Process for obtaining granules of a stable adduct of sodium sulphate and hydrogen peroxide containing sodium chloride

Abstract: The procedure comprises the direct granulation of an intimate mixture of Na 2 SO 2 and NaCl, with H 2 O 2 , in the appropriate stoichiometric proportion. The granulated adduct [4Na 2 SO 4 .2H 2 O 2 .NaCl] obtained is intrinsically very stable, and, once dry, only contains up to 4.6 % of active oxygen. Said adduct can be used as a whitener in detergent formulations.

Description: FIELD OF THE INVENTION The invention relates to a procedure for obtaining a stable adduct of Na 2 SO 4 and H 2 O 2 containing NaCl, in the form of granules, that comprises the direct mixing and granulation of the reactants. BACKGROUND OF THE INVENTION Traditionally, sodium perborate (SPB) has been used as a whitener in washing powders. To the benefit of the manufacturers of washing powders, SPB has ceded terrain in the market to sodium percarbonate (SPC), which is less stable but cheaper. Both persalts, in aqueous solution, give rise to a strong alkalinity that limits their field of application. To palliate this disadvantage, it was decided to obtain adducts of H 2 O 2 and neutral salts such as Na 2 SO 4 . Thus, various adducts are known, of generic formula Na 2 SO 4 .mH 2 O 2 .nH 2 O, where m = 0.5 -2 and n = 0 - 1, the stability of which is insufficient to give them a practical use. 4Na 2 SO 4 .2H 2 O 2 .NaCl (SPS) is an adduct obtained in aqueous solution by reaction of Na 2 SO 4 , NaCl and H 2 O 2 , as described in the US patents US 3.979.313 and US 4.005.182. This crystalline compound has good properties as a whitener, dissolves quickly and its solubility in cold water is high. In addition, it is prepared from non-toxic salts that are abundant, cheap and used regularly in washing powder formulations. Nevertheless, it is not directly applicable to washing powders because it is unstable in the presence of alkaline salts. FMC Corp. (US 4.323.465) have stabilised the adduct to allow its application in alkaline media through the incorporation of traces of organic polyphosphonates, for example, DEQUEST 2000 and DEQUEST 2010, into its crystalline structure. In other subsequent patents, such as US 4.618.445, other stabilisation procedures were mentioned that, either are complex, such as coating with aluminium stearate, or suppose the addition of significant quantities of phosphates, for example, up to 10% of sodium hexamethaphosphate during the crystallisation stage. These practices give rise to a product with less whitening power and to other limitations in its application derived from the compatibility with the added compounds. The known procedures for obtaining the adduct 4Na 2 SO 4 .2H 2 O 2 .NaCl are based on crystallisation, such as the one described in the US patent 4.005.182. There is no knowledge that the adduct is obtained on an industrial scale with this procedure. The current tendencies in washing powders in terms of reducing the energy costs through "post-dosing", the introduction of new more compact presentations (tablets, granules) and an improvement in public health (ecotoxicity, toxicity when inhaled) suggest that the use of formulations should be fostered that are based on innocuous compounds presented in granular form. The invention provides a response to said demand that comprises the production of 4Na 2 SO 4 .2H 2 O 2 .NaCl by direct granulation. It is known from the state of the art the patent US 3979312 where a process for the production of 4Na 2 SO 4 .2H 2 O 2 .NaCl is disclosed, in which a sodium sulphate and sodium chloride were mixed with an aqueous solution of 30% hydrogen peroxide. The temperature was kept at 40°C. Now the object of our invention is to improve the process for obtaining said product, avoiding the use of any solution, being used instead of a solution a granulating process where the solids are granulated under addition of hydrogen peroxide. DETAILED DESCRIPTION OF THE INVENTION The invention provides a procedure for obtaining an adduct of the formula 4Na 2 SO 4 .2H 2 O 2 .NaCl, in the form of granules, hereinafter, the procedure of the invention, which comprises the mixing and direct granulation of the reactants (Na 2 SO 4 , NaCl and H 2 O 2 ). In a particular embodiment of the invention, the procedure of the invention comprises the direct granulation of an intimate mixture of Na 2 SO 4 and NaCI finely ground with H 2 O 2 . In the case that one or both raw materials do not have the appropriate grain size, the procedure of the invention includes a previous stage of grinding and cutting up of the raw material or materials, using conventional methods, until the appropriate grain size is attained. To perform the procedure of the invention, the grain size of the raw materials should be less than 100 µm, preferably lying between 0 and 50 µm. In the case that the raw materials Na 2 SO 4 and NaCl had to be ground up, these could be ground up separately or, alternatively, together. Therefore, when both raw materials have the appropriate grain size, the procedure of the invention can be performed by means of direct granulation of an intimate mixture of Na 2 SO 4 and NaCl with the appropriate grain size, with H 2 O 2 . Alternatively, when one or the two raw materials (Na 2 SO 4 and NaCl) do not have the appropriate grain size, the procedure of the invention may be performed mixing said raw materials and grinding them up together until the appropriate grain size is obtained. In a particular embodiment, the mean size of the particles of the raw materials (Na 2 SO 4 and NaCl) is equal to or less than 50 µm. Once the mixture of raw materials has been prepared with the appropriate grain size, said mixture of raw materials is brought into contact with H 2 O 2 . The H 2 O 2 can be found, in a particular embodiment, in the form of an aqueous solution of H 2 O 2 , in a variable concentration, for example, between 50 % and 70 %. Once the mixture of raw materials and H 2 O 2 is brought into contact, the direct granulation is performed from the resulting mass in appropriate equipment, and the granulates obtained are dried. In another particular embodiment, the procedure of the invention comprises the direct granulation of Na 2 SO 4 with the appropriate grain size, by means of spraying said raw material with a solution of NaCl and H 2 O 2 with subsequent drying of the granulates obtained. In a particular embodiment, the Na 2 SO 4 has a grain size equal to or lower than 50 µm, for which it has been necessary to perform a previous grinding of the raw material, and said solution of NaCl in H 2 O 2 is a solution of NaCl in aqueous solution of H 2 O 2 , in which the content of NaCl lies between 10 % and 28 %, and is preferably 24 %, and the content of H 2 O 2 lies between 10 and 50 %, and is preferably 40 %. In another particular embodiment, the procedure of the invention comprises the addition of NaCI, solid or in solution, to a paste formed by Na 2 SO 4 and H 2 O 2 and then performing a direct granulation from the resulting mixture and finally drying the granulate obtained. The mixing of the components can be performed by any method of agglomeration or physical mixing of all or part of the components. The raw materials (Na 2 SO 4 and NaCl) are neutral salts that are non-toxic and available at a low price. Both salts are used in washing powder formulations used for washing clothes. As raw materials of the procedure of the invention, any source of Na 2 SO 4 and NaCl can be used. In a particular embodiment, the source of Na 2 SO 4 is chosen from, Thenardite (a natural product that contains Na 2 SO 4 ), Na 2 SO 4 , fines of Na 2 SO 4 [d50 (size of particle corresponding to 50 % on the accumulated curve) equal to or less than 30 µm], and mixtures thereof. In another particular embodiment, the source of NaCl is chosen from Halite (a natural product that contains NaCl), NaCl and mixtures thereof. The procedure of the invention means that it is not necessary to perform a prior purification of the raw materials. For the embodiment of the invention, the different components of Na 2 SO 4 , NaCI and H 2 O 2 are mixed at a molar ratio lying between 4.0 ± 0.2:1.0 ± 0.2:2.0 ± 0.2 (Na 2 SO 4 :NaCl:H 2 O 2 ), preferably in a molar proportion of 4.0:1.0:2.0. The procedure of the invention is performed at a temperature lying between 10° C and 100° C, preferably between 35° C and 50° C. The granulated adduct obtained by means of the procedure of the invention is intrinsically more stable, as it does not decompose until 180° C. This adduct, once dry, contains up to 4.6% of active oxygen. In order to find an application as a whitener in alkaline washing powder formulations, it needs a stabilising additive, phosphonates, etc. Therefore, in a particular embodiment, the procedure of the invention comprises, in addition, the step of adding said stabilising additive accompanying the H 2 O 2 or the other components (Na 2 SO 4 , NaCl). In a particular embodiment, said stabilising additive is of the polyphosphonate type, for example, DEQUEST 2060 from FMC Corp., which is preferably incorporated dissolved in the H 2 O 2 used for the granulation. The procedure of the invention allows the manufacture on an industrial scale of SPS, optionally stabilised, for application in washing powders with full use made of the raw materials. The procedure is energy efficient and is completely compatible with the environment. Compared to the procedures based on the crystallisation of the SPS adduct, direct granulation simplifies the installations and equipment necessary as only a conventional drying stage is required to evaporate off the water introduced with the H 2 O 2 . This process does not generate effluents or imply a risk of accidental release of liquids in the process given than solutions are not handled, as is the case in the crystallisation processes. In addition, in the procedure of the invention, there are no losses of yield linked to the individual solubility of the reactant salts or to the impurities of the raw materials themselves, which in solution might lead to the decomposition of H 2 O 2 . The incorporation of the stabilising additive dissolved in H 2 O 2 guarantees that maximum use is made and that there is a perfect distribution thereof in the SPS. By way of summary, the invention provides a procedure for the manufacture, on an industrial scale, of granules of innocuous whitener, in a simple, economical and ecological fashion, because, since it does not generate any residue, it is fully compatible with the most rigorous present or future environmental legislation. EXAMPLES By way of example, several typical examples are described of the operational procedure applied. In Examples 1 to 6, the salts are used in solid form. In Examples 7 and 9, the NaCl is incorporated dissolved in H 2 O 2 . In Examples 1, 3 and 7, the pilot granulator was used which consisted of a rotating cylinder inclined at 30° to the vertical equipped with an internal scraper stirrer, the motors of which have several speed regulators, which even allows verification of the sizes of the final granulates avoiding the mechanical reprocessing thereof. In Examples 4 and 5, a rotating trommel and a basket type extruder, respectively, were used. In Example 6, the mixture was performed directly in a fluid-bed drier. In Examples 8 and 9, a malaxator was used in which a paste of Na 2 SO 4 /H 2 O 2 was prepared, to which NaCl was subsequently added. To dry the products, a fluid bed drier was used (Examples 1 to 7) and a flash type one (Examples 8 and 9). EXAMPLE 1 Example 1A For this test, 1.701 g of anhydrous Na 2 SO 4 and 177 g of NaCl were weighed, both of technical quality. They were mixed together and dry blended such that a powder was obtained with a d50 of 30 µm. Once this had been transferred to the granulator and once this was in operation, the powder cloud was sprayed with 300 g of H 2 O 2 70 %. 5 minutes after finishing the addition of H 2 O 2 , a drying operation was carried out to give 2.090 of SPS with a content of active oxygen of 4.58 %. Example 1B That indicated in Example 1A was repeated, but dissolving 0.2 g of DEQUEST 2060 in H 2 O 2 before addition. EXAMPLE 2 For this test, 1.710 g of anhydrous Na 2 SO 4 taken directly at the output of an industrial Na 2 SO 4 drier, from the fraction denominated fine (< 50 µm), were weighed. The manufacturers of washing powders reject this grain size and it has to be reprocessed. It was mixed in the granulator with 177 g of technical NaCl previously ground (d50 = 50 µm) and then the mixture was sprayed with 300 g of H 2 O 2 70 % with 0.2 g of DEQUEST 2060 added. After drying, 2.087 g of SPS were recovered with a content of active oxygen of 4.54 %. EXAMPLE 3 For this test, Thenardite and Halite were used, both natural products extracted from the mines, with an Na 2 SO 4 content (d50 = 30 µm) and NaCl (d50 = 30 µm) of 99.1 % and 99.4 %, respectively. The impurities are mainly CaSO 4 (partially as plaster), MgSO 4 , Fe oxides and clay. Thenardite and Halite were ground and milled together. The test was performed with the quantities referred to in the previous Examples 1 and 2, including the polyphosphonate necessary for stabilisation in alkaline media. The SPS obtained had an active oxygen content of 4.50 %. EXAMPLE 4 For this test, fines of Na 2 SO 4 were used (as indicated in Example 2) and ground Halite (d50 = 30 µm). Both salts were mixed in-line by a thread from which a rotating trommel was fed in which axial tubing was inserted with two sprayers through which the H 2 O 2 was projected. The dosing of Na 2 SO 4 and NaCl (halite) was performed ensuring that the variations between the two did not exceed 10 % of the stoichiometric ratio. H 2 O 2 50% was used in an excess of 5 % with respect to the stoichiometry. MgSO 4 was used as a stabilising agent. It was incorporated dissolved in the H 2 O 2 . The pilot installation, with a SPS production capacity of 20 Kg/h, was kept in operation with no interruptions for 5 days, during which time samples of product were taken every 2 hours. On average, the product, once dry, had an active oxygen content of 4.46 ± 0.12 % and a verified grain-size distribution between 50 and 500 µm. Sieving the product through a stack of meshes of 100, 160, 250 and 325 µm showed a maximum difference of 0.2 % in the content of active oxygen between fractions. EXAMPLE 5 For this test, a mixture of Thenardite and Halite ground up together were used (d50 = 30 µm) in the stoichiometric ratio of Na 2 SO 4 /NaCl. The mixture, at a rhythm of 5 kg/h, fed a mixing thread over which H 2 O 2 70 % was sprayed, supplying the peroxide with a 10 % excess with respect to the stoichiometry. Sodium stannate was used as a stabiliser dissolved in H 2 O 2 . The mixture wetted with H 2 O 2 was unloaded in a shoot, the size of which ensured a residence time of 5 min, from where an extruder of the basket type was fed, said extruder being fitted with a 1 mm mesh. The operation was kept in motion for 16 hours, with samples being taken every 2 hours. On average, the dry product showed an active oxygen content of 4.51 ± 0.02. EXAMPLE 6 Starting from the mixture of Thenardite (Th) and Halite (H) from Example 5 and using H 2 O 2 50 % (10 % in excess with respect to the stoichiometry) to which MgSO 4 had been added as a stabiliser, a direct granulation test was performed in a batch fluid-bed drier. The drier in question has a porous bed which allows the passage of hot air, which is extracted, via an internal arm filter, through the upper part of the equipment. The mixture of Th + H (and in the previous examples the moist SPS) is introduced through a side hole. The H 2 O 2 is projected by means of a double fluid nozzle towards the bottom. At the end of the test, the drier is emptied through the bed, which, designed as a butterfly valve, rotates 90° and allows the product to drop onto a lid with a fast-closing lid, like a pressure cooker. In the test corresponding to the example, 2 kg of SPS were obtained. The addition of the mixture of Th + H was performed in one stage. The temperature of the incoming air was kept at 90 ± 5° C. In these conditions, the final product turned out to have a content of active oxygen of 4.56% and a grain size with a d99 (size of particle corresponding to 99% of the cumulative curve) of 230 µm. EXAMPLE 7 For this example, fines of Na 2 SO 4 were used (as indicated in Example 2) and HCl which had been previously dissolved in H 2 O 2 . On the one hand, 1.710 g of anhydrous Na 2 SO 4 were weighed. These were transferred to the granulator. On the other, a solution of NaCl was prepared containing 177 g of the salt in 520 g of H 2 O 2 40%. DEQUEST 2060 was applied as stabiliser, which was added dissolved in H 2 O 2 . The adduct was obtained by spraying the Na 2 SO 4 with the aforementioned solution. Once dry, the content of active oxygen turned out to be 4.58%. EXAMPLE 8 For this example, fine grains of Na 2 SO 4 (as indicated in Example 2) and ground Halite (d50 = 30 µm) were used. In this case, 5.130 g of Na 2 SO 4 and 20 g of MgSO 4 (used as a stabiliser) were mixed in a malaxator with 2.050 g of H 2 O 2 30 % forming a paste over which 535 g of NaCl were subsequently powdered. The final mixture, once dried by spin-flash, gave rise to an active oxygen content of 4.50 %. EXAMPLE 9 For this example, the procedure and the quantities of raw materials indicated in Example 8 were used but with the exception of introducing NaCI and MgSO 4 dissolved in 1.375 g of H 2 O. The final product, after being dried in a spin-flash apparatus, gave an active oxygen content of 4.56 %. Stability of the SPS Two types of study have been performed, one to evaluate the intrinsic stability of SPS, using the sample obtained in Example 1A, and another to determine the stability of SPS in alkaline media, using the sample obtained in Example 1B The intrinsic stability of SPS is very high, as it does not decompose until it reaches a temperature of 180° C. In normalised tests normally applied to SPB and PCS, it showed greater stability than those persalts. This is logical as Na 2 SO 4 and NaCl are neutral salts. In order to test the stability in alkaline medium, the fact that the incorporation of SPS would have a typical washing powder formulation with a content of 20 % of Na 2 SO 4 and 10 % of SPB.4H 2 O was taken into account. Thus, to provide both active oxygen and SPB.4H 2 O, 25% of SPS was required which, in turn, is equivalent to a little more than 20% of Na 2 SO 4 . On eliminating the SPB, the washing powder also loses the alkalinity provided by the borate. Thus, to compensate for this deficiency, and, at the same time, to restore the initial mass (30% of the washing powder) it was decided to add 5% of anhydrous Na 2 CO 3 . The SPS mixture (sample from Example 1B) + anhydrous Na 2 CO 3 was prepared by joint milling, obtaining a powder with a d50 of 30 µm. In a second stage, this powder was dry blended and ground up with a dry base of alkaline detergent prepared by atomisation. The final formulation, the composition of which was: 25% of SPS, 20% of Na 2 CO 3 , 20 % of sodium tripolyphosphate (TPF), 15% of sodium silicate (molar ratio 2:1), 15% of alkyl (C12) sodium benzenosulphonate, 4% of H 2 O, 1% of carboxymethyl cellulose (CMC), was introduced into semi-permeable bags and submitted, in an incubator, to a temperature of 50° C and a relative humidity of 90 % for three months. Each week, the residual activity was checked, showing a loss of less than 10% at the end of the test. The sample from Example 1B, lacking the stabilising additive, on being submitted to the same test as the sample from Example 1B, completely lost its activity before the end of the 3 rd week of incubation.

Claims: A process for obtaining an intrinsically stable adduct of formula 4Na 2 SO 4 .2H 2 O 2 .NaCl selected from A) direct granulation of an intimate blend of Na 2 SO 4 and NaCl, with a grain size equal to or less than 100 µm, with H 2 O 2 ; B) direct granulation of Na 2 SO 4 , with a grain size equal to or less than 100 µm, by means of spraying with a solution of NaCl in H 2 O 2 ; and C) direct granulation of a blend formed by addition of NaCl, solid or in solution, to a paste formed by Na 2 SO 4 and H 2 O 2 . A process according to claim 1, alternative A), in which said intimate blend of Na 2 SO 4 and NaCI has a grain size lying between 0 and 50 µm. A process according to any of claims 1, alternative A), or 2, in which the H 2 O 2 is found in the form of an aqueous solution, with a concentration lying between 50 % and 70 %. A process according to claim 1, alternative B), in which said Na 2 SO 4 has a grain size lying between 0 and 50 µm. A process according to any of claims 1, alternative B), or 4, in which the NaCl content in said solution of NaCI in H 2 O 2 lies between 10 % and 28 %, and is preferably 24 %, and the content of H 2 O 2 in said solution of NaCI lies between 10 % and 50 %, and is preferably 40 %. A process according to any of the previous claims, in which the molar proportions between Na 2 SO 4 , NaCl and H 2 O 2 lie between 4.0 ± 0.2:1.0 ± 0.2:2.0 ± 0.2, and is preferably the proportion 4.0:1.0:2.0. A process according to any of the previous claims, in which the temperature lies between 10° C and 100° C, and is preferably between 35° C and 50° C. A process according to any of the previous claims, which comprises the agglomeration or physical mixing of all or part of the components. A process according to any of the previous claims, in which the raw materials comprise Thenardite and Halite. A process according to any of claims 1 to 8, in which the raw materials comprise fines of Na 2 SO 4 and NaCl. A process according to any of claims 1 to 8, in which prior purification of the raw materials is not required. A process according to any of the previous claims that comprises, in addition, the stabilisation of the adduct obtained by means of the incorporation of a stabilising additive. A process according to claim 12, in which the incorporation of said stabilising adduct is performed accompanying the H 2 O 2 or Na 2 SO 2 or the NaCl A process according to any of claims 12 or 13, in which said stabilising additive is selected from salts of Mg, stannates, dipicolinic acid, phosphonates and mixtures thereof.

Inventors: ARTIGAS PUERTO RAMON, FORNER BENITO JUAN, ARTIGAS PUERTO, RAMON, FORNER BENITO, JUAN
Applicants: FMC FORET S A, FMC FORET S.A.
Application year: 2001
Main classification: C01B15
IPC codes: C01B15