Scum De-sweetening Process
The Three Stage Scum Process recovers sugar from Calcium Phosphate Scum produced by the Melt Clarification Process. This involves three consecutive stages of counter-current extraction, i.e., scum flow through the process is from the 1st to 2nd to the 3rd clarifier whilst dilution water flows from the 3rd to the 2nd to the 1st. The following simplified block diagram shows a typical example of the extraction likely to be experienced.
To achieve good flotation separation in the clarifiers, variations in process parameters must be minimised e.g. scum feed rate, dilution feed rate, flocculant feed rate and dilution water temperature.
Scum from the Melt Clarifier is fed to the scum feed tank. From here it is pumped up to the 1st stage mixing tank where it is diluted with clarified sweetwater from the 2nd stage clarifier, i.e., mid-brix sweetwater.
The scum and sweet water are thoroughly mixed together by the mixing tank agitator to produce a homogeneous solution. This solution then flows by gravity into the 1st stage clarifier. Prior to solution entry into the clarifier, air and flocculant are added.
The flocculant is dosed into the system by the dosing pumps from the holding tank.
The aerated stream is produced by recirculating clarified sweet water through the aeration pump which has a supply of compressed air fed into the eye of the pump impeller. This air is dissolved into the sweet water (due to the high outlet pressure) whereby it is re-introduced back into the process stream just prior to the addition of the flocculant.
The precipitate forms into large aerated flocs, which float to the surface of the clarifier to form a scum.
The scum is removed from the sweet water surface by a rotating scraper arm and discharged into the scum trough, whereby it is washed away with low brix sweet water to the 2nd stage mixing tank.
The clarified sweet water from the 1st stage clarifier flows by gravity to the sweet water tank at the melting station.
The processing steps and operation for the 2nd and 3rd stages are identical to that described as above for 1st stage.
However, when the 3rd stage clarification step is reached, the final scum is discarded whilst the dilution water feed is pumped into the system from the dilution water tank.
Lime sucrate solution is added to the second stage clarifier scum trough from where it flows into the third stage mixing tank and clarifier. This raises the pH of the sweet water and hence helps promote good flocculation. This sweet water from the third stage later passes to the second and finally the first stage. This helps raise the pH in all the stages.
The pH in all three clarifiers should be maintained above 7.