Raw Sugar Melting
Melting is simply the process of completely dissolving the Raw Sugar so that is can then be purified by the processes described earlier, all of which, apart from Affination require the sugar to be in solution. If an affination station is installed then the melting is placed directly after it, otherwise it is the Melting that is the first process in line.
Melting will be followed by Phosphatation, it is important that the melting produces a fully dissolved sugar at a defined brix and temperature. It is important that there is a control system that measures the brix of the sugar leaving the melter and the dosing of sweet water and fresh water is controlled to maintain the correct density. Phosphatation will operate at a fixed brix of 65. If there is a problem filtering the sugar, it is usual to reduce the brix since this will reduce the viscosity.
Similarly Phosphatation run at high temperature, around 85 - 90oC and this has to be controlled to maintain good operation .It is usual to operate theMelting at around 70C and do the rest of the temperature increase in the next process since this will minimise the time at high temperature and reduce the risk of sugar loss.
It is normal to have a continuous Melter rather than Batch Melters and there would be an initial coarse brix control to make the solution down to 67 brix and then fine brix control to give 65 brix to an accuracy of +/- 0.5 brix. The coarse brix control requires an approximate weight of Raw Sugar so that a controlled weight of sweetwater can be added.
It is usually necessary to screen the sugar liquor before sending it forward to Phopho Clarification Process. It is possible to use a vibrating screen.
Steam is used for heating the Melter Liquor up to 70C. Use of direct injection of steam since it is simpler than a coil and condensate recovery and dilution of the sugar is required anyway.
The water used to melt the sugar will predominantly be sweetwater from Scum Desweetening and any Decolourising Processes plus any return streams such as dust and lumps from the Drying Process. The design will be to use these sweetwaters first and only add fresh water if these is not available. It is important that sweetwater is kept hot – above 70C otherwise microbial contamination will take place, which will drop the pH and cause sugar loss through inversion. A small steam coil or steam blowers are usually fitted to the Sweetwater Tank.
Raw Sugar Weighing
The important point to consider for Melting is whether the sugar should be accurately weighed into the Refining Process. In order to control the brix, there will have to be at least a simple weighing system to initially apportion the sweetwater at the right ratio to the sugar.
If it is considered important to accurately measure the sugar input into the Refinery, then standard way of weighing the sugar into the Process is with an automatic batch weighing scale. This is a basket on load cells that weighs a measured weight of sugar on a regular repeating cycle.