Paper360 - May/June 2015 - (Page 38)

techlink | SluDGe DeWAteRInG Dewatering Fibrous Sludge with Soy Protein For mills that generate a mostly fibrous sludge, use of the soy additive can be justified on both a cost and a green basis. rObiNa HOGaN aNd SUJiT baNErJEE As much as 4 percent of the feedstock that enters a paper mill is discarded as sludge. This adds up to several million tons of waste material that is dumped into landfills, spread over terrain or burned every year. Dewatering sludge with a belt, screw press or centrifuge squeezes some of the liquid out and makes the resulting cake less costly to transport. Compacted cakes also take up less space in landfills. If the sludge is burned, fuel costs decrease as cake solids increase due to the lower water content of the material. can increase cake solids during belt pressing of fiber or fibrous sludge. Displacing petroleum-based polymers with renewable, sustainable soy protein provides additional environmental benefits. For more than a decade, USB has helped fund the development of many successful new uses for soybeans in products, including soy-based wood adhesives, plastics, foams, rubber, lubricants, soy methyl esters and soybased coatings and inks. Research to find new applications for these products continues in an effort to utilize more U.S. soy. DeWATerInG WITH SOY PrOTeIn Cake solids can be increased by more intensive pressing and/or by using agglomerants to promote dewatering. Agglomerants, such as cationic polyacrylamides (c-PAMs), neutralize the negative charge on the sludge particles, which reduces their mutual repulsion and allows them to consolidate. Research conducted at Georgia Tech and supported by the United Soybean Board (USB) demonstrates that c-PAMs can be partially substituted with soy protein for conditioning fibrous paper mill sludge prior to dewatering. Cheaper than the petroleumbased polymers presently used, soy proteins SUCCeSSFUL PAPer MILL DeWATerInG TrIALS Results from dewatering three paper mill sludges with mixtures of c-PAM and crude soy protein are illustrated in Figure 1. The c-PAM and soy doses were optimized. The crude protein was prepared by stirring soy flour in water at pH 10 and adding the resulting paste directly to the sludge. The pH can be lowered to 7.5 if warm water is used. For mill B sludge, the soy/c-PAM combination provides the same cake solids as the c-PAM alone, but at a much lower c-PAM dose, which translates to a lower cost. Mill P does not use any sludge conditioning polymers, but is interested in obtaining higher cake solids. The soy protein does this quite well and the application is particularly well suited for mills of this type. The results for Mill W sludge are similar to those obtained for Mill B in that the soy protein reduces the c-PAM dose. When the trials were conducted in 2013, the c-PAM was nominally priced at $4.80/kg and the soy protein at $0.66/kg. Consider the results of Mill B. The chemical cost for the conventional case with c-PAM alone is $2.64/ tonne. The c-PAM/soy combination costs $1.60, for a 39 percent reduction in cost. The corresponding numbers for mill W are $17.28 and $11.96, respectively, for a 30 percent cost reduction. For mill P, the value is provided by the greatly increased sludge solids. The result was unexpected because both the sludge particles and the soy protein are negatively charged and they should repel each other rather than bind together. Conventional sludge conditioners are typically cationic. Because soy proteins are isoelectric at pH ~4.5 they would be negatively charged under typical dewatering conditions where the filtrate is essentially neutral. However, soy protein contains a sufficient number of cationic FIGURE 1. Dewatering sludge with hydrolyzed soy flour. 38 Paper360ยบ MAY/JUNE 2015

Table of Contents for the Digital Edition of Paper360 - May/June 2015

Over the Wire
Centennial Wrap-Up
An Interview with James Hannan
Hydraulic Accumulators
Special Capabilities at Lincoln Paper
TAPPI Journal Summaries
Dewatering Fibrous Sludge with Soy Protein
Consolidation Watch
Association News
Online Exclusives
Index of Advertisers

Paper360 - May/June 2015