WO2005062800A2 - Lignosulfonate production process & product - Google Patents

Abstract

A plant for producing lignosulfonate as a co-product of alcohol, sugar or pulp production receives a feed stream of derived from plant waste and applies electrodialysis to produce a specialty lignosulfonate. The product may be a low-chloride product from vinasse for use as a cement- condition, a demineralized product stream that resists corrosion, crystallization or scaling of downstream process equipment, or other demineralized product wherein the species removal is tailored to an intended use. The electrodialysed feed stream is preferably dewatered by a step such as nanofiltration which may also further reduce its mineral content, allowing a high grade lignosulfonate product to be produced at a commercial valuable solids concentration above about 50% with a smaller sized evaporator or other concentrator.

Description

LIGNOSULFONATE PRODUCTION PROCESS & PRODUCT The present invention relates to materials and materials processing, and relates to lignosulfonate production from process or distillery waste material, for example, from Kraft process effluent or from vinasse, and relates to the lignosulfonate materials so produced.

Lignin, a product or by-product of wood or plant processing mills of various types, is either termed "Kraft lignin" or "lignosulfonate". Kraft Lignin and lignosulfonate typically possess weight average molecular weights M_w of from about 3,000 to 15,000 g per mole and an oxygen content of about 20% to about 30% by total weight. Simplistically, these lignin derivatives are often described as complex polymers of phenylpropane monomer units linked by oxygen atoms. Kraft lignin and lignosulfonate are sometimes described by the empirical formulae: Kraft lignin, C₉H₆₇O₂.₂S.₁(OCH₃) ,₉, and Lignosulfonate, C H_8.2O ₆(OCH₃) ,₉₄. In these formulae, the sulfonate groups - -SO₃ have been omitted in order to illustrate the number of oxygen molecules in the lignin structure. Other large-scale processing of fruit or vegetable matter has also been exploited to produce lignosulfonate product or co-product.

Lignosulfonates have many uses. Traditionally, they have seen extensive use as an inexpensive and widely-available additive for improving the plasticity of concrete while increasing its density or strength. While the requisite processing or concentration and recovery processes may vary somewhat, other applications as diverse as fertilizer and soil additives, feed pelletizing or binding agents, paper coating, adjuvants, supplements and conditioners of various types also consume significant quantities of lignosulfonate. When derived from vegetable milling or other industrial plant processes, the feed stream for lignosulfonate production almost always contains small organic molecules, such as sugars or their acids, small-molecule polymers of sugars, and a spectrum of other compounds which may have detrimental effects in the intended use, such as adversely affecting the setting and hardening, or the surface properties, when added to concrete. If the starting material is vinasse, e.g., the post-fermentation residue of an alcohol or yeast production process, a major part of the sugar may already be eliminated. However other components may present problems. The addition of higher amounts of even purified lignosulfonates to condition a concrete mixture may retard the hardening of the concrete, although this effect may be minimized by using gypsum-free cement as binder, and certain components of the material may impose other limitations in use. The presence of excess amount of potassium and sulfate in the vinasse can interfere with the process of lignosulfonate production (for example, by causing deposits or scale to form in evaporator or other concentration equipment.) When the upstream processes do not involve vigorous chemical conditions, the presence of potentially fouling organics in the stream may need to be addressed with regard to the equipment or exchange media used in a lignosulfonate production line. In addition, excessive amounts of other commonly encountered components may impair the value of a lignosulfonate product for particular uses. For example chloride, commonly present in a high concentration in a vinasse, can cause long-term corrosion or other problems such that the amount of such high chloride lignosulfonate added to a concrete employed for steel-reinforced concrete construction, must be limited.

Typical processes to provide a lignosulfonate co-product commonly include some form of separation of stream components, by filtration, by capture in and elution from exchange beds, or by other industrial treatment and separation processes. A number of minerals are commonly present in the feed stock thereby produced, either as a result of the initial industrial process (such as caustic addition, sulfite digestion in a paper-making plant, or the like) or as an ingredient added to promote cleaving, cross-linking, sulfonation or other chemical modification of the feed material for industrial lignosulfonate production. In the wood industry, a lignin product may, for example, be obtained from alkaline cooking of so-called sulphate cellulose and that has been sulfonated afterwards. This sulfate lignin product has the advantage that, as a rule, it does not contain sugars, but instead it does contain quite an abundance of sugar acids. The Soviet scientists V. L. Popova, H. N. Gripanova, R. G. Gimasbeva, and R. K. Boyarskaja (Gidroliznaja i Lasokhomicheskaja Promyslennost, No. 4 pp 13-14, 1977) have studied the effect of the purity of lignin on its dispersing property, and they have noticed that alcohol fermentation improves the dispersing property of spent liquor. The above scientists have further noticed that, when spent liquor is evaporated to a higher concentration, the product functions as an even better dispersing agent, at least when used to condition kaolin. The scientists explain that this results from the fact that furfural, which is always present to some extent in the spent fermentation liquor, promotes polymerization, and the larger-molecule product produced in this way operates as a better dispersing agent than the same product containing smaller molecules. In other publications the relationship between the effects of alkali metal lignosulfonate and earth- alkali metal lignosulfonate has also been discussed. Thus, e.g., in the book M. R. Rixom, Chemical Admixtures for Concrete, E & F. N. Spon Ltd, London 1978, it is stated on page 8 that Na-lignosulfonates are, owing to their higher solubility and higher ionizability, more advantageous than Ca-lignosulfonates, operating better as a dispersing agent. A great many other variables have been explored, but of necessity these investigations proceed, typically, from the available feed stock of a specific installation (paper mill, sugar refinery, etc.) that may practice cellulose digestion or fermentation with quite specific but different starting materials, underlying physical or biological treatment processes, and different types and amounts of chemical additives.

Writing more than twenty years ago on the production of lignosulfonate from conifer wood for use as a concrete conditioner, one writer (in U.S.Patent 4,450,106) wrote "When all the affecting factors in the lignin molecule obtained from conifer wood have been studied systematically and, further, which combinations of these factors give the best properties, the following observations have been reached: In contrast to what is generally believed, a low sulfonation degree has proved best. Further, it has been established that a very low sulfonation degree of the lignin together with a very high molecular weight gives superb properties. In addition to these discoveries, it has proved advantageous if molecules with low molecular weight have been eliminated from the product. A product with a high molecular weight and low sulfonation degree is, as a rule, most advantageous as a Na-salt, because the solubility of Mg- and Ca-salts is lower. Such products with a naturally low sulfonation degree are obtained from pulp cooking in which the pH- value of the cooking is rather high, i.e. the quantity of sulphur dioxide is low. Such pulp cookings are also increasing to-day, because attempts are made to promote the achievement of a high fibre yield by means of a high pH, even a pH being on the alkaline side. Such a pulp cooking is typically either a so-called Na- sulphite cook, the pH of the spent liquor obtained from it being 4.5 to 5.5, whereas it is in a typical Ca-bisulphite spent liquor of the order of 2.5 and always lower than 4.0, or such an advantageous cook is such an alkali sulphite cook in which pH is higher than 7, typically 7 to 13. In these alkali sulphite cooks, wherein the pH is on the alkaline side, to-day some such catalyst accelerating the cooking is used more and more commonly as, despite the high pH, yields rapid cooking and as typically frequently functions as such an oxydating-reducing catalyst as protects the splitting of the cellulose chain and especially of hemicellulose. If, in the cooking of cellulose, in addition to low SO₂ content (=high pH), some catalyst protecting the cellulose and hemicellulose has been used, such as amines or some quinone or anthraquinone derivatives or combinations of same, a spent liquor has been produced that right from the beginning contains a very low extent of dissolved sugars. This is why the purification treatment of this solution is easier and less expensive that that of some other, corresponding solution that contains a higher extent of sugars and dissolved polysaccharides. Such lignin products, preferably of a low sulfonation degree, are best separated from the spent liquor so that the small-molecule components are at the same time eliminated by using ultrafiltration, electrodialysis, or ion exchange for the fractioning of the spent liquor. Ultrafiltration is a method in which molecules are filtered through such a membrane with micropores as is sufficiently dense to hold molecules of large size. In ultrafiltration, difference in pressure makes molecules pass through the membranes, whereas in electrodialysis electrical current is used as an aid. Typically, such a membrane holds materials of a molecular weight of 5,000 to 50,000. " As will be apparent from the foregoing discussion, the quality and properties of lignosulfonate made as a co-product of the processing or treatment of vegetable matter, or even of resin production, is a complex matter, and may vary depending on the type of starting material, other co-products being produced at the plant, and the intended characteristics or end-use of the lignosulfonate. Nonetheless, lignosulfonate is a bulk material of commerce, valued in pennies per kilogram, rather than dollars. While the economic value of adding a lignosulfonate co-production line to an existing vegetable matter mill may be determined in part by the considering savings achieved by utilization of what would otherwise be a voluminous waste stream requiring costly disposal, the lignosulfonate production process must also be capable of inexpensively concentrating the product and/or removing unwanted components, since the savings in waste disposal cost will also typically be below a few cents per kilogram. A commercially viable lignosulfonate production process must further be able to operate dependably for long periods of time on large volumes of feed stock. Furthermore, its product must be well suited for its intended market; thus the process may need to address, in an economical fashion, the presence of specific components in the feed.

Over the last few decades, synthetic polymers such as polynaphthalenesulfonic acid-based and other "superplasticizer" materials, have been developed to supplant lignosulfonates in addressing some major applications, e.g., concrete conditioning applications. However, with a history extending back more than half a century, the lignosulfonates have established their position in the formularies for a wide range of industrial and feed materials and fabrication processes. Moreover, the need to address industrial waste streams from which they are derived - forest and agricultural material process waste streams - has not abated. This results in a continuing need for processes of lignosulfonate production.

It is therefore desirable to provide new processes to produce lignosulfonate. Summary of the Invention One or more of these and other desirable traits are achieved in a plant for production of a lignosulfonate product wherein a lignosulfonate-containing feed stream of industrial residue, such as effluent from a sulfite cook, or vinasse from an alcohol-producing or other fermentation process, is treated by electrodialysis and filtration to remove solids and certain small molecules and ionizable components present in the feed, to form a lignosulfonate product stream of enhanced quality. In one embodiment, the plant includes a plurality of electrodeionization units, each unit being comprised of a cathode, an anode, and cation exchange and anion exchange membranes which define a plurality of dilute and concentrate chambers within the unit. The feed is passed through dilute chambers of a unit, and the electrodes maintain a field across the dilute chambers that transports certain ionizable components from the stream through the adjacent membranes and into the concentrate chambers, reducing the concentration of chloride, sulfate or other target component in the product stream below a low threshold value. This refines the product stream that exits the dilute chambers. The feed may have relatively high total dissolved solids, e.g., up to 8-10%, including high levels of salts (e.g., .5 - 4% KC1), and may operate at somewhat elevated temperatures such as 30-55 °C. In one embodiment electrodialysis treatment of the lingo-containing feed reduces chloride by more than 90%, and preferably 95-98% or more. The stream may be filtered, e.g., with a nanofiltration membrane unit, which may be placed before or after the electrodialysis units. Subgroups of the electrodialysis units may be periodically shut down and subjected to a cleaning procedure.

Treatment of the feed stock may proceed continuously, or in batches with the stream from a batch tank recirculating through the electrodialysis units until a desired quality set point, such as a conductivity below 3-5 microSiemens/centimeter, is attained. Portions of the concentrate stream may be returned to an upstream fermentation process to maintain electrolytes or nutrients in the process fluid. The electrodialysis stage may be controlled to selectively remove two or more components, and a subsequent nanofiltration may be operated to further reduce the level of one or more of these components. The procedure is particularly effective for lowering species that affect utility of the lignosulfonate as a plasticizer.

Brief Description of the Drawings The invention will be understood from the specification and claims presented herewith, taken together with the drawings of representative embodiments, wherein: Figure 1 is a schematic representation of a lignosulfonate production system in accordance with the present invention.

Detailed Description Figure 1 schematically shows a lignosulfonate production system 10 in accordance with the present invention, which illustratively receives its feed from an upstream plant P, and produces a lignosulfonate product as a co-product or by-product of the upstream production process. As shown, the feed provided by the plant P, which may be the vinasse or residue after distillation from a beet- or cane-sugar molasses fermentation process, is treated in one or more conditioning steps C. The conditioning C may include a first or pre-filtration step (e.g., with a suitable 50 micrometer filter, to remove large suspended particles) and clarification, so that the feed to the process is presented as a substantially particle-free and non-fouling liquid feed that is compatible with and may be treated by electrodialysis in flow-through, multi-celled electrodialysis units. Preferably the level of suspended solids remaining in the prefϊltered feed, as measured, for example, by centrifugal sedimentation, amounts to less than about 0.2%. The pretreatment may also include pH adjustment or other steps to address components in the feed that could otherwise bind to or foul the downstream membranes. The pretreated feed may in general have about 5-10%) dissolved solids, comprised largely of lignins, with sugars or and other small organic molecules, and dissolved minerals such as K, Na, Cl, Mg, Ca and sulfate, with the dissolved minerals typically constituting about one third of the dissolved solids. Water may be added to dilute the feed stream to an appropriate viscosity level or solids concentration. This pretreated feed is passed to an electrodialysis (ED) section 12, where it flows through the dilute cells of one or more ED devices and is further refined. The feed stock may, for example, have an initial conductivity of about fifteen to about fifty microSiemens/centimeter or higher, and is treated by the ED units 12 (for example, recirculating through the units from a batch storage tank, not shown) until its conductivity is reduced to a desired level such as seven, five or about two microSiemens/centimeter, or less. The treatment is preferably carried out to remove at least those species that are deemed undesirable components in lignosulfonate prepared for a particular market (such as a concrete plasticizer market, an animal feed additive, pottery slip conditioner, coating, binder or other lignosulfonate end-use market). Preferably the ED is operated to remove a substantial portion of the total dissolved mineral burden. Small organic acids and acid salts will also be removed. Flow rate, electrode potential and/or batch recycle times may be adjusted to achieve the desired endpoint, and once the removal profiles for given treatment conditions have been acscertained, the process may be run automatically by a controller, based on feedback detection from a few process stream sensors. A target conductivity level may initially be determined by performing an analysis of the dissolved mineral content of the feed, and then measuring corresponding levels of relevant minerals in samples taken at each level of conductivity during continued electrodialysis, to determine the treated feed conductivity level that corresponds to the desired demineralization level and species distribution. That conductivity level may then be employed as a set point for the ED processing stage 12.

The electrodialysed product is preferably substantially demineralized, meaning that 90% or more of the relevant dissolved minerals are removed. Thus, with an initial feed having 9% dissolved solids, one-third being dissolved minerals, the treated stream will have a low mineral content and 5-7% other dissolved solids,^' rimarily lignins. Small ionizable organics may also be removed. Initial studies indicate that this electrodialysis treatment is particularly effective at chloride removal, reducing the chloride by 96% even with a relatively high conductivity set point of 7.5 microSiemens/centimeter. Other common minerals were at least 80% removed at that level, and removal rates approaching or exceeding 95% were achieved when electrodialysis was continued down to a 2.5 microSiemens/centimeter set point. Thus a chloride content of (0.7)% was quickly reduced to below (0.02)%. Even lower target levels of chloride may be achieved in accordance with the present invention by applying a chloride-reducing pretreatment to reduce the incoming level of chloride in the bulk feed, or by adjusting the ED processing to further enhance removal, or by providing a further chloride removal process downstream.

As further shown in Figure 1, the electrodialysed product 13 from the ED stage 12 is preferably passed for concentration to a nanofiltration (NF) unit 14 that de-waters and thus concentrates the refined ED product. Nanofiltration is particularly advantageous when applied to a vinasse stream; unit 14 produces a lignosulfonate stream output 16, having increased dissolved solids concentration of about 15-20% or more, and it further passes some components into the waste stream 15, which contains water and the removed ionic components. Advantageously, stage 14 further reduces the level of chloride that is present in the product stream. By operating the system to provide chloride levels below 1%, and preferably below about 0.1 %, the resulting low chloride lignosulfonate product becomes suitable as a high grade cement plasticizer. The final low-chloride product may be used at higher levels in concrete formulations to achieve greater plasticity and compatibility with reinforced constructions - e.g., enhamced penetration around rebars, as well as enhanced resistance to corrosion and enhanced strength. The post-NF stream may also be further concentrated, by an evaporator or the like, to increase the ^• lignosulfonate concentration to about 50%. Notably, when this treatment is added to an existing lignosulfonate line, the use of ED followed by NF, not only improves the quality improve, but the NF de- watering or preconcentration reduces the size of any required evaporator/ concentrator equipment.

In one contemplated plant using the process of the present invention, the concentrated stream 15 is next further concentrated to produce a commercial grade (e.g., 50%) lignosulfonate product. This may be done in an evaporator (not shown). When the process is added to an existing plant to introduce, or to improve production of a lignosulfonate co-product, the evaporator may be an existing installation that previously serviced a different co-product (such as a feed supplement or fertilizer co-product), or may be implemented in part with existing equipment (such as heat exchangers from existing digester or distillation equipment). Thus, the ED/filtration equipment offers a highly efficient production add-in or front end for developing value in an existing vegetable-based lignosulfonate line or other industrial waste stream operation.

In the above-described embodiment, the ED stage 12 may be implemented with either ED, or by using suitable electrical and fluid path control units, with electrodialysis reversal (EDR) stacks. Table I shows operating parameters and fluid characteristics over time for one feasibility study in which an eight liter sample of cane sugar distillery vinasse was treated in a single ED stack having ten cell pairs and assembled with electrodialysis exchange membranes manufactured by Ionics, Incorporated of Watertown, Massachusetts, USA.

10 cell pairs of AR204 SZRA / CRS7 HMR Heavy CR64 LMR at Anode and Cathode Electrode Solution Na2S04 @ 2 16 pH (initially)

Analysis Initial Feed Solution 10 2 Bπx Final Dilute Solution 7 2 Bnx Final Concentrate Solution 5 2 Box Table 1

During electrodialysis, the conductivity was monitored and samples of the processed fluid were taken initially and at conductivity thresholds of 7.5, 5, and 2.5 microSiemens. Analyses of the mineral content of the samples revealed exceptionally effective demineralization, with an early and high degree of chloride removal, producing a refined product which, after concentration, would be a high-grade low-chloride cement- conditioning lignosulfonate. The ED stage employed in the treatment process offers the further prospect of tailoring the treatment conditions by control of flow, voltage and treatment cycle parameters to achieve other desired mineralization targets. These may, for example, be set to remove certain minerals so as to avoid a number of particular process problems of lignosulfonate production, such as scaling in evaporators or corrosion in fluid lines; or to address environmental, health or other regulatory limitations on components present in the intended lignosulfonate product; or to achieve desired physical characteristics such as the above-mentioned chloride removal. By way of example, when producing a lignosulfonate intended for use as a pelletizer or binder for animal feed, the ED stage may be operated to remove toxic metals, or to de-bitter the flow. ED operation may also be controlled to more efficiently remove a set of two or more components, potentially replacing ion exchange, or conventional chemical treatment processes such as acidification, precipitation or the like, with unit operation that addresses several components at once. The ED may operated to separate certain species from the feed, e.g., returning a portion of the concentrate stream to an upstream stage (for example, returning removed nutrients or ionic species to an upstream fermentation process or chemical treatment stage), or simply serving to provide a smaller waste stream of the separated components.

The invention may also be applied to treatment of lignosulfonate-bearing streams derived form pulp or paper making processes, such as sulphite cooking operations, or may be applied to produce different types of lignosulfonate intended for other specialty uses, such as paper-coating or adhesive-enhancement applications. Upstream conditioning or pre-treatment of the feed stock may include separation of the lignin-rich components from dissolved salts by ultrafiltration, various rinse, liquid/liquid extraction or other steps to provide a clarified and particle-free ligno feed stock to the ED stage. Suitable pretreatments may be adapted from existing treatment processes, and corresponding modifications of the ED treatment may be made to tailor the distribution of species left in the lingo product.

The invention being thus disclosed, it will be readily adapted by those skilled in the art to the lignin streams or lignosulfonate co-product processes of diverse industrial plants, and variations and modifications thereof will become apparent to form existing grades of or to create new specialty lignosulfonates. All such variations and modifications are considered to be within the scope of the invention as described herein and in the claims appended hereto.

What is claimed is:

Claims

Claims 1. A system for producing lignosulfonate from a lignin-bearing material such as Ninasse or a sulfite liquor, where the lignin-bearing material is processed in electrodialysis units to demineralize the stream to a commercial quality threshold level.

2. The system of claim 1, wherein the material processed in electrodialysis units is passed to a membrane dewatering/concentrating unit to form a concentrated and further demineralized product.

3. The system of claim 2, wherein the dewatering/concentrating unit includes a nanofiltration unit and said nanofiltration unit further removes chloride.

4. The system of claim 2 or 3, wherein output of the dewatering concentrating unit is further concentrated to produce a commercial lignosulfonate product having dissolved solids content of approximately 50%.

5. A method for preparing lignosulfonate product from a plant-derived industrial liquor, wherein the liquor is pretreated to be compatible with electrodialysis, treated by electrodialysis to remove target dissolved species and form a demineralized ED output, and then concentrated to a commercial grade lignosulfonate product having solids content above about 20%.

6. The method of claim 5, further wherein said electrodialysis is carried out to a set point effective to remove species that cause adverse scaling or crystallization in downstream concentrator or heat stages.

7. The method of claim 5, wherein said plant-derived industrial liquor is a pretreated or clarified liquor from a sulfite pulp operation, and the electrodialysis is performed to remove residual sulfate.

8. The method of claim 5, wherein said plant-derived industrial liquor is a pretreated or clarified molasses or vinasse liquor, and the electrodialysis is performed to remove residual minerals and form a specialty lignosulfonate suitable for a specialty use such as a food use or reinforced concrete plasticizer use.