Title of Invention

METHOD FOR INHIBITING CALCIUM SALT SCALE

Abstract

The present invention relates to a method for inhibiting calcium salt scale formation in chemical pulping processes comprising adding an effective scale inhibiting amount of at least one phosphonate compound to the alkaline aqueous mixture in the digester of said chemical pulping process, wherein said at least one phosphonate compound is selected from compounds having the formula: X<SUB>2</SUB>NCH<SUB>5</SUB>PO<SUB>3</SUB>M<SUB>2</SUB> (I), compounds having the formula: R,' l I Y -C-OH (II), I PO3M2 amine oxides of phosphonates of formula I), or mixtures thereof; wherein M is independently selected from hydrogen, alkali metal, alkaline earth metal or ammonium, X is independently selected from H; R, or -CH<SUB>2</SUB>PO<SUB>3</SUB>M<SUB>2</SUB>, wherein R is an alkyl group or -NX<SUB>2</SUB> substituted alkyl group having 2 to 6 carbon atoms, R' is an alkyl group having I to 17 carbon atoms and R' is optionally branched and optionally unsaturated, and Y is selected from -PO<SUB>3</SUB>M<SUB>2</SUB>, H or R'; with the proviso that: ' a) said scale inhibiting composition does not contain a non ionic surfactant, and b) that when said phosphonate compound is only N(CH<SUB>2</SUB>PO<SUB>3</SUB>M<SUB>2</SUB>)<SUB>3</SUB>, the amount of said phosphonate compound on an active acid basis is in the range of from 500 to 1000 ppm based on the weight of total liquor charged to said digester.

Full Text

METHOD FOR CALCIUM SALT SCALE FIOLD OF THE INVENTION This invention relates to compositions and methods for inhibiting scale formation in aqueous alkaline systems of chemical pulping processes. More particularly, this invention relates to compositions and methods for inhibiting formation, deposition and adherence of calcium salt scale deposits in chemical pulping process equipment. BACKGROUND OF THE INVENTION Paper is widely used worldwide in commerce and in homes and has a variety of uses. Pulp making is thus carried out on a large industrial scale worldwide to produce sufficient quantities of paper. Accordingly it is highly desirable that such pulp making operations be carried out in a cost effective, efficient operation with minimum manufacturing equipment downtime and minimum periods of reduced pulp making process equipment efficiency. The basic steps in industrial pulp making arc to convert plant fiber into chips, convert chips into pulp, (optionally) bleach the pulp, wash the pulp, and transform the pulp into suitable paper which can be used in paper products such as writing paper, newsprint and paper for documents. Typically, several chemical pulping processes arc used in industrial pulp making operations. Well known industrial alkaline chemical pulping processes include the Kraft (or sulfate), soda and alkaline sulfite processes. The Kraft process makes the strongest fibers of any pulp producing process and is the most commonly used pulp making process in part due to its efficient recovery process for the cooking chemicals. While the present invention has applicability to any of the above alkaline chemical pulping processes, it is particularly useful with the Kraft process and, as such, the Kraft process is described in more detail below. Initially, suitable trees are harvested, debarked and then chipped into suitable size flakes or chips. These wood chips are sorted with the small and the large chips being removed. The remaining suitable wood chips are then charged to a digester (which is a vessel or tank for holding the chips and an aqueous digesting composition, such tanks can be designed for either batch or continuous operation). Illustratively, in a batch type digester, wood chips and a mixture of "weak black liquor," the spent liquor from a previous digester cook, and "white liquor," a solution of sodium hydroxide and sodium sulfide, that is either fresh or from the chemical recovery plant, is pumped into the digester. In the cooking process lignin. which binds the wood fiber together, is dissolved in the white liquor forming pulp and black liquor. The digester is sealed and the digester composition is heated to a suitable cook temperature under high pressure. After an allotted cooking time at a particular temperature and pressure (H-factor) in the digester, the digester contents (pulp and black liquor) are transferred to a holding tank. The pulp in the holding tank is transferred to brown stock washers while the liquid (black liquor formed in the digester) is sent to the black liquor recovery area, i.e. black liquor evaporators. The black liquor is evaporated to a high solids content, usually 60-80% solids, using a multiple effect evaporator, for example. The higher the solids content, the more difficult it is to pump the black liquor and the more scale problems the pulp mill will have. One of the most troublesome is calcium carbonate scale which forms in various areas of the pulp mill, including the digester, the black liquor evaporator area, and the brown stock washing area. Most commercial paper mills use multiple effect evaporators (MEE) as the black liquor evaporators. These evaporators generally range from four to eight effects in length. Generally, undesirable calcium carbonate scaling occurs in only one or two effects. Currendy, most mills do not use any scale inhibitor but rather contend with the scale problem by shutting down the black liquor evaporator section and washing out the calcium carbonate scale with hot acid, i.e. acid cleaning. Tries hot acid boil out adversely affects papyri production and is a concern because the acid used is corrosive to mill piping and equipment. The Kraft cook is highly alkaline, usually having a pH of 10 to 14, more ancillary 12 to 14. The digester composition contains a large amount of sodium sulfide, which is used as an accelerant to increase the dezincification rate of the cook. This works to release the lignin in the wood chips and thus the cellulose becomes available as pulp. The combination of operating conditions in the Kraft process is conducive to scale formation and deposition and increases the propensity of the calcium carbonate scale to form, deposit and adhere to metallic and other surfaces within which it comes in contact. Under such process conditions, calcium present in the water and leached from the wood in the Kraft process can react with carbonate and produce rather rapid scaling with the deposition of calcium carbonate scale. Such scale is frequently deposited in the digester, piping, heat exchangers etc., all of which have surfaces on which the calcium carbonate can deposit and adhere. Such deposition builds up over time and can result in undesirable premature shutdowns downstream on the pulp making manufacturing line to remove scale deposits by hot acid washing. Several patents and a technical article disclose problems of scaling. In 'An Effective Sequestrant For Use In Controlling Digester Scale," R.H. Windbags, Paper Trade Journal, pp. 42-44, November 5, 1973, the use of small quantities of mono-amino ethylene phosphonic acid (ATM?) as a calcium carbonate scale inhibitor in a digester to inhibit scale deposition from the digester cooking liquor is disclosed. U.S. Patent No. 4,799,995 (issued to Deuce K. Crump et al. on January 24, 1989) discloses that inhibition of calcium scale under conditions found in pulp digesters has been accomplished by employing mixtures of polyamino(polyalkyIenephosphonic) acids with non-ionic surfactants added to the pulp liquor. This U.S. patent also discloses that phosphonites such as nitrilotris(mcthylenephosphonic acid) ("NTMP"or "ATMP'), 1-hydroxycthane-l,!-diphosphonic acid ("HEDP") and sodium l-hydroxyethane-l,l-diphosphonate ("NaHEDP") are said to have been commonly used to control scale. However, the '995 patent discloses that the use of HEDP in black liquor actually promoted scale and use of diethylenetriamine penta(methylencphosphonic acid) ("DTPMP") in black liquor without the presence of a nonionic surfactant resulted in only limited scale reduction. While the '995 patent discloses the use of nonionic surfactants to improve scale reduction, it is preferred to avoid the use of surfactants in chemical pulp processes, particularly in the digester. The compositions of the present invention when added to an alkaline chemical pulp process digester are effective at inhibiting calcium salt scale in chemical pulp processes without the need for a nonionic surfactant. Canadian Patent No. 1,069,800 (Philip S. Davis ct al., Jan. 15, 1980) discloses the addition of blends of organophosphonates, e.g. l-hydroxyethyIidcne-1,1-diphosphonic acid (HEDP), with amino-organo phosphonates, e.g. amino tri(methylenephosphonic acid) (AMP), ethylenediamine tetra(methylenephosphonic acid) (EDTPA) and hexaraethylenediamine tetra(methylenephosphonic acid) (HMDTA), to black liquor to reduce calcium carbonate scale in a black liquor evaporator system at a pH above 9. This patent also discloses that use of individual (single) phosphonates, instead of the disclosed blends, were not effective at a pH above 9 to inhibit calcium carbonate crystallization. U.S. Patent No. 4,851,490 (issued to Fu Chen et al. on July 25,1989) discloses water soluble polymers containing hydroxyalkyleneaminoalkylene phosphonate functions which are said to have utility as deposit control agents effective in a number of water systems such as cooling, boilers, conversion coating, paper and pulp processing and gas scrubbing. U.S. Patent No. 5,534,157 (issued to Craig D. Iman et al. on July 9,1996) discloses a method for inhibiting the formation, deposition and adherency of scale-forming salts in process waters at high pH utilizing polyethers polyamine methylene phosphonates. At column 4, lines 35-51 thereof, this U.S. patent discloses that inhibitors such as HEDP and ATM? are useless as scale inhibitors at alkaline pH conditions. U.S. Patent No. 5,562,830 (issued to Davor F. Zidovec et al. on October 8, 1996) discloses a method of inhibiting corrosion and scale formation and deposition in aqueous systems by adding a combination of a polycpoxysuccinic acid or salts thereof and a phosphonocarboxylic acid or salts thereof. U.S. Patent No. 5,552,018 (issued to Johan Devenyns on September 3,1996) discloses a process in which a peroxyacid is employed to improve the selectivity of the dezincification of a chemical paper pulp that has already undergone a dclignifying treatment in the presence of chemical reagents, i.e. a Kraft cook. Phosphonates are disclosed as stabilizers in this process. Despite the aforementioned patents and technical article, enhanced methods and compositions for inhibiting the formation, deposition and adherence of scale to metallic surfaces particularly in commercial chemical pulp processing equipment is highly desired. SUMMARY OF THE INVENTION It is an object of this invention to provide a composition for inhibiting the formation, deposition and adherence of calcium salt scale to metallic and other surfaces in the equipment, vessels and/or piping of a chemical pulp process facility. It is yet another object of this invention to provide a method for inhibiting the formation, deposition and adherence of calcium salt scale to surfaces in the equipment, vessels and/or piping of a chemical pulp process facility. These and other objects are achieved in the invention which is described in more nonlimiting detail hereinafter. According to the invention, a scale inhibiting composition for inhibiting calcium salt scale formation in alkaline aqueous mixtures of chemical pulping processes is provided, wherein the composition is added to the digester of a chemical pulping process, the composition comprising an effective scale inhibiting amount of at least one phosphonate selected from compounds having the formula: amine oxides of the phosphonates of formula (I), or mixtures thereof; wherein M is independently selected from hydrogen, alkali metal, alkaline earth metal or ammonium, X is independency selected from H, R, or -CH.POjM, wherein R is an alk7l group or -NX, substituted alkyl group having 2 to 6 carbon atoms, R' is an alkyl group having 1 to 17 carbon atoms and R' is optionally branched and optionally unsaturated, and Y is selected from -PO^M^, H or R'; with the proviso that when the phosphonate is N(CI LPO.M.),, the amount of the phosphonate on an active acid basis is greater than 25 ppm based on the weight of total liquor charged to the digester. Further according to the invention, a method for inhibiting calcium salt scale formation in chemical pulping processes is provided comprising admixing an effective scale inhibiting amount of the above composition with the alkaline aqueous mixture in the digester of the chemical pulping process. Still further according to the invention, a method for inhibiting calcium salt scale formation in an aqueous system in a chemical pulping process having a sufficient quantity of available calcium cations and anions selected from carbonate and sulfate to form said calcium salt scale is provided, comprising admixing an effective scale inhibiting amount of at least one phosphonate with the aqueous system in the digester of the chemical pulping process maintained in a temperature range to inhibit calcium salt scale formation, wherein the at least one phosphonate is as defined above. DETAILED DESCRIPTION OF THE DRAWINGS NOT APPLICABLE. DETAILED DESCRIPTION OF THE INVENTION A first embodiment of the invention relates to a scale inhibiting coraposidon for inhibiting calcium salt scale formation in alkaline aqueous mixtures of chemical pulping processes, wherein the composition is added to the digester of a chemical pulping process, the composition comprising an effective scale inhibiting amount of at least one phosphonate selected from compounds having the formula: amine oxides of phosphonates of formula (I), or mixtures thereof; wherein M is independently selected from hydrogen, alkali metal, alkaline earth metal or ammonium, X is independently selected from H, R, or -CH2PO3M2 wherein R is an alkyl group or -^JX2 substituted alkyl group having 2 to 6 carbon atoms, R' is an alkyl group having 1 to 17 carbon atoms and R' is optionally branched and optionally unsaturated, and Y is selected from -Poem,, H or R'; with the proviso that when the phosphonate is N(CH2P03M2)3, the amount of the phosphonate on an active acid basis is greater than 25 ppm based on the weight of total liquor charged to the digester. In the phosphonates of the invention, M is preferably hydrogen or alkali metal, and the alkali metal is preferably sodium and potassium, X is preferably R or -CH2PO3M;, Y is preferably -PO3M., and R' is preferably an alkyl group having 1 to 5 carbon atoms. Examples of suitable phosphonates include, but are not limited to, the phosphonates in Table 1 below. Table I below provides formulas for representative phosphonates of formulas (I) and (II)- The phosphonates in Tabic 1 are available from Solaria Inc., 575 Maryville Centre Drive, St. Louis, MO under the trademark Dequest® phosphonates and are identified by their Dequest® phosphonate product number. A series of blends of phosphonates which may be used according to the invention were prepared for testing- The blends were prepared as concentrates having 30% total active acid content and were then diluted to the desired concentration for use. These blends (as described below) were tested as calcium salt scale inhibitors in a simulated Kraft cook according to the procedure described in the Examples. The weight ratios of these various blends are shown in Table 2 below. ethylenediamine tetra(metliylcnephosphonic acid), diethylenetriaminepcnta(methylenephosphonic acid) or salts thereof with another phosphonate selected from the phosphonates of formulas (I) and blends of N,N'-bis(3-aminopropyl)ethylenediaraine-hexa(methylenephosphonic acid) or salts thereof with a phosphonate selected from the phosphonates of formula (11). An effective amount of phosphonate or mixtures of phosphonates is employed in making and using the scale inhibiting composition of this invention. That effective amount depends on the particular phosphonate(s) employed in practicing this invention and other factors including, but not limited to, the digester composition, the operating conditions (i.e. H-factor) of tlie digester, the composition and operating conditions in the brown stock washing area and black liquor recovery area, as well as other factors and conditions known to those of ordinary skill in the art. Selection of the effective amount of phosphonate will be readily apparent to one of ordinary skill in the art after reading this specification. The scale inhibiting composition of the invention include, but are not limited to, at least one phosphonate of formula (I), at least one phosphonate of formula (11), at least one amine oxide of a phosphonate of formula (I), a mixture of at least two pliosphonates of formula (I), a mixture of at least one phosphonate of formula (1) or an amine oxide of a phosphonate of formula (I) and at least one phosphonate of formula (IF), a mixture of at least one phosphonate of formula (I) and at least one amine oxide of a phosphonate of formula (I), or a mixture of at least two phosphonates of formula (II), Preferably, the scale inhibiting composition of the invention is at least one phosphonate of formula (I), a McClure of at least two phosphonates of formula (I), or a mixture of at least one phosphonate of formula (1) and at least one phosphonate of formula (II). When the scale inhibiting composition of the invention is at least one phosphonate of formula (I), the phosphonate(s) and the effective scale inhibiting amount of each is as follows. As used herein, the ppm usage level of scale inhibitor is based on the weight of total liquor charged with the liquor assumed to have a density of 1 g/mL. When the phosphonate is N(CH.P03M;)3, the effective scale inhibiting amount of phosphonate on an active acid basis is about 500 to about 1000 ppm, and preferably about 600 to about 800 ppm, based on the weight of total liquor charged to the digester. When the phosphonate is (M.03PCH.):NCH.CH,N(CH.P03M2):, the effective amount of the phosphonate on an active acid basis is about 10 to about 1000 ppm. Further according to the second embodiment of the invention, the invention is also a method for inhibiting calcium salt scale formation in an aqueous system in a chemical pulping process having a sufficient quantity of available calcium cations and anions selected from carbonate and sulfate susceptible to form said calcium salt scale, comprising admixing an effective scale inhibiting amount of at least one phosphonate with the aqueous system in the digester of the chemical pulping process maintained in a temperature range of about 1 to about 180°C, preferably about 150°C to about ITS C, to inhibit calcium salt scale formation, wherein the phosphonate is as described above. In the practice of the metalloid of this invention in a chemical pulping process, e.g. a Kraft process, the aqueous phosphonate composition of the invention is adduced with an alkaline, aqueous composition in the digester. The aqueous phosphonate composition of the invention can be added to the digester using any conventional means known to those of ordinary skill in the art. In addition, the aqueous phosphonate composition of the invention can be added directly to the digester composition or it can be introduced into one of the aqueous feed compositions being charged to the digester prior to charging of that aqueous feed composition. The pH in the digester of an alkaline chemical pulping process is at least 9. In the case of a Kraft process, the pH in the digester is preferably about 10 to about 14, and more preferably about 12 to about 14. The aqueous phosphonate composition of the invention can be added in a batch digester in any conventional manner known to one of ordinary skill in the art. For example, in a batch digester operation, the addition of the aqueous phosphonate composition of the invention can be a bulk addition at the beginning of the digester cook cycle or during the digester cook cycle, or it can be added in multiple charges throughout the digestion cycle or continuously throughout the digester cook cycle. It is currently preferred to add the aqueous phosphonate composition of the invention as a bulk charge at or near the beginning of the digester cook cycle. In the case of a continuous digester operation, the addition of the aqueous phosphonate composition of the invention will typically be added continuously to maintain the effective concentration of phosphonate. The amount of a scale inhibiting composition of this invention employed is an effective amount which is that amount that is sufficient to provide an effective scale inhibiting concentration of phosphonate in the digester over time at which the formation, deposition and adherence of calcium salt scale, particularly calcium carbonate or calcium sulfate scale, is satisfactorily inhibited in the digester, brown stock washers and/or black liquor recover}* area. One of ordinary skill in the art using this invention will know the acceptable level of calcium salt scale in the digester, brown stock washing area, and black liquor recovery area of the particular chemical pulping facility, and will be able to readily select an appropriate phosphonate and concentration for addition to the digester to achieve the desired scale inhibition for the required time based on the disclosure of this specification. It will be apparent to those of skill in the art after reading this specification that many factors of the type which have been mentioned herein and others, will determine the amount of the phosphonate of the invention needed to achieve the desired inhibition. The determination of these amounts is within the ordinary skill of the artisan in this field without undue experimentation considering the direction provided herein. A third embodiment of the invention relates to a method for inhibiting calcium salt scale formation in an aqueous system in a selected chemical pulping process comprising (a) identifying the calcium salt scale inhibition capability required by the selected chemical pulping process based on the process operating conditions of time, temperature and pressure, and the aqueous digester composition, (b) selecting the appropriate phosphonate composition and phosphonate use concentration to effectively inhibit calcium salt scale formation in the selected chemical pulping process when the phosphonate is admixed with the aqueous digester composition in the selected chemical pulping process based on step (a) and the calcium salt scale inhibition profiles of phosphonate concentration and process temperature as a function of time for phosphonate compositions admixed with the aqueous digester composition in a chemical pulping process digester, and (c) admixing the selected phosphonate composition with the aqueous digester composition in the selected chemical pulping process during the digestion stage of the chemical pulping process; wherein the selected phosphonate composition is as defined above for this invention. A fourth embodiment of the invention relates to a method for inhibiting calcium salt scale fonn ation in an aqueous system in a selected chemical pulping process comprising (a) determining the calcium salt scale inhibition profiles of phosphonate concentration and process temperature as a function of time for phosphonate compositions admixed with the aqueous digester composition in a chemical pulping process digester, (b) identifying the calcium salt scale inhibition capability required by the selected chemical pulping process based on the process operating conditions of time, temperature and pressure, and the aqueous digester composition, (c) selecting the appropriate phosphonate composition and phosphonate use concentration to effectively inhibit calcium salt scale formation in the selected chemical pulping process when the phosphonate is admixed with the aqueous digester composition in the selected chemical pulping process based on steps (a) and (b), and (d) arraying the selected phosphonate composition with the aqueous digester composition in the selected chemical pulping process during the digestion stage of the chemical pulping process; wherein the selected phosphonate composition is as defined above for this invention. In the third and fourth embodiments of the invention, the calcium salt scale inhibition profiles of phosphonate concentration and process temperature as a function of time for phosphonate compositions admixed with the aqueous digester composition in a chemical pulping process digester can be determined by conducting laboratory experiments, such as described herein, or by conducting larger scale testing. As each chemical pulping process will vary depending on the type of wood being processed, the specific operating conditions used, the composition in the digester, and the like, the specific phosphonate or phosphonate blend and ti)c required use concentration of same necessary to achieve the desired scale inhibition will be dependent upon the specific chemical pulping process. By utilizing the calcium salt scale inhibition profiles in conjunction with the calcium salt scale inhibition capability required by the selected chemical pulping process based on its process operating conditions of time, temperature and pressure, and the aqueous digester composition, one of ordinary skill in the art may select the appropriate phosphonate composition and phosphonate use concentration to effectively inhibit calcium salt scale formation in the selected chemical pulping process when the phosphonate is admixed with the aqueous digester composition in the sealed chemical pulping process. The invention is further described in the following Examples which are not intended to limit or restrict the invention. Unless otherwise indicated all quantities are expressed in weight. EXAMPLES A Kraft cook test was employed in the following examples and illustrates the use of the compositions of this invention in the process of this invention. In carrying out these tests, samples were taken of a composition of the digester at selected times during the cook. The concentration of total calcium and inhibited calcium were determined analytically using Atomic Absorption Spectroscopy (AA). The general procedure described below was followed. Additionally, the tests were generally carried out at inhibitor levels of 10,50,100 and 500 parts per million (ppm) active acid based on the amount of total liquor charged to the digester, for each phosphonate composition tested, and also with no inhibitor present. As used herein, the active acid level is that amount of free acid which is equimolar to the amount of phosphonic that was actually added. Unless otherwise specified, use of "%" is on a weight basis. KRAFT COOK TEST Tlie craft Cook Test used herein was developed to gauge the pcrfoniiance of scale inhibition of compositions of this invention in a simulated digester composition wherein calcium is slowly extracted from the wood chips into the Kraft system. Tlie test was a standard Kraft cook with a 5:1 liquor to wood ratio in a MK Systems Inc. minim ill laboratory digester. Tic digester aqueous composition temperature was ramped from ambient temperature to ISO in one hour and then maintained at 1S0°C for an additional one to two hours. Samples were taken from the digester using a liquid cooled extractor at various time intervals under high pressure and temperature during the cook to monitor calcium concentrations by AA as described in the "Monitoring Calcium Release During Kraft Cook" section below. Drying of wood chips: Pine wood chips were passed through a 12.5 mm slotted screen, with the small pins being removed. The chips were sorted by hand to remove any bark or knots, and the wood chips dried at 1 lOPC for 12 hours. This was done ie reduce variability with moisture and extractives. The wood chips were stored in a container with desiccant and allowed to cool to room temperature. Preparation of White Liquor/Charize of Digester: A liquor to wood ratio of 5:1 was prepared with 18.5% effective alkali, having a 25% solidity and 5 grams per liter of sodium carbonate. The sodium carbonate introduced into the white liquor was representative of that which is typically carried over in the recovery process in a Kraft mill. The charge of phosphonate employed was based upon the weight of total liquor charged to the digester to give the desired equivalent ppm of active acid in the digester. White liquor was prepared according to the following procedure. Approximately 2 liters of double-demonized water were transferred to a 4 liter volumetric flask. 322.99 g of 50% sodium hydroxide, 163.76 g Na,S-9H,0, and 20.0 g anhydrous sodium carbonate were added to the 4 liter flask and dissolved, enough inhibitor was added to reach the desired concentration, and double demonized water added to fill to the mark. Prior to running the test, the digester was acid cleaned using a 10% sulfuric acid solution to remove any existing deposits. After the acid cleaning, the digester was rinsed with distilled water. 800 grams of dried Pine wood chips, prepared as described above, were added to the wood chip holder. White liquor (4L) and wood chips were transferred to the digester and the infidel temperature and time recorded. Monitoring Calcium Release During Kraft Cook: A 5-mL sample was taken for AA analysis and the heating sequence in the digester was initiated. (The AA analysis is done by atomic absorption by flame photometry using a Perkin Elmer model 100 spectrometer; see generally. Instrumental Methods of Analysis, Hobart H. Willard, Lynn L. Merritt, Jr.; John A Dean, 4"' Edition, D. Van Nostrand Company, Inc. August 1965) Quantitatively one milliliter (mL) of the sample was transferred to a centrifuge tube with 5 mL of 4% HCl solution and AA was used to determine the calcium content of the sample, i.e. Total Calcium. The remaining sample was drawn into a 10 mL syringe and filtered through a 0.45-um .syringe filter. Quantitatively one mLof the tiltrate was transferred to a centrifuge tube with 5 mL of 4% HCl solution and AA was used to determine the calcium content of the tilt rate, i.e. Inhibited Calcium. Every 15 minutes for the length of the test, e.g. appro.ximately 2-4 hours, the liquor in the condenser line was purged, a temporary measurement was made, and a 5 mL liquor sample was pulled. The AA analysis procedure as described above was then repeated. At the end of the test, the calcium content and temperature data were plotted versus time. Each example below was carried out according to the general procedure recited above. In most examples, the phosphonates were tested at four concentration levels. All levels are given in parts per million phosphonate on an active acid basis by weight total liquor. Except as specified herein, chemicals used in the examples were obtained from Fisher Scientific. Dequest phosphonates, used individually and in blends in the examples, were obtained from Solutia Inc. (St. Louis, MO). 4iSnFIMP was prepared according to the procedure described herein. Tables 3-96 hereinafter following provide the data for a series of test runs performed on the digester at various levels of phosphonates and mixtures of phosphonates. The phosphonate or blend tested are identified by product name (as defined in Tables 1 and 2 herein) in the header of each Tabic below. The temperature is in degrees Celsius. Parts per million (ppm) of calcium is in parts per million by weight based on the total liquor. Example 1 Dequest 2006 was tested in the Kraft Cook Test described in the Examples section at 500,100,50 and 10 ppm active acid. The results are given in Tables 4-7 below. In addition, a control experiment with no added inhibitor was run and the results are given below in Table 3. The data in Tabic 3 can be used as the control for Examples 1-8. Acquits 2046 and 4 fHMP in the use range of about 20 to about 1000 ppm would be effective to inhibit calcium salt scale according to the invention. The preceding description is for illustration and should not be taken as limiting. Various modifications and alterations will be readily suggested to persons skilled in the art. It is intended, therefore, that the foregoing be considered as exemplary only and that the scope of the invention be ascertained from the following claims. It is further intended that each and every claim limitation be literally construed to include any and all variants which are insubstantially different from what is literally recited except variants which are in the prior art. CLAIMS WHAT IS CLAIMED IS: 1. A scale inhibiting composition for inhibiting calcium salt scale formation in alkaline aqueous mixtures of chemical pulping processes, wherein said composition is added to the digester of said chemical pulping process, said composition comprising an effective scale inhibiting amount of at least one phosphonate selected from compounds having the formula; amine oxides of phosphonates of formula (1), or mixtures thereof; wherein M is independently selected from hydrogen, alkali metal, alkaline earth metal or ammonium, X is independently selected from H, R, or -CH.PO,M, wherein R is an alkyl group or –NX substituted alkyl group having 2 to 6 carbon atoms, R' is an alkyl group having 1 to 17 carbon atoms and R' is optionally branched and optionally unsaturated, and Y is selected from -PO,M., H or R'; with the proviso that when said phosphonate is N(CH2P03M,)3, the amount of said phosphonate on an active acid basis is greater than 25 ppm based on the weight of total liquor charged to said digester. 2. The composition of claim 1 wherein M is independently selected from hydrogen or an alkali metal. 3. The composition of claim 2 wherein M is sodium or potassium when M is an alkali metal. -(CHJXNX'J, wherein n is an integer from 2 to 6 and X' is independency selected from R or -CH2PO3M2. 7. The composition of claim 1 wherein Y is -PO3M2. 8. Tiie composition of claim 7 wherein R' is an alkyl group having 1 to 5 carbon atoms. 9. The composition of claim 1 wherein said phosphonate is at least one phosphonate of formula (I). 10. The composition of claim 1 wherein said phosphonate is at least one phosphonate of formula (II). 11. The composition of claim I wherein said phosphonate is at least one amine oxide of phosphonates of formula (I). 12. The composition of claim 1 wherein said phosphonate is a mixture of at least two phosphonates of formula (I). 13. The composition of claim 1 wherein said phosphonate is a mbcture of at least one phosphonate of formula (I) and at least one phosphonate of formula (II). 14. The composition of claim 1 wherein said phosphonate is a mbcture of at least two phosphonates of formula (II). 15. The composition of claim 9 wherein said phosphonate is N(CH2P03M2)3 and the amount of said phosphonate on an active acid basis is about 500 to about 1000 ppm based on the weight of total liquor charged to said digester. 16. The composition of claim 10 wherein said phosphonate is CH3C(OH)(P03M2).. 17. The composition of claim 16 wherein the amount of said phosphonate on an active acid basis is about 20 to about 200 ppm based on the weight of total liquor charged to said digester. 18. The composition of claim 9 wherein said phosphonate is (M.03PCH.),NCH2CH.N(CH3P03M2)2. 89. The method of claim 88 wherein said calcium salt is calcium carbonate. 90. The method of claim 43 wherein the pH of said alkaline aqueous mixture is at least 9, 91. A method for inhibiting calcium salt scale formation in an aqueous system in a chemical pulping process having a sufficient quantity of available calcium cations and anions selected from carbonate and sulfate susceptible to form said calcium salt scale, comprising admixing an effective scale inhibiting amount of at least one phosphonate with said aqueous system in the digester of said chemical pulping process maintained in a temperature range to inhibit calcium salt scale formation; and wherein said phosphonate is selected from compounds having the formula; 93. Tlie method of claim 55 wherein said phosphonate is a mixture of N(CH.P03M2)3, and CH3C(OH)(P03M2)2, and the amount of said mixture on an active acid basis is about 30 to about 500 ppm based on the weight of total liquor charged to said digester. A scale inhibiting composition substantially as herein described and exemplified.

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