WO2015006799A1 - Recovery of super absorbent polymers - Google Patents

Recovery of super absorbent polymers Download PDF

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Publication number
WO2015006799A1
WO2015006799A1 PCT/AU2014/000716 AU2014000716W WO2015006799A1 WO 2015006799 A1 WO2015006799 A1 WO 2015006799A1 AU 2014000716 W AU2014000716 W AU 2014000716W WO 2015006799 A1 WO2015006799 A1 WO 2015006799A1
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WIPO (PCT)
Prior art keywords
mol
super absorbent
salt
restoration
dehydration
Prior art date
Application number
PCT/AU2014/000716
Other languages
French (fr)
Inventor
Mark Dunn
Clarke KEILAND
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Relivit Pty Ltd
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Filing date
Publication date
Priority claimed from AU2013902612A external-priority patent/AU2013902612A0/en
Application filed by Relivit Pty Ltd filed Critical Relivit Pty Ltd
Publication of WO2015006799A1 publication Critical patent/WO2015006799A1/en

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29B—PREPARATION OR PRETREATMENT OF THE MATERIAL TO BE SHAPED; MAKING GRANULES OR PREFORMS; RECOVERY OF PLASTICS OR OTHER CONSTITUENTS OF WASTE MATERIAL CONTAINING PLASTICS
    • B29B17/00—Recovery of plastics or other constituents of waste material containing plastics
    • B29B17/02—Separating plastics from other materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/22—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising organic material
    • B01J20/26—Synthetic macromolecular compounds
    • B01J20/265—Synthetic macromolecular compounds modified or post-treated polymers
    • B01J20/267—Cross-linked polymers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/30—Processes for preparing, regenerating, or reactivating
    • B01J20/34—Regenerating or reactivating
    • B01J20/345—Regenerating or reactivating using a particular desorbing compound or mixture
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/30—Processes for preparing, regenerating, or reactivating
    • B01J20/34—Regenerating or reactivating
    • B01J20/3483—Regenerating or reactivating by thermal treatment not covered by groups B01J20/3441 - B01J20/3475, e.g. by heating or cooling
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J11/00—Recovery or working-up of waste materials
    • C08J11/04—Recovery or working-up of waste materials of polymers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J2220/00—Aspects relating to sorbent materials
    • B01J2220/50—Aspects relating to the use of sorbent or filter aid materials
    • B01J2220/68—Superabsorbents
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29L—INDEXING SCHEME ASSOCIATED WITH SUBCLASS B29C, RELATING TO PARTICULAR ARTICLES
    • B29L2031/00—Other particular articles
    • B29L2031/48—Wearing apparel
    • B29L2031/4871—Underwear
    • B29L2031/4878—Diapers, napkins
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2300/00—Characterised by the use of unspecified polymers
    • C08J2300/14—Water soluble or water swellable polymers, e.g. aqueous gels
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P20/00—Technologies relating to chemical industry
    • Y02P20/141—Feedstock
    • Y02P20/143—Feedstock the feedstock being recycled material, e.g. plastics
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
    • Y02W30/00—Technologies for solid waste management
    • Y02W30/50—Reuse, recycling or recovery technologies
    • Y02W30/62—Plastics recycling; Rubber recycling

Definitions

  • This invention relates to a process for recovering super absorbent polymers in a form suitable for further use, and a plant for working the process.
  • SAPs Super Absorbent Polymers
  • a hydrogel water absorbing SAP
  • SAPs are made of curled polymer chains that carry carboxyl groups (COOH) such as modified starches, polyacrylamide, po!yacrylates or other hydrophilic components.
  • COOH carboxyl groups
  • the polymer chains are cross- linked by a variety of chemicals and to varying extents. These cross-links allow the curled structure to straighten (and swell), but not to fall apart, and determine its dimensional stability, resistance to mechanical forces and hydrophil!c/hydrophobic nature.
  • MPIs monovalent cations
  • the choice of MP! typically depends on the end use of the SAPs.
  • the MPI is typically sodium (Na + ).
  • K + potassium
  • ammonium NH 4 + ).
  • SAPs are used in the manufacture of sanitary absorbent articles such as disposable nappies, incontinence pads and sanitary napkins. SAPs are also used in agriculture to improve water retention of soils and to seal leaking dams. In horticulture and forestry they provide a water and fertiliser reserve for seedlings. In the mining industry, SAPs are used for dewatering. In industry more generally, they find application in the clean-up of toxic and non-toxic spills, to remove flood water e.g. from a building site, and to scavange heavy metals, such as uranium, from waste water.
  • SAPs are embedded within other materials or may become embedded as a consequence of their application, such as grit and soil when used for spill control.
  • SAPs Once swollen with absorbed liquids SAPs have adhesive characteristics that bind together such materials, and so the release of such liquids is an important prerequisite to their extraction from such mixtures.
  • SAPs are essentially single use only, and effectively function as a medium to capture and transport the absorbed content to a landfill for long term storage/disposal.
  • SAPs are relatively expensive chemicals to manufacture so there is incentive to recover SAPs in a form permitting reuse.
  • the absorbed liquids are relatively benign, e.g. flood waters, or have well established treatment or recycling routes, e.g. urine. Accordingly, there is the potential to improve the utilisation of SAPs if methods were available to release the liquids from the SAPs, without permanently and substantially impairing their absorbency characteristics.
  • SAPs are dehydrated in the presence of chemical salts.
  • the salt content of urine (0.9% wt) may reduce the absorbency of typical SAPs used in absorbent hygiene products from 400 g/g to typically 40 g/g. Consequently, absorbency of synthetic urine is a standard performance measure of SAPs used for this purpose.
  • ammonium, alkali and alkali earth metals, aluminium, copper, iron and zinc salts (dehydration salts) given sufficient time and concentration, can reduce absorbency to below 10g/g, and in the process, reduce the swollen and adhesive 'bubbles' of SAP to a rubbery grit.
  • this process is postulated to occur by both osmotic pressures and by the substitution of the original MP! by mu!ti-vending cations in the dehydration salts, such as calcium (Ca 2* ) or aluminium (Al 3+ ). It is further thought that the multi-valent ions provide an additional level of cross-linking within the SAPs, by bridging between more than one carboxyl group, thus preventing the expansion required to take in additional water molecules.
  • Dehydration salts are known to exhibit varying degrees of effectiveness, longevity and reversibility depending on the choice of cation and the concentration.
  • the suppressive effects of monovalent cations only exist as long as the salt concentration remains high. If the concentration is reduced, the SAPs will reabsorb. In contrast, bivalent cations such as calcium or magnesium have an ongoing effect, in that the SAPs will not reabsorb once removed from a solution containing these ions. However if these bivalent ions can be extracted, substantive absorbency is restored.
  • SAPs from other material in United States patent number 5558745 This process separates SAP's from absorbent sanitary paper products by using a salt solution (alkali metal, alkaline earth metal, aluminium, copper (II), iron (III) and zinc). Th treated SAP is then dried and separated from the other materials using a vibrating screen.
  • the salts utilized in this patent also permanently deactivate the SAP.
  • anion is also important from a purchase cost, in -process management and a waste water disposal perspective.
  • the invention resides in a process for recovering super absorbent poiymer in a form suitable for re-use from a material containing super absorbent poiymers including the steps of:
  • the process may also include the initial step of shredding or otherwise pre-processing the material to ensure access to the super absorbent poiymers by the dehydration salt.
  • the concentration of the dehydration salt is suitably from 0.05 mol/L to 1.0 moi/L, and preferably selected from 0.05 mol/L, 0.1 mol/L, 0.15 mol/L, 0.2 mol/L, 0.25 mol/L, 0.3 mol/L, 0.35 mol/L, 0.4 mol/L, 0.45 mol/L, 0.5 moi/L, 0.55 mol/L, 0.6 mol/L, 0.65 mol/L, 0.7 mol/L, 0.75 mol/L, 0.8 mol/L, 0.85 mol/L, 0.9 mol/L, 0.95 mol/L, or 1.0 mol/L and most suitably about 0.2 mol/L.
  • about Q.2moi/L it is meant that the concentration is nominally 0.2 mol/L but the actual concentration may vary from this precise concentration to an unimportant degree.
  • the dehydration salt may be an alkaline earth salt such as: magnesium sulphate (MgSO-i); magnesium chloride ( gCla); magnesium nitrate (MgfNC te); calcium chloride (CaCfe); calcium nitrate (CaCNOsfe).
  • the dehydration salt is preferably an aqueous solution of magnesium sulphate (MgS0 4 ).
  • the dehydration salt may also be added dry so that the moisture content of the material forms an aqueous solution.
  • the super absorbent polymers are separated by using methods that exploit the size, shape, density, magnetic properties and conductive properties. Properties such as size and shape may be exploited by filtration or screening; the density may be exploited by flotation or cycloning; the magnetic properties may be exploited by passing the super absorbent polymer through a magnetic field and the conductive properties may be exploited by passing the super absorbent polymer through an electric field. Separation may take place whilst the materials are relatively dry or in a water suspension.
  • the restoration salt is a salt containing a anion that will react with the cation of the dehydration salt to form a precipitate. This neutralises this ion and eliminates its dehydrating effects on the SAP.
  • Suitable anions include carbonate (C0 3 -), a variety of phosphates (H 2 PQ 4 " , HP0 4 2" , ⁇ NH 4 P0 4 2” ), hydroxide (OH * ), sulphate (SO4 2* ), fluoride (P), oxalate (C 2 0 4 2 ⁇ ) or citrate (CeHsQ? 3* ).
  • the concentration of the restoration salt is suitably from 0.1 moi/L to 1.5 mol/L and preferably selected from 0.1 mol/L, 0.2 moi/L, 0.3 moi/L, 0.4 mol/L, 0.5 moi/L, 0.6 mol/L, 0.7 mol/L, 0.8 moi/L, 0.9 moi/L, 1.0 moi/L, 1.1 mol/L, 1.2 moi/L, 1.3 mol/L, 1 A mol/L or 1.5 mol/L and most suitably about 0.3 mol/L.
  • about 0.3 mol/L it is meant that the concentration is nominally 0.3 mol/L but the actual concentration may vary from this precise concentration to an unimportant degree.
  • the restoration salt may suitably be an aqueous solution of alkali and ammonium salts, including: sodium carbonate ( a ⁇ COs); trisodium phosphate (Na 3 p0 4 ); sodium hydroxide (NaOH); potassium carbonate ⁇ K2CO3); potassium hydroxide (KOH); diammonium phosphate (( H4)2HPG 4 ).
  • the restoration salt may suitably be an aqueous solution of sodium carbonate (Na 2 C0 3 ).
  • the ratio of the restoration salt to dehydration salt is suitably from 1.0 to 1.5 times the stoichiometric requirement and preferably selected from 1.1 times, 1.2 times, 1.3 times, 1.4 times or 1.5 times.
  • the process may further include one or more post-processing steps including: rinsing the SAPs; dosing with additional chemicals; drying the SAPs.
  • FiG 1 is a flow chart of the steps of a process for recovering and restoring super absorbent polymers from a material
  • FIG 2 is a schematic of a plant for working the process of FiG 1.
  • Embodiments of the present invention reside primarily in a process and plant for recovering and restoring super absorbent polymers from a material. Accordingly, the steps have been illustrated in concise schematic form in the drawings, showing only those specific details that are necessary for understanding the embodiments of the present invention, but so as not to obscure the disclosure with excessive detail that will be readily apparent to those of ordinary skill in the art having the benefit of the present description.
  • adjectives such as first and second, left and right, and the like may be used solely to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order.
  • Words such as “comprises” or “includes” are intended to define a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed, including elements that are inherent to such a process, method, article, or apparatus.
  • FIG 1 there is shown a flow chart outlining the steps in a process to recover super absorbent polymers from a material containing super absorbent poiymers and at least partially restore the functionality of super absorbent poiymers to absorb liquids and multi-valent cations.
  • the material containing the SAP is shredded or otherwise processed to open enclosed pockets and separate components so that chemicals applied in the process steps are able to penetrate the material and treat the SAP.
  • the first step of the process is to apply a dehydration salt to the SAPs so that they release absorbed liquids, lose adhesive characteristics and shrink to near original size.
  • the dehydration salt may be applied as an aqueous solution (dehydration wash) or it may be applied dry, in which case an aqueous solution forms with liquid released from the SAP.
  • the dehydration salt is an alkaline earth salt such as: magnesium sulphate (lV!gS0 4 ); magnesium chloride ( gCIa); magnesium nitrate (MgiNOafe); calcium chloride (CaCfe); calcium nitrate (Ca(N0 3 )2)
  • the dehydrated SAP is separated from the rest of the material using conventional separation techniques.
  • SAP may be separated from the rest of the material using differences such as size, shape, density, magnetic or conductive properties. For example, separation may be achieved by sieving if the SAP particles are a different size or shape from the rest of the material. Another example may be by dropping material through a water flow or by using flotation to separate according to density. If the material contains any magnetic or conductive properties separation can be achieved using techniques that exploit these properties, such as by passing through a magnetic or electric field.
  • Recovered dehydration wash may be recycled to be reused in the dehydration step,
  • the concentration of the dehydration solution is monitored to maintain the dehydration salts in a target range.
  • the SAPs are then treated with a restoration salt to restore absorbency.
  • the restoration salt contains monovalent cations such as sodium, potassium or ammonium.
  • the restoration salt may be applied dry and mechanically mixed through the SAPs, or applied as an aqueous solution (restoration wash).
  • the anion in the restoration salt is selected so that it forms a precipitate with the cation of the dehydration salt at appropriate pH and temperature conditions.
  • Suitable anions include hydroxide (OH “ ), carbonate (COa ⁇ ), a variety of phosphates (H 2 PQ 4 " , HP0 4 2 ⁇ , PO4 3" ,NH 4 PO 4 2” ), sulphate (S0 4 2” ), fluoride (F “ ), oxalate (C2O2 2” ) and citrate (CeHsOr 3” ).
  • the anions may be introduced to the solution by means of dissociation.
  • the carbonate (CO3 2" ) may dissociate from salts selected from a group comprising H2CO3, NaHCOa, a2COs, KHCO3 or K2CO3.
  • the carbonate may also be introduced by bubbling carbon dioxide through water to form carbonic acid (H2CO 3 ), which can then dissociate into bicarbonate (HCO 3 " ) and carbonate (C0 3 2 ⁇ ).
  • Suitable combinations of anions and SAP absorbed cations from the dehydration salt include:
  • the cations of the dehydration salt are drawn out of the SAP by osmotic pressure.
  • This osmotic pressure is maintained by the scavenging of the cations by the anions of the restoration salt to form precipitate. Forming precipitate effectively removes any osmotic back pressure that would otherwise slow the removal of cations from the SAPs.
  • the SAPs may be washed to remove the excess restoration salt and precipitate.
  • the precipitate may be removed by filtration and the restoration solution may be reused.
  • the precipitate may be used as landfill or may find other environmentally friendly economic uses.
  • the extracted and washed SAPs may be dosed with further chemicals to promote drying to, say, less than 5 g/g moisture, or less than 4 g/g moisture, or less than 3 g/g moisture, or less than 2 g/g moisture, or less than 1 g/g moisture or nominally no moisture.
  • the SAPs could also be dosed with other chemicals for use in different applications, such as fertiiizer.
  • the SAPs are then dried using any combination of natural or thermally assisted techniques to a level optimised for shipping.
  • the super absorbent polymers in disposable nappies are recovered and restored.
  • the nappies are pre-processed by being shredded to expose the SAPs and partially separate the SAPs from other components.
  • Disposable nappies, and many other absorbent sanitary products typically consist of an inner layer of liquid permeable material, an outer layer of liquid impermeable material, and an adsorbent or absorbent core of filler that contains the SAP.
  • the liquid permeable material may be a non-woven sheet formed from polypropylene or polyethylene, or a woven product formed from cotton or rayon.
  • the liquid impermeabl material may b polyethylene, polypropylene, starch based degradable plastic films, woven cloth or rubber.
  • the adsorbent or absorbent core ma be air laid wood pulp fluff, commonly referred to as air felt, and/or synthetic fibres including polypropylene or polyethylene filaments.
  • the core also contains a super absorbent polymer (SAP) material, which is typically a polyacrylate, polyacrylamide, cross-linked starch or other hydrophilic component, which may be synthetic, and ma be in granular, fibrous or laminate form.
  • SAP super absorbent polymer
  • the shredding step breaks away the layers and fillers to expose the SAPs to further processing. Processing continues on the mixed materials of the SAPs with the other components.
  • the mixed materials are treated with an aqueous solution of magnesium sulphate (Dehydration Wash) of 0.5 to 5 moi/L and preferably 2 mol/L.
  • Magnesium sulphate is also known as Epsom salts. It is low in price, is relatively benign from an environmental perspective, with both magnesium and sulphur being important micro-nutrients.
  • the dehydration salt may be added to the mixed materials in a dry form, reliant on the moisture content of the mixed materials to form an aqueous solution.
  • the dehydration salt and mixed materials are maintained in contact for a sufficient period in order to achieve adequate dehydration of the SAPs to permit subsequent separation. This contact period is reduced by agitating the materials to improve the rate of contact between the salt and the SAPs, and also by elevated temperatures and pressures, such as use of hot water or processing in an autoclave.
  • This treatment has the effect of dehydrating the SAPs by osmotic pressure and also substituting magnesium ions (Mg 2+ ) for the monovalent cations originally in the SAPs or simply introducing additional ions.
  • This treatment reduces the absorbency to less than 10 g/g, and shrinks the SAP 'bubbles', however the SAP 'bubbles' are still larger than the dry particles. This is advantageous for the process as the size of the SAP makes them easier to separate from the other material.
  • this method i used on sodium poiy-acrylate, the treated particles have a consistency of hard rubber.
  • Dehydrated and shrunken SAPs are then separated from other materials using a variety of techniques that exploit differences in size, shape, density, magnetic and conductivity properties. For mixed materials such as absorbent hygiene products, this may involve screening, flotation and cycloning.
  • the Dehydration Wash is removed by methods such as screening and pressing, with the recovered solution returned to the previous part of the process. This solution is monitored and dosed to maintain the target concentration of dehydration salts.
  • the SAPs are treated with an aqueous solution of sodium carbonate (Restoration Wash) of 0.5 to 5 mol/L and preferably 2 moi/L and in sufficient quantity to provide at least 1.0 to 1.5 times the stoichiometric requirement.
  • sodium carbonate is also known as washing soda and soda ash. It is low in price and relatively benign from an environmental perspective. The most significant effect is the potential of the carbonate ions to raise the pH of the waste water - a factor readily measured and corrected before eventual discharge.
  • the magnesium ions are drawn out of the SAP by osmotic pressure.
  • This osmotic pressure is maintained by the scavenging of magnesium ions by the carbonate ions in the restoring solution, to form magnesium carbonate which precipitates under near neutral or high pH conditions.
  • SAPs originally containing sodium ions such as those used in absorbent hygiene products manufacture, this largely restores their original chemistry and absorbency characteristics.
  • the SAPs are washed to remove the excess Restoration Wash, and loose particles of magnesium carbonate. These particles are removed by a fine screen, with the restoring solution returned to the previous stage of the process.
  • the magnesium carbonate is collected as sludge for drying and eventual disposal to landfill, or as a raw material in its own right, for example in brick manufacture.
  • this process has shown to extract sodium poly-acrylate SAPs from sanitary absorbent articles and restore absorbency as follows:
  • the SAP was treated with a 0.2 mol/L solution of Epsom Salts for 15 minutes at room temperature.
  • the treated SAP was then found to have an absorbed water content SAP of 10 g/g.
  • Soda ash was then applied to the SAP as a powder at a stiochiometric ratio of 1.2 (CO 3 2" to ! lg 2 *).
  • the SAP was then rinsed to remove the remaining soda ash and magnesium carbonate precipitate and oven dried.
  • the absorbency of the recovered SAP was in excess of 10Og/g of tap water.
  • the restoring solution may use potassium carbonate instead of sodium carbonate.
  • potassium carbonate instead of sodium carbonate.
  • potassium salts are a common component of basic chemical fertilisers, which provide a balance of nitrogen, phosphorus and potassium (NPK).
  • the cleaned SAPs may then be dosed with chemicals to promote further drying, i.e. to less than 5 g/g moisture, or to be absorbed into the SAPs for deployment in a new application, for example the application of fertilisers.
  • the additives must be environmentally acceptable in the new application.
  • SAPs used to scavenge water based liquid spills are recovered and restored.
  • the SAPs are likely to be mixed in with other detritus, soils and similar materials.
  • the mixed materials are treated with an aqueous solution of magnesium sulphate (Dehydration Wash) of 0.05 to 1 mol/L and preferably 0,2 mol/L.
  • the dehydration salt may also be added to the mixed matertais in a dry form, reliant on the moisture content of the mixed materials to form an aqueous solution.
  • the dehydration salt and mixed materials are maintained in contact for a sufficient period in order to achieve adequate dehydration of the SAPs to permit subsequent separation.
  • This contact period is reduced by agitating the materials to improve the rate of contact between the salt and the SAPs, and also by elevated temperatures and pressures, such as use of hot water or processing in an autoclave.
  • Dehydrated and shrunken SAPs are then separated from other materials using a variety of techniques that exploit differences in size, shape, density, magnetic and conductivity properties, such as settling in water to remove dense particles, and screening to remove large items such as wood or cloth.
  • the SAPs are screened to remove excess water. This water may be reused for the treatment of future batches, o treated and discarded.
  • the SAPs are treated with an aqueous solution of sodium carbonate (Restoration Wash) of 0.05 to 1.5 mol/L and preferably 0.3 mol/L, and in a minimum quantity of 1 to 1.5 times the stoichiometric requirement.
  • the sodium carbonate may be added in a dry form, reliant on the moisture content of the mixed materials to form an aqueous solution.
  • the magnesium ions are drawn out of the SAP by osmotic pressure. Once this ion exchange has progressed to an acceptable point, the SAPs are washed to remove the excess Restoration Wash, other soluble contaminants, and loose particles of magnesium carbonate. These particles are removed by a fine screen.
  • the cleaned SAPs may then be dosed with chemicals to promote further drying, i.e. to less than 5 g/g moisture, or to be absorbed into the SAPs for deployment in a new application, for example the application of fertilisers.
  • the additives must be environmentally acceptable in the new application.
  • Material containing SAPs to be processed are loaded into a hopper 20.
  • the materials are loaded from the hopper 20 into a shredder 21 which may be, for example, rotating blades, teeth or overlapping sharp edged disks, or a grinder which combines both extrusion and cutting actions.
  • a shredder 21 which may be, for example, rotating blades, teeth or overlapping sharp edged disks, or a grinder which combines both extrusion and cutting actions.
  • Other suitable mechanisms for exposing the SAPs will be evident to persons skilled in the field.
  • the shredded material is transferred to a dehydrator 22 where the materia! is treated with dehydration wash.
  • the dehydration wash may be an aqueous solution of magnesium sulphate or one of the other dehydration salts described above, in the simplest form the dehydrator 22 may be a vat but in other embodiments it may be an autoclave that also applies pressure and heat
  • the material is resident in the dehydrator 22 for sufficient time for the SAPs to release the absorbed moisture, reducing absorbency to below 15g/g. Absorbency may be reduced to as low as 10 g/g or even 5 g/g.
  • the dehydration wash may be recycled and is monitored for correct concentration of dehydration salts.
  • the separator 23 may be a sieve, flotation separator, air flow separator, magnetic separator, electric separator or other device capable of separating the SAPs from the remaining material or combinations of these.
  • the remaining material is discarded as waste.
  • 'waste' it is meant that the material is not of further use in the process, but may nonetheless have other uses.
  • the SAPs are then sent to the restorer 24 for treatment with a restoration wash to restore absorbency to the SAPs.
  • the restoration wash may be an aqueous solution of sodium carbonate or one of the other restoration salts described above.
  • the restoration wash may be recycled and is monitored for correct concentration of restoration salts.
  • the restored SAPs may be rinsed in a rinser 25 and dried in a dryer 26.
  • Rinse from the rinser 25 is considered as waste to the process but may be useful elsewhere.
  • the plant of FIG 2 produces recovered and restored SAPs which can be reused.

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  • Processing Of Solid Wastes (AREA)

Abstract

A process for recovering and reactivating a super absorbent polymer from a material, such as a hygiene product, containing a super absorbent polymer. The process includes the steps of: treating the super absorbent polymer with a dehydration salt to release absorbed liquid; separating the super absorbent polymer from the material; and treating the super absorbent polymer with a restoration salt to restore absorbency. The dehydration salt is suitably MgSO4 and the restoration salt is suitably Na2CO3. The invention also includes a plant for working the process for recovering and reactivating a super absorbent polymer.

Description

TITLE
RECOVERY OF SUPER ABSORBENT POLYMERS
FIELD OF THE INVENTION
This invention relates to a process for recovering super absorbent polymers in a form suitable for further use, and a plant for working the process.
BACKGROUND TO THE INVENTION
Super Absorbent Polymers (SAPs) are polymers that can absorb and retain many times their own mass of a liquid. For example, a hydrogel (water absorbing SAP) may be able to hold and retain up to 500 times its own weight in deionised and distilled water. SAPs are made of curled polymer chains that carry carboxyl groups (COOH) such as modified starches, polyacrylamide, po!yacrylates or other hydrophilic components. The polymer chains are cross- linked by a variety of chemicals and to varying extents. These cross-links allow the curled structure to straighten (and swell), but not to fall apart, and determine its dimensional stability, resistance to mechanical forces and hydrophil!c/hydrophobic nature. Finally, monovalent cations (sometimes referred to as MPIs - monovalent positive ions) are introduced into the structure, attracted by the negative charge on the carboxyl ion. The choice of MP! typically depends on the end use of the SAPs. For sanitary absorbent articles the MPI is typically sodium (Na+). in agricultural or industrial (outdoor) use, it is frequently potassium (K+) as this leaves a more environmentally acceptable residue once the SAP particle breaks down. In food and cosmetics, it is usually ammonium (NH4 +).
It is estimated that 70% to 80% of all SAPs are used in the manufacture of sanitary absorbent articles such as disposable nappies, incontinence pads and sanitary napkins. SAPs are also used in agriculture to improve water retention of soils and to seal leaking dams. In horticulture and forestry they provide a water and fertiliser reserve for seedlings. In the mining industry, SAPs are used for dewatering. In industry more generally, they find application in the clean-up of toxic and non-toxic spills, to remove flood water e.g. from a building site, and to scavange heavy metals, such as uranium, from waste water. In a number of applications, such as in the manufacture of sanitary absorbent articles, the SAPs are embedded within other materials or may become embedded as a consequence of their application, such as grit and soil when used for spill control. Once swollen with absorbed liquids SAPs have adhesive characteristics that bind together such materials, and so the release of such liquids is an important prerequisite to their extraction from such mixtures.
Apart from the agrarian applications, the SAPs are essentially single use only, and effectively function as a medium to capture and transport the absorbed content to a landfill for long term storage/disposal. SAPs are relatively expensive chemicals to manufacture so there is incentive to recover SAPs in a form permitting reuse.
In many cases, the absorbed liquids are relatively benign, e.g. flood waters, or have well established treatment or recycling routes, e.g. urine. Accordingly, there is the potential to improve the utilisation of SAPs if methods were available to release the liquids from the SAPs, without permanently and substantially impairing their absorbency characteristics.
It has been established that SAPs are dehydrated in the presence of chemical salts. In one practical application, the salt content of urine (0.9% wt) may reduce the absorbency of typical SAPs used in absorbent hygiene products from 400 g/g to typically 40 g/g. Consequently, absorbency of synthetic urine is a standard performance measure of SAPs used for this purpose. Elsewhere it has been shown that ammonium, alkali and alkali earth metals, aluminium, copper, iron and zinc salts (dehydration salts), given sufficient time and concentration, can reduce absorbency to below 10g/g, and in the process, reduce the swollen and adhesive 'bubbles' of SAP to a rubbery grit. At a chemical level, this process is postulated to occur by both osmotic pressures and by the substitution of the original MP! by mu!ti-vaient cations in the dehydration salts, such as calcium (Ca2*) or aluminium (Al3+). It is further thought that the multi-valent ions provide an additional level of cross-linking within the SAPs, by bridging between more than one carboxyl group, thus preventing the expansion required to take in additional water molecules. Dehydration salts are known to exhibit varying degrees of effectiveness, longevity and reversibility depending on the choice of cation and the concentration.
Specifically, the suppressive effects of monovalent cations only exist as long as the salt concentration remains high. If the concentration is reduced, the SAPs will reabsorb. In contrast, bivalent cations such as calcium or magnesium have an ongoing effect, in that the SAPs will not reabsorb once removed from a solution containing these ions. However if these bivalent ions can be extracted, substantive absorbency is restored.
It is further understood that when cations with a valency of three or more, such as aluminium or iron are used for dehydration, the effects are effectively permanent, as they cannot be extracted. This may be desirable when the objective is primarily to recover the other materials associated with the SAPs, and thus ensuring thai there is no residue of functional SAPs. However, the SAPs themselves would then have little future use. Accordingly, it is necessary to identify a dehydration process and choice of salts that is both effective and reversible.
One prior art example of a process for separating SAPs from other materials is found in United States patent number 8177151 assigned to Knowaste International LLC. The Knowaste process separates SAPs from fibres and other plastics by shredding the source material (such as an absorbent hygiene product), adding salt to the material, agitating and drying the material. The dried material is sifted to separate the plastic from fibre, SAP and other waste. This patent identifies alum which contains aluminium ions as the preferred salt which, as mentioned above, permanently deactivates the SAP. Knowaste Internationa! has another patent for the process of separating
SAPs from other material in United States patent number 5558745. This process separates SAP's from absorbent sanitary paper products by using a salt solution (alkali metal, alkaline earth metal, aluminium, copper (II), iron (III) and zinc). Th treated SAP is then dried and separated from the other materials using a vibrating screen. The salts utilized in this patent also permanently deactivate the SAP.
Another prior art example of a process for separating SAP from other materials is found in the international Application WO2014/077619, assigned to Kimberly-Clark Worldwide, Inc and Yuhan-Kimberiy Limited. This application discloses a method of recycling superabsorbent polymers (SAP) in sanitary absorbent articles. The method used to belch the liquid from the sanitary absorbent articles is submersion of the swollen SAP in sea water and CaC (0.5% to 3%) to remove the waste and moisture from the SAP (80-90% discharged). The SAP is then filtered and dried using thermally aided methods (105-110°C for 12 hrs). The described process is effectively a dehydration process that overlaps the Knowaste patents. There is no description of reactivating the SAPs and there is no data on the performance of the recovered SAPs. The absorbency is permanently reduced to a fatal degree.
Another prior art example is United States patent number 8318306 which is assigned to Evonik Stockhausen, LLC. This patent describes a process for triggering the absorbency of SAP. The Evonik process describes the use of triggers to control the absorbency and distribution of liquid over SAPs. The first trigger uses cations with a valency of two or more which causes the SAP to release the absorbed liquid and the second trigger contains an anion which forms a relatively insoluble salt with the cations of the first trigger to restore absorbency. This process utilizes SAPs with anionic functional groups. It does not describe a process for recovery and reactivation of SAPs. It is also doubtful that the process is economically viable as it uses a large volume of expensive chemicals.
Liquids released from the SAP and associated materials typically require treatment before final disposal. In commercial implementations of such a process, it is necessary to use an excess of salts in order to achieve the desired results in an acceptable time frame. Accordingly, these waste liquids are relatively high in salt content. In many jurisdictions, there are tight restrictions on the content of waste waters discharged to the environment or to the sewer. As a consequence, a range of salts, such as those containing aluminium, copper, iron, manganese, nickei, tin and zinc, alum, may be effectively precluded from use, geographically restricted, or would incur substantial costs to remove them from the waste water prior to discharge. When also factoring in the sa!t purchase costs, the range of useful salts is restricted to those containing ammonium, sodium, potassium, calcium and magnesium. Of these, only calcium and magnesium are bivalent and thus provide ongoing suppression of SAP hydration.
The choice of anion is also important from a purchase cost, in -process management and a waste water disposal perspective.
SUMMARY OF THE INVENTION
In one form, although it need not be the only or indeed the broadest form, the invention resides in a process for recovering super absorbent poiymer in a form suitable for re-use from a material containing super absorbent poiymers including the steps of:
treating the super absorbent polymers with a dehydration salt to release absorbed liquid;
separating the super absorbent poiymers from the material; and
treating the super absorbent polymers with a restoration salt to restore functionality to the super absorbent polymers.
The process may also include the initial step of shredding or otherwise pre-processing the material to ensure access to the super absorbent poiymers by the dehydration salt.
The concentration of the dehydration salt is suitably from 0.05 mol/L to 1.0 moi/L, and preferably selected from 0.05 mol/L, 0.1 mol/L, 0.15 mol/L, 0.2 mol/L, 0.25 mol/L, 0.3 mol/L, 0.35 mol/L, 0.4 mol/L, 0.45 mol/L, 0.5 moi/L, 0.55 mol/L, 0.6 mol/L, 0.65 mol/L, 0.7 mol/L, 0.75 mol/L, 0.8 mol/L, 0.85 mol/L, 0.9 mol/L, 0.95 mol/L, or 1.0 mol/L and most suitably about 0.2 mol/L. By "about Q.2moi/L" it is meant that the concentration is nominally 0.2 mol/L but the actual concentration may vary from this precise concentration to an unimportant degree.
The dehydration salt may be an alkaline earth salt such as: magnesium sulphate (MgSO-i); magnesium chloride ( gCla); magnesium nitrate (MgfNC te); calcium chloride (CaCfe); calcium nitrate (CaCNOsfe). The dehydration salt is preferably an aqueous solution of magnesium sulphate (MgS04).
The dehydration salt may also be added dry so that the moisture content of the material forms an aqueous solution.
The super absorbent polymers are separated by using methods that exploit the size, shape, density, magnetic properties and conductive properties. Properties such as size and shape may be exploited by filtration or screening; the density may be exploited by flotation or cycloning; the magnetic properties may be exploited by passing the super absorbent polymer through a magnetic field and the conductive properties may be exploited by passing the super absorbent polymer through an electric field. Separation may take place whilst the materials are relatively dry or in a water suspension.
The restoration salt is a salt containing a anion that will react with the cation of the dehydration salt to form a precipitate. This neutralises this ion and eliminates its dehydrating effects on the SAP. Suitable anions include carbonate (C03-), a variety of phosphates (H2PQ4 ", HP04 2", Ρθ NH4P04 2"), hydroxide (OH*), sulphate (SO42*), fluoride (P), oxalate (C204 2~) or citrate (CeHsQ?3*).
The concentration of the restoration salt is suitably from 0.1 moi/L to 1.5 mol/L and preferably selected from 0.1 mol/L, 0.2 moi/L, 0.3 moi/L, 0.4 mol/L, 0.5 moi/L, 0.6 mol/L, 0.7 mol/L, 0.8 moi/L, 0.9 moi/L, 1.0 moi/L, 1.1 mol/L, 1.2 moi/L, 1.3 mol/L, 1 A mol/L or 1.5 mol/L and most suitably about 0.3 mol/L. By "about 0.3 mol/L" it is meant that the concentration is nominally 0.3 mol/L but the actual concentration may vary from this precise concentration to an unimportant degree. The restoration salt may suitably be an aqueous solution of alkali and ammonium salts, including: sodium carbonate ( a^COs); trisodium phosphate (Na3p04); sodium hydroxide (NaOH); potassium carbonate {K2CO3); potassium hydroxide (KOH); diammonium phosphate (( H4)2HPG4). The restoration salt may suitably be an aqueous solution of sodium carbonate (Na2C03).
The ratio of the restoration salt to dehydration salt is suitably from 1.0 to 1.5 times the stoichiometric requirement and preferably selected from 1.1 times, 1.2 times, 1.3 times, 1.4 times or 1.5 times.
The process may further include one or more post-processing steps including: rinsing the SAPs; dosing with additional chemicals; drying the SAPs.
Further features and advantages of the present invention will become apparent from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
To assist in understanding the invention and to enable a person skilled in the art to put the invention into practical effect, preferred embodiments of the invention will be described by way of example only with reference to the accompanying drawings, in which:
FiG 1 is a flow chart of the steps of a process for recovering and restoring super absorbent polymers from a material; and
FIG 2 is a schematic of a plant for working the process of FiG 1.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention reside primarily in a process and plant for recovering and restoring super absorbent polymers from a material. Accordingly, the steps have been illustrated in concise schematic form in the drawings, showing only those specific details that are necessary for understanding the embodiments of the present invention, but so as not to obscure the disclosure with excessive detail that will be readily apparent to those of ordinary skill in the art having the benefit of the present description. In this specification, adjectives such as first and second, left and right, and the like may be used solely to distinguish one element or action from another element or action without necessarily requiring or implying any actual such relationship or order. Words such as "comprises" or "includes" are intended to define a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed, including elements that are inherent to such a process, method, article, or apparatus.
Referring to FIG 1 there is shown a flow chart outlining the steps in a process to recover super absorbent polymers from a material containing super absorbent poiymers and at least partially restore the functionality of super absorbent poiymers to absorb liquids and multi-valent cations. In a preprocessing step the material containing the SAP is shredded or otherwise processed to open enclosed pockets and separate components so that chemicals applied in the process steps are able to penetrate the material and treat the SAP.
The first step of the process is to apply a dehydration salt to the SAPs so that they release absorbed liquids, lose adhesive characteristics and shrink to near original size. The dehydration salt may be applied as an aqueous solution (dehydration wash) or it may be applied dry, in which case an aqueous solution forms with liquid released from the SAP. The dehydration salt is an alkaline earth salt such as: magnesium sulphate (lV!gS04); magnesium chloride ( gCIa); magnesium nitrate (MgiNOafe); calcium chloride (CaCfe); calcium nitrate (Ca(N03)2) The dehydrated SAP is separated from the rest of the material using conventional separation techniques. SAP may be separated from the rest of the material using differences such as size, shape, density, magnetic or conductive properties. For example, separation may be achieved by sieving if the SAP particles are a different size or shape from the rest of the material. Another example may be by dropping material through a water flow or by using flotation to separate according to density. If the material contains any magnetic or conductive properties separation can be achieved using techniques that exploit these properties, such as by passing through a magnetic or electric field.
Recovered dehydration wash may be recycled to be reused in the dehydration step, The concentration of the dehydration solution is monitored to maintain the dehydration salts in a target range.
The SAPs are then treated with a restoration salt to restore absorbency. The restoration salt contains monovalent cations such as sodium, potassium or ammonium. The restoration salt may be applied dry and mechanically mixed through the SAPs, or applied as an aqueous solution (restoration wash). The anion in the restoration salt is selected so that it forms a precipitate with the cation of the dehydration salt at appropriate pH and temperature conditions. Suitable anions include hydroxide (OH"), carbonate (COa^), a variety of phosphates (H2PQ4 ", HP04 2~, PO43",NH4PO4 2"), sulphate (S04 2"), fluoride (F"), oxalate (C2O22") and citrate (CeHsOr3"). The anions may be introduced to the solution by means of dissociation.
For example, the carbonate (CO32") may dissociate from salts selected from a group comprising H2CO3, NaHCOa, a2COs, KHCO3 or K2CO3. The carbonate may also be introduced by bubbling carbon dioxide through water to form carbonic acid (H2CO3), which can then dissociate into bicarbonate (HCO3 ") and carbonate (C03 2~).
Suitable combinations of anions and SAP absorbed cations from the dehydration salt include:
Suitable Anion
CO32" OH" PO43" SO4 2" HPO4 2~ NH4PO42" F- Oxalate Citrate I combination with
Ca + ✓ ✓ ✓ s ■ ✓
g2* ✓ V x x < J V x where indicates a suitable combination and * indicates the combination is not suitable. These suitable anions may be obtained from salts, in combination with the accompanying ions shown in the following table:
Suitable Anion
CO32" OH" PO4 3~ S04 2' HPO42" NH4PO42" F- Oxalate in combination with
NH4 + ■f V / ■S · ÷ y'" X - <
Figure imgf000012_0001
where v indicates a suitable combination and * indicates the combination is not suitable.
During this step, the cations of the dehydration salt are drawn out of the SAP by osmotic pressure. This osmotic pressure is maintained by the scavenging of the cations by the anions of the restoration salt to form precipitate. Forming precipitate effectively removes any osmotic back pressure that would otherwise slow the removal of cations from the SAPs.
Once the ion exchange has proceeded to an acceptable point the SAPs may be washed to remove the excess restoration salt and precipitate. The precipitate may be removed by filtration and the restoration solution may be reused. The precipitate may be used as landfill or may find other environmentally friendly economic uses.
The extracted and washed SAPs may be dosed with further chemicals to promote drying to, say, less than 5 g/g moisture, or less than 4 g/g moisture, or less than 3 g/g moisture, or less than 2 g/g moisture, or less than 1 g/g moisture or nominally no moisture. The SAPs could also be dosed with other chemicals for use in different applications, such as fertiiizer. The SAPs are then dried using any combination of natural or thermally assisted techniques to a level optimised for shipping.
To further assist with understanding the invention a number of typical examples will now be presented, it should be understood that these examples are not meant to be limiting, only indicative. Example 1
In one example the super absorbent polymers in disposable nappies are recovered and restored. The nappies are pre-processed by being shredded to expose the SAPs and partially separate the SAPs from other components. Disposable nappies, and many other absorbent sanitary products, typically consist of an inner layer of liquid permeable material, an outer layer of liquid impermeable material, and an adsorbent or absorbent core of filler that contains the SAP. The liquid permeable material may be a non-woven sheet formed from polypropylene or polyethylene, or a woven product formed from cotton or rayon. The liquid impermeabl material may b polyethylene, polypropylene, starch based degradable plastic films, woven cloth or rubber. The adsorbent or absorbent core ma be air laid wood pulp fluff, commonly referred to as air felt, and/or synthetic fibres including polypropylene or polyethylene filaments. The core also contains a super absorbent polymer (SAP) material, which is typically a polyacrylate, polyacrylamide, cross-linked starch or other hydrophilic component, which may be synthetic, and ma be in granular, fibrous or laminate form.
The shredding step breaks away the layers and fillers to expose the SAPs to further processing. Processing continues on the mixed materials of the SAPs with the other components.
The mixed materials are treated with an aqueous solution of magnesium sulphate (Dehydration Wash) of 0.5 to 5 moi/L and preferably 2 mol/L. Magnesium sulphate is also known as Epsom salts. It is low in price, is relatively benign from an environmental perspective, with both magnesium and sulphur being important micro-nutrients. In another embodiment, the dehydration salt may be added to the mixed materials in a dry form, reliant on the moisture content of the mixed materials to form an aqueous solution. The dehydration salt and mixed materials are maintained in contact for a sufficient period in order to achieve adequate dehydration of the SAPs to permit subsequent separation. This contact period is reduced by agitating the materials to improve the rate of contact between the salt and the SAPs, and also by elevated temperatures and pressures, such as use of hot water or processing in an autoclave.
This treatment has the effect of dehydrating the SAPs by osmotic pressure and also substituting magnesium ions (Mg2+) for the monovalent cations originally in the SAPs or simply introducing additional ions. This treatment reduces the absorbency to less than 10 g/g, and shrinks the SAP 'bubbles', however the SAP 'bubbles' are still larger than the dry particles. This is advantageous for the process as the size of the SAP makes them easier to separate from the other material. In one example, where this method i used on sodium poiy-acrylate, the treated particles have a consistency of hard rubber. These effects eliminate the entrapment of other materials between SAP particles and also eliminate the adhesive nature of the SAPs.
Dehydrated and shrunken SAPs are then separated from other materials using a variety of techniques that exploit differences in size, shape, density, magnetic and conductivity properties. For mixed materials such as absorbent hygiene products, this may involve screening, flotation and cycloning. The Dehydration Wash is removed by methods such as screening and pressing, with the recovered solution returned to the previous part of the process. This solution is monitored and dosed to maintain the target concentration of dehydration salts. Once extracted from the mixed materials the SAPs are treated with an aqueous solution of sodium carbonate (Restoration Wash) of 0.5 to 5 mol/L and preferably 2 moi/L and in sufficient quantity to provide at least 1.0 to 1.5 times the stoichiometric requirement. Sodium carbonate is also known as washing soda and soda ash. It is low in price and relatively benign from an environmental perspective. The most significant effect is the potential of the carbonate ions to raise the pH of the waste water - a factor readily measured and corrected before eventual discharge.
During this step, the magnesium ions are drawn out of the SAP by osmotic pressure. This osmotic pressure is maintained by the scavenging of magnesium ions by the carbonate ions in the restoring solution, to form magnesium carbonate which precipitates under near neutral or high pH conditions. This effectively removes any magnesium ion osmotic back pressure that would otherwise slow the removal of magnesium from the SAPs. For SAPs originally containing sodium ions, such as those used in absorbent hygiene products manufacture, this largely restores their original chemistry and absorbency characteristics.
Once this ion exchange has progressed to an acceptable point, the SAPs are washed to remove the excess Restoration Wash, and loose particles of magnesium carbonate. These particles are removed by a fine screen, with the restoring solution returned to the previous stage of the process. The magnesium carbonate is collected as sludge for drying and eventual disposal to landfill, or as a raw material in its own right, for example in brick manufacture.
In experiments, this process has shown to extract sodium poly-acrylate SAPs from sanitary absorbent articles and restore absorbency as follows: The SAP was treated with a 0.2 mol/L solution of Epsom Salts for 15 minutes at room temperature. The treated SAP was then found to have an absorbed water content SAP of 10 g/g. Soda ash was then applied to the SAP as a powder at a stiochiometric ratio of 1.2 (CO3 2" to ! lg2*). The SAP was then rinsed to remove the remaining soda ash and magnesium carbonate precipitate and oven dried. The absorbency of the recovered SAP was in excess of 10Og/g of tap water.
In other embodiments, the restoring solution may use potassium carbonate instead of sodium carbonate. This has the effect of creating a potassium poly-acrylate SAP - a preferred chemistry in many applications. For example potassium salts are a common component of basic chemical fertilisers, which provide a balance of nitrogen, phosphorus and potassium (NPK).
Optionally, the cleaned SAPs may then be dosed with chemicals to promote further drying, i.e. to less than 5 g/g moisture, or to be absorbed into the SAPs for deployment in a new application, for example the application of fertilisers. In all cases, the additives must be environmentally acceptable in the new application. Exampie 2
In another example SAPs used to scavenge water based liquid spills are recovered and restored. The SAPs are likely to be mixed in with other detritus, soils and similar materials. The mixed materials are treated with an aqueous solution of magnesium sulphate (Dehydration Wash) of 0.05 to 1 mol/L and preferably 0,2 mol/L. The dehydration salt may also be added to the mixed matertais in a dry form, reliant on the moisture content of the mixed materials to form an aqueous solution. The dehydration salt and mixed materials are maintained in contact for a sufficient period in order to achieve adequate dehydration of the SAPs to permit subsequent separation. This contact period is reduced by agitating the materials to improve the rate of contact between the salt and the SAPs, and also by elevated temperatures and pressures, such as use of hot water or processing in an autoclave. Dehydrated and shrunken SAPs are then separated from other materials using a variety of techniques that exploit differences in size, shape, density, magnetic and conductivity properties, such as settling in water to remove dense particles, and screening to remove large items such as wood or cloth. Finally, the SAPs are screened to remove excess water. This water may be reused for the treatment of future batches, o treated and discarded.
Once extracted from the mixed materials the SAPs are treated with an aqueous solution of sodium carbonate (Restoration Wash) of 0.05 to 1.5 mol/L and preferably 0.3 mol/L, and in a minimum quantity of 1 to 1.5 times the stoichiometric requirement. Alternatively, the sodium carbonate may be added in a dry form, reliant on the moisture content of the mixed materials to form an aqueous solution. During this step, the magnesium ions are drawn out of the SAP by osmotic pressure. Once this ion exchange has progressed to an acceptable point, the SAPs are washed to remove the excess Restoration Wash, other soluble contaminants, and loose particles of magnesium carbonate. These particles are removed by a fine screen. Optionally, the cleaned SAPs may then be dosed with chemicals to promote further drying, i.e. to less than 5 g/g moisture, or to be absorbed into the SAPs for deployment in a new application, for example the application of fertilisers. In ali cases, the additives must be environmentally acceptable in the new application.
Plant
Referring to F!G 2, there is shown a schematic of a plant that is useful to work the method described above. Material containing SAPs to be processed are loaded into a hopper 20. The materials are loaded from the hopper 20 into a shredder 21 which may be, for example, rotating blades, teeth or overlapping sharp edged disks, or a grinder which combines both extrusion and cutting actions. Other suitable mechanisms for exposing the SAPs will be evident to persons skilled in the field.
The shredded material is transferred to a dehydrator 22 where the materia! is treated with dehydration wash. The dehydration wash may be an aqueous solution of magnesium sulphate or one of the other dehydration salts described above, in the simplest form the dehydrator 22 may be a vat but in other embodiments it may be an autoclave that also applies pressure and heat The material is resident in the dehydrator 22 for sufficient time for the SAPs to release the absorbed moisture, reducing absorbency to below 15g/g. Absorbency may be reduced to as low as 10 g/g or even 5 g/g. The dehydration wash may be recycled and is monitored for correct concentration of dehydration salts.
Once the SAPs have been adequately dehydrated the content of the dehydrator 22 is transferred to a separator 23 to separate the SAPs from other material. The separator 23 may be a sieve, flotation separator, air flow separator, magnetic separator, electric separator or other device capable of separating the SAPs from the remaining material or combinations of these. The remaining material is discarded as waste. By 'waste' it is meant that the material is not of further use in the process, but may nonetheless have other uses.
The SAPs are then sent to the restorer 24 for treatment with a restoration wash to restore absorbency to the SAPs. The restoration wash may be an aqueous solution of sodium carbonate or one of the other restoration salts described above. The restoration wash may be recycled and is monitored for correct concentration of restoration salts.
The restored SAPs may be rinsed in a rinser 25 and dried in a dryer 26.
Rinse from the rinser 25 is considered as waste to the process but may be useful elsewhere.
The plant of FIG 2 produces recovered and restored SAPs which can be reused.
The above description of various embodiments of the present invention is provided for purposes of description to one of ordinary skill in the related art. It is not intended to be exhaustive or to limit the invention to a single disclosed embodiment. As mentioned above, numerous alternatives and variations to the present invention will be apparent to those skilled in the art of the above teaching. Accordingly, while some alternative embodiments have been discussed specifically, other embodiments wilt be apparent or relatively easil developed by those of ordinary skill in the art. Accordingly, this invention is intended to embrace all alternatives, modifications and variations of the present invention that have been discussed herein, and other embodiments thai fall within the spirit and scope of the above described invention.

Claims

1. A process for recovering a super absorbent polymer in a form suitable for re-use from a material containing a super absorbent polymer including the steps of:
treating the super absorbent polymer with a dehydration salt to release absorbed liquid;
separating the super absorbent poiymer from the material; and
treating the super absorbent polymer with a restoration salt to restore absorbency.
2. The process of claim 1 further including the preliminar step of shredding or otherwise pre-processing the material to ensure access to the super absorbent polymer by the dehydration salt.
3. The process of claim 1 wherein the dehydration salt comprises Mg + or Ca2+ with a eounter-anion .
4. The process of claim 3 wherein the negative counter-ion is selected from SO42", CI- or NO3".
5. The process of claim 1 , wherein the dehydration salt is MgS04.
6. The process of any one of the preceding claims wherein the dehydration sait has a concentration of 0.05 moi/L to 1.0 moi/L.
7. The process of claim 8 wherein the dehydration salt has a concentration selected from: 0.05 mol/L; 0.1 mol/L; 0.15 mol/L; 0.2 mol/L; 0.25 moi/L; 0.3 moi/L; 0.35 mol/L; 0.4 moi/L; 0.45 mol/L 0.5 mol/L; 0.55 moi/L; 0.6 mol/L; 0.65 mol/L; 0,7 moi/L; 0.75 mol/L; 0.8 moi/L; 0.85 mol/L; 0.9 moi/L; 0.95 mol/L; or 1.0 moi/L.
8. The process of claim 1 wherein the dehydration salt has a concentration of 0.2 mol/L.
9. The process of claim 1 wherein the step of treating the super absorbent poiymer with the dehydration sait includes agitating the materials at an elevated temperature and pressure,
10. The process of claim 1 wherein the step of separating the super absorbent polymer from the material includes exploiting difference in size, shape, density, magnetic or conductive properties by:
if the super absorbent poiymer may be separated by exploiting the size and shape, filtration or screening;
if the super absorbent polymer may be separated by exploiting the density, flotation or cycloning;
if the super absorbent poiymer may be separated by exploiting the magnetic properties, by passing the super absorbent polymer through a magnetic field; and
if the super absorbent poiymer may be separated by exploiting the conductive properties, by passing the super absorbent polymer through an electric field.
11. The process of claim 1 wherein the restoration sait comprises OH", CO32", H2PO4 HPO42", P04 3", ΝΗ4ΡΟΛ SO42', F", oxalate (C2O42") or citrate (CeHsO?3"} with a counter-cation.
12. The process of claim 3 wherein, if the dehydration salt consists of Ca2+, the restoration sait comprises OH", CO32", H2P04\ HPO4 2", PO*3", NH4PO42", SO42", F" T oxalate (C2O42") and citrate {C6H5O73") with a counter-cation.
13. The process of claim 3 wherein, if the dehydration salt consists of g2+, the restoration salt comprises OH', CO32', H2PO4 , PO43", NH4PO42", P and oxalate (C2O42") with a counter-cation.
14. The process of any one of claims 11 to 13 wherein the counter-cation is selected from Na÷, NH4 + or K+.
15. The process of any one of claims 11 to 13 wherein, if the restoration salt consists of NH4PO2", the counter-cation is NH4 ÷
16. The process of any one of claims 11 to 13, wherein if the restoration salt consists of OH", the counter-cation comprises K+ or Na+.
17. The process of claim 1 , wherein the restoration salt is aaCOg.
18. The process of claim 1, wherein the restoration sait has a concentration of 0.1 moi/L to 1.5 mol/L.
19. The process of claim 18 wherein the restoration salt has a concentration selected from 0.1 mol/L, 0.2 moi/L, 0.3 mol/L, 0.4 mol/L, 0.5 moi/L, 0.6 mol/L, 0.7 moi/L, 0.8 mol/L, 0.9 mol/L, 1.0 mol/L, 1.1 mol/L, 1.2 moi/L, 1.3 mol/L, 1.4 mol/L or 1.5 mol/L.
20. The process of claim 1 wherein the restoration salt has a concentration of 0.3 moi/L.
21 . The process of ciaim 1 wherein the ratio of restoration salt to dehydration sait is in the range 1.0 to 1.5 times the stoichiometric amount.
22. The process of claim 21 wherein the ratio of restoration salt to dehydration salt is selected from 1.0 times, 1 .1 times, 1.2 times, 1.3 times, 1.4 times or 1.5 times.
23. The process of claim 1 , wherein the dehydration salt is MgS04 and the restoration salt is a2COs.
24. The process of claim 1 , wherein the dehydration salt and the restoration sait are applied to the super absorbent polymer as a solution or a soiid.
25. A plant for recovery and reactivation of super absorbent polymer from a materia! containing super absorbent polymer, the plant comprising:
a hopper in which a material containing super absorbent polymer is loaded;
a shredder that receives the material from th shredder and processes the materiai to expose the super absorbent polymer; a dehydrator that treats the exposed super absorbent poiymer with a dehydration wash;
a separator that separates the treated super absorbent poiymer from the material;
a restorer that teats the super absorbent poiymer with a restoration wash to reactivate absorbency;
a rinser that rinses the reactivated super absorbent poiymer with water to remove excess restoration sait; and
a dryer that dries the reactivated super absorbent poiymer.
PCT/AU2014/000716 2013-07-15 2014-07-14 Recovery of super absorbent polymers WO2015006799A1 (en)

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