Recovery of surfactant from papermaking process

Details Recovery of surfactant from papermaking process Recovery of surfactant from papermaking process

1991-12-17

2001-09-11

Chin, Peter (Department: 1731)

Paper making and fiber liberation

Processes and products

Pore forming in situ

C162S190000

Reexamination Certificate

active

06287417

ABSTRACT:

Replacing water with foam provides many advantages in papermaking. However, in addition to substantial problems involved in merely making that substitution, environmental problems involved with use of large amounts of surfactants have become increasingly severe as environmental regulations become more stringent. The combination of both sets of problems have done much to impede large scale commercial implementation of most, if not all, of the papermaking processes replacing water with foam which have been suggested over the last quarter century. The present invention largely obviates the environmental problems previously associated with foam substitution as it provides an economical, practical means of concentrating the surfactant in streams recovered from the process. In many cases, the surfactant, the stream bearing it or both, may be purified to such an extent that they may be reused, thus moving us many steps closer to the common dream of the papermaker and the ecologist—a closed process. Thus, broadly this invention relates to a method for recovery of surfactant from an aqueous admixture. In one of its more specific aspects, this invention relates to recovery of reusable surfactant from discarded foamed liquid in a foam forming papermaking process and reuse of the recovered surfactant in the process. In still another of its more specific aspects, this invention relates to a wet web forming process wherein a foamed fiber furnish is prepared by mixing a water slurry of fibers with a foamed liquid comprising water, air, and surfactant, a fibrous web is formed on a moving felt or wire on a papermaking machine with recovery of foamed liquid therefrom, part of the foamed liquid is recycled to make up further amounts of furnish and part of the foamed liquid is processed for recovery of surfactant for reuse in the process. Water from which the surfactant is recovered by the method of this invention may be so processed to the extent necessary to meet environmental purity requirements for discharge into lakes and streams, as well as for reuse or recycle into the papermaking process. The process may be used also in conjunction with other papermaking processes employing foamable admixture of surfactants, for example, a foam assisted dewatering process.
BACKGROUND OF THE INVENTION
The manufacture of high grade paper products such as towels and tissue, especially facial tissue and bathroom tissue, by forming a dispersion of wood fibers in a foamed aqueous carrier and depositing the resulting foam-fiber furnish on a forming wire of a papermaking machine is known in the art. A disclosure of pertinent prior art relating to foam forming processes is included in U.S. Pat. No. 4,443,297 incorporated herein by reference. In such processes, all of the foamable liquid recovered from the forming wire is recycled in the process. In a coassigned copending patent application of John H. Dwiggins and Dinesh M. Bhat, filed concurrently herewith, an improved process for foam forming is disclosed in which the papermaking fibers are dispersed in water prior to addition to foamed liquid with the result that excess liquid must be discarded from the system as a foamed liquid containing surfactants.
SUMMARY OF THE INVENTION
The invention is directed to a method and apparatus for the recovery of surfactant, e.g., an alpha olefin sulfonate or a peg-6 lauramide, from a foamed or foamable aqueous admixture containing surfactant, usually of the order of 150 to 1000 parts per million (ppm) surfactant by weight.
In accordance with this invention, foamed liquid is separated into reusable surfactant admixture and a purified aqueous stream by a multi-stage process including drainage, aeration, and foam resolution. First, foamed liquid is drained creating a more-dense fraction of foamable liquid impoverished in surfactant and a less-dense fraction of foam enriched in surfactant. Subsequently, these fractions are separated from each other and the dense fraction from the drainage stage, optionally combined with foamable liquid from other sources in the papermaking operation, is aerated with air bubbles, preferably of a uniform size range of from about 1 to 5 mm, to form a stable, free draining foam enriched in surfactant from which a more-dense phase of surfactant-depleted liquid is drained, separated from the foam, and upon which other aeration stages may be imposed if desired or necessary. The resulting foams may be broken or resolved to remove air therefrom, and the resulting surfactant-enriched foamed liquid or foamable liquid stream recycled to the process. Alternatively, in some cases, foam may be recycled without being resolved.
The surfactant content of a foamed aqueous liquid containing of the order of 500 ppm or more surfactant may be reduced to non-foamable water containing about 10 parts per million or less, and as low as about 3 ppm, by repeating the aeration, drainage and separation steps in sequential stages. Foaming of the water containing surfactant usually ceases at about 3 to 10 ppm. In many cases, it will be advantageous to recycle surfactant-depleted water to other operations in the web forming process such as, for example, pulping, repulping, showers, washers, and other applications for which water of this quality is suitable.
In some applications wherein surfactant is to be recovered primarily from foamable liquid rather than from a foamed liquid, the initial drainage step may be omitted. The drainage step may occur in the foamable liquid return lines without requiring a separate drainage apparatus.


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