Sodium-based spent liquors

Table 3.5s4 shows the retention of sodium base spent sulfite liquor by a series of UF membranes. The multistage UF was carried out in succession from the largest to the smallest pore size-i.e., the permeate from the first stage became the feed for the second stage. Although 90% of the sugar passed through membranes between 100,000 and 20,000 MWCO, the retention increases for 10,000 and 500 MWCO membranes up to 36%. It has been estimated that about 76 of the sugars are bound in complexes with molecular weights above 10,000 and an additional 10% in complexes above 500 daltons. In other words, A of all the monosaccharides present in spent sulfite liquor are bound. On the other hand, 100,000 MWCO membranes retained over 50% of the lignosulfonates. It appears (from Table 3.5)M that about 70% of the lignosulfonate compounds have a MW over 50,000 only 7.2% had a MW less than 10,000.

Table 3.5 UF of Sodium Base Spent Sulfite Liquor-Retention of Sugars ...

The spent sodium-base sulfite liquor is burned under reducing conditions, as in the kraft process, yielding a smelt of sodium sulfide. 

Chemical recovery ia sodium-based sulfite pulpiag is more complicated, and a large number of processes have been proposed. The most common process iavolves liquor iaciaeration under reduciag conditions to give a smelt, which is dissolved to produce a kraft-type green liquor. Sulfide is stripped from the liquor as H2S after the pH is lowered by CO2. The H2S is oxidized to sulfur ia a separate stream by reaction with SO2, and the sulfur is subsequendy burned to reform SO2. Alternatively, ia a pyrolysis process such as SCA-Bidemd, the H2S gas is burned direcdy to SO2. A rather novel approach is the Sonoco process, ia which alumina is added to the spent liquors which are then burned ia a kiln to form sodium aluminate. In anther method, used particulady ia neutral sulfite semichemical processes, fluidized-bed combustion is employed to give a mixture of sodium carbonate and sodium sulfate, which can be sold to kraft mills as makeup chemical. 

After conversion to the use of sodium-based chemicals, spent liquor could be incinerated, and sulfur dioxide, sodium sulfate, carbonate, or sulfide could be recovered. These compounds could be sold for use at nearby kraft mills or for other industrial uses. 

Reducing smelting furnaces that produce a high-sulfidity, kraft-like green liquor are now employed at sodium-based sulfite mills. U.S. EPA anticipates that it would be necessary to replace the existing recovery boilers at ammonia-based mills if chemical substitution to a sodium base were employed. Additionally, it is likely that, because the heat value of sodium spent liquor is lower than ammonia spent liquor, evaporator modification may he required if excess capacity does not already exist. 

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