Ash gain due to lactic acid formation during carbonatation.

Hulett Refinery had been experiencing problems with higher than normal ash gains since the 1987/88 season. Investigations into the composition .ofthe as~ sho,,:ed the major cause of the ash gam to be an mcr~~se in calcium. It ~as proposed that the calcium was solubl1~sed by reaction WIth lactic acid to form calcium lactate WhICh was not removed during carbonatation. This proposal was later.c~)Ofirmed ~y laboratory carbonatations with controlled additions oflactic acid. A programme of lactic acid analysis was started and the results showed that the major cause of lactic acid formation was chemical destruction of reducing sugars under alkaline conditions during liming and not as a byproduct of microbial action as originally surmised. Laboratory ~i!Uu­ lations of the process were done under a range of conditions with temperature, time, lime addition and reducing sugars as the variables, and a model equation was developed from the laboratory results. This enable~ predictions ~o be ma~e of the extent of lactic acid formation under vanous conditions. Modifications of the process conditions based on this equation led to a reduction in lactic acid formation an~ a consequent reduction in the ash gain across the refimng process. Introduction Hulett Refinery processes approximately 2 000 tons of . VHP sugar per day using carbonatation, ion-exchange and crystallisation as the main refining tools. The products of the refinery are basically white sugar and molasses plus a small quantity of speciality products which may be considered as blends ofsugar and molasses. Soluble ash introduced into the refinery has the undesirable effect of increasing the solubility of sucrose in molasses, rendering an additional amount ofsucrose unrecoverable and causing a considerable financial loss. The quantity of sucrose lost due to ash in a Refinery may be estimated from the Rendement or Commercial Yield (Honig,2 Lyle,3 Meade and Chen) : Yield = Pol (4,5 X Ash + RS) . The references given have the ash factor ranging between 3 and 5 and the reducing sugar factor between I and 7. The particular values in the above formula have been found to be reliable in predicting the yield from raw sugar at Hulett Refinery and show that the unrecoverable s.ucrose due. to ash is 45 times the weight ofash. A conservative calculation shows that for every 10%ofash gained approximately R350 000 is lost per year. It was thus a cause for concern when Hulett Refinery began experiencing unacceptably high ash gains between the ash "in" in raw sugar and the ash "out" in sugar and molasses. Initially no positive link could be made with any likely cause except for the possibility that it was tied in to either the commissioning of the new ion-exchange plant or to low pH levels in remelting. Measures ~hat were !aken on these assumptions had no effect and so It was decided to form a project team to study the problem in a more systematic way. The team initially consisted of members from the refinery 217 and the Sugar Milling Research Institute (SMRI) who instituted an extensive sampling and analysis programme to identify the main constituents ofthe soluble ash and perform mass balances on these components. Later Tongaat-Hulett STD staff joined the project team, and using the data from the earlier work put the emphasis onto establishing why the ash gain was occurring and finding solutions. This paper gives an outline of the investigation programme and highlights the important findings that were crucial to the success of the project. Data from laboratory experiments are presented which confirm the results from the factory survey and enable a model to be derived to predict performance under various conditions. Finally a co~­ siderable reduction in the ash gain at Hulett Refinery IS shown from the time the recommendations from the project findings were implemented. Factory Investigations Ash Balance and Constituents It was necessary to establish which ash constituents were the major contributors towards the ash gain. The two most likely components were calcium and .sodium, becal;lse calcium is added directly as Ca(OH)2dunng carbonatation and sodium as NaCl is used in large amounts during the regeneration of decolourising resin. A programme of analysing weekly composite samples of the major refinery str~ams for calcium sodium and potassium was started. PotaSSIUm was included as it constitutes a large portion of the incoming ash. The average results covering an eight week period are given in Table 1 and a diagrammatic representation of the total ash and its composition change through the process IS shown in Figure 1. Table 1 Refinery ash balance Raw Brown Fine MolasSugar Liquor Liquor ses Tons Solids 204