consumption and trouble-free operation.
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Crystallizer
Crystallisation of the produced sodium sulphate in the form of Glauber's salt (Na2SO4 x 10HO)
To recover sodium sulphate from the spin bath, the solution concentrated in the evaporator is transferred to a crystallizer.
It is designed as a vacuum crystallizer in which the spin bath is continuously cooled by expansion in a vacuum.
The concentrated/standard spin bath is first fed through a precooling unit where its temperature is significantly reduced.
Subsequent cooling with crystallisation takes place in the crystallization drum at a saturation/crystallization point of approx. +9°C.
The vapours produced in the process are removed from the individual crystallisation drum units and condensed or absorbed in the connected condensers by means of a cooling medium.
Gases and vapours that cannot be condensed are released through steam jet units and precipitated and condensed in the downstream mixing and auxiliary condensers operated with cooling water.
The remaining gases are removed from the system by means of a vacuum pump, whereby the exhaust gases are fed to a downstream recovery system.
The salt crystals produced in the crystallization drum are removed from the individual drum stages by built-in air coils.
The salt sludge is transferred to the evaporator and processed in centrifuges where the Glauber's salt is separated from the spin bath.
The spin bath is returned to the evaporator where the residual Glauber's salt is removed.The recovered Glauber's salt in the centrifuges is transferred through a melting vessel for further processing in the calcination unit.
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Calcination unit
Processing of Glauber's salt to anhydrous sodium sulphate (Na2SO4) that can be sold
To produce high-quality sodium sulphate, the Glauber's salt must be dehydrated. This is done by processing the Glauber's salt solution in a calcination unit consisting of an evaporator crystallizer, heat exchangers operated with fresh steam and recirculation pumps.
The already concentrated Glauber's salt solution is circulated by the recirculation pumps along the heat exchanger, which are heated with fresh steam. In the evaporator crystallizer, the liquid phase is released in the form of vapour that is extracted and used as a heating medium elsewhere in the process (e.g. for the operation of a preheater in the calcination unit).
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Salt dryer
The concentrated sodium sulphate suspension flows across the overflow of the evaporator crystallizer to the evaporator and from there to screen centrifuges where the crystals are mechanically separated from the liquid phase.
When a residual moisture content of approx. 3.5% w/w is reached, the sodium sulphate crystals are transferred by a conveyor system to the dryer.
