The first stage at which filtration plays an important part is at the time of first racking. This filtration step is usually carried out using kieselguhr as the filter medium and its purpose is to remove heavy metals and improve chemical stability. Typical addition rates for kieselguhr in this application are 40 to 120 grammes per hectolitre, depending on the condition of the unfiltered wine. A plate and frame press which is fitted with Carlson's high performance W2N backing sheets is used to contain the kieselguhr. The sheets are normally pre-coated and typical pre-coat rate in this application is 500 grammes per square metre.
Specially designed for this type of application, W2N sheets offer a number of benefits to the wine producer:
- Because W2N sheets are manufactured from pure cellulose incorporating carefully chosen wet strength resins within the cellulose matrix, the sheet has a high wet strength and is very durable. This allows the sheet to be used time and time again, so minimising costs.
- The pore size distribution of the sheet is carefully controlled so that there is even build up of the filter cake on the sheet. This ensures that filter capacity is maximised and that there is minimum risk of bleed through of even the finest of fines, whilst minimising the pressure drop through the filter.
- W2N sheets are very tolerant of pressure transients during the filtration cycle because the formulation used in their manufacture, coupled with the manufacturing process produces an extremely durable yet flexible sheet.