Rosin sizes deposits

Calcium carbonate cannot be used in the same low pH papermaking process that contains alum and rosin size. The presence of carbonate in an acid papermaking process causes severe problems such as foam, excess alum demand, inefficient sizing conditions with rosin size, and troublesome deposits. The alkaline system, therefore, enables the paper-maker to use calcium carbonate as a primary filler as well as to use more recycled paper. 

Barium sulphate is also becoming a rather common scale, particularly in closed pulp mills. When it occurs, the problem of its formation is exbemely troublesome as it is a salt with very low solubility. Because barium sulphate is a salt of such low solubility, barium sulphate scale may form even if barium concenbation, relative to other cations, is low, particularly when concentration in the system of sulphate ions is high. Barium is dissolved from the wood in the digester. The sulphate ion comes into the system through additives such as alum, and high sulphate ion concenbation may develop as a result of accumulation of the sulphate ion from papermaker s alum (used in the rosin sizing process) or from sulphuric acid used for pH conbol. A particular problem is Barium sulphate deposition in the headbox of Kraft liner machines. 

The main reason for deposits with rosin size is due to the presence of alkalinity or high pH. The Na and Mg + caused by the alkalinity react with rosin size, forming small floes which are no longer cationic in nature, as stated previously. They are thus less likely to be retained, but grow to form macroflocs which can be deposited in the stock approach system on the wire. These macroflocs being organic in nature are often associated with oil from defoamers and antifoams and, of course, with any fillers that are in the system. 

Deposits can also occur at high pH due to a lack of AF species with alum. Alum may have reacted with rosin, but there is not sufficient charge on the particle to retain it. It then will be deposited in the stock approach system as basic aluminium rosinate. This is aluminium rosinate where the cationic part of the aluminium has been neutralised by the hydroxyl ion. This tends to occur more with soap sizes than with rosin sizes as the alum needs to react with the rosin soap and then be retained. With rosin dispersions this effect is less. 

These rosin-based sizes, whether paste, Hquid, or emulsions, can be used to size all grades of paper that are produced at acid pH. The latter include bleached or unbleached kraft Hnerboard and bag paper, bleached printing and writing grades, and cylinder board. In addition, polyaluminum compounds have been used in place of alum, most notably, polyaluminum chloride (48), which can reduce barium deposits where these have been a problem. The barium chloride by-product is more water-soluble than barium sulfate. Other polyaluminum compounds such as polyhydroxylated forms of alum and polyaluminum siHcosulfate have been evaluated as alum replacements. 

With ASA, good first pass retention of ASA is paramount to good sizing efficiency, as any ASA not retained in the first pass can be easily hydrolysed in the white-water system. This is because the ASA dispersion is not as stable in the wet-end as that of rosin, or AKD, as these products are designed to be stable for a period of months. This hydrolysis product can react with calcium and/or magnesium to form sticky salts, which can deposit and cause many problems. If aluminium is not added to the ASA dispersion, then it should be added to the white-water system to react with any unretained, hydrolysed, ASA to prevent the formation of these salts, by formation of the less/non-sticky aluminium salt. There are appUcations of ASA that do not use aluminium species, where first pass ASA retention is optimised, but these tend to be in the minority. 

All of these effects are enhanced by other effects, such as high stock temperature. The lack of alum may also cause deposits at the presses and dryers, as the size precipitate will not be firmly anchored to the fibre and fines and can be removed, at both press and dryers, with fines and fillers. If there is also a chemical imbalance in the system, the fines and fillers can be removed at the presses. In this case, there will always be a high level of fmes/fiUer to rosin ratio. 

The reasons for deposits with synthetic sizes are much the same as for rosin, although the scale of the deposits tends to be smaller due to the lower levels of chemicals anployed and, in general, better retention of these sizes. 

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