FR2985200A1 - PHOSPHATE - BASED GRANULAR MATERIAL, METHOD FOR MANUFACTURING THE SAME AND USE THEREOF IN A DEVICE FOR DEPHOSPHATION OF WASTEWATER. - Google Patents

Abstract

The subject of the present invention is a phosphate-based granular material, its method of manufacture and its use in a device for dephosphating wastewater. According to the invention, this granular material is in the form of granules having a size of between 2 and 10 mm, said granules comprising, expressed as a percentage by weight, based on the total weight of the granules: from 65 to 97% by weight a natural phosphate preferably of sedimentary origin; from 0 to 30% by weight of at least one additional component, preferably a component capable of increasing the crush resistance of said granules; from 1 to 10% by weight of a hydraulic binder, preferably chosen from cements; from 0 to 10% by weight of at least one specific granulation binder other than water; from 2 to 15% by weight of water. This material is intended to capture phosphates dissolved in wastewater, and may especially be used in horizontal flow type filters.

Description

The present invention relates to a phosphate-based granular material for the dephosphatation of wastewater which can in particular be used in filtration filters. horizontal flow type. It also relates to a method of manufacturing this granular material and an installation for the implementation of this method, as well as the use of this granular material in a wastewater dephosphatation device. The presence of too much phosphorus in the wastewater is the main factor involved in the eutrophication phenomenon of the aquatic environment which leads, in particular, to an excessive proliferation of the vegetation and limits the possibilities of use of these 15 waters. The presence of phosphorus in wastewater comes mainly from domestic discharges resulting, for example, from the use of washing powders, industrial discharges as well as agricultural discharges. This phosphorus is in the form of phosphates, especially orthophosphates, soluble in water. Much research has been conducted, particularly in recent years, to solve pollution problems related to the presence of phosphorus in wastewater. Several routes have thus been envisaged to allow the uptake of phosphorus in the wastewater, using: - either Macrophyte plants; - to microorganisms; or to specific materials allowing the phosphorus to be fixed by adsorption / precipitation. The use of Macrophyte plants leads to good retention of phosphorus during the growing season of plants. However, the release of phosphorus into the water that inevitably occurs at the death of plants limits the interest. The use of microorganisms is also not entirely satisfactory because of the relatively low level of phosphorus retention and possible salting out after bacterial decomposition.

Among the many materials that have been tested for their ability to fix phosphorus, especially by adsorption / precipitation effects, we can cite bauxites, zeolites, clays, iron-rich materials such as magnetite, carbon-rich substrates and phosphate materials such as apatites or struvites. Natural phosphate rocks have excellent physicochemical properties, their high adsorption capacity and percolation rate make them particularly interesting for solving pollution problems related to the presence of phosphorus in wastewater. In French Patent 2,873,678, it has been proposed a device for dephosphating wastewater, type "horizontal flow filter" comprising a bed of granular material consisting of natural phosphate rocks of sedimentary origin. This granular material makes it possible to retain, by adsorption-precipitation, the phosphates dissolved in the water to be treated passing through the filter. It is stated without further specification that the granulometry of the granular material must be chosen to ensure proper hydraulic operation of the device.

Nevertheless, it has been found that the efficiency of a granular material exclusively made up of natural phosphate rocks decreases relatively rapidly over time, this material having a tendency to degrade by forming particles which block the pores inter or even neatly. , altering the adsorption / precipitation properties and slowing the flow of water to be treated. In addition, the quality of the discharges to be treated can also disturb the kinetics of retention of phosphorus on the granular material and give rise to anaerobic zones that can chemically alter phosphate adsorption / precipitation phenomena.

In this context, the object of the present invention is to solve the technical problem of providing a novel phosphate-based granular material which exhibits excellent integrity in the aqueous environment; - which allows easy and constant hydraulic flow of wastewater into the filters where this material is used; 2 9 85200 3 - which exhibits high and stable dephosphating performance throughout the life of the filter or pond operation, generally in the order of 5 to 10 years. It has been discovered, and this is the basis of the present invention, that it was possible to solve this technical problem in a relatively simple and usable way on an industrial scale, by combining a natural phosphate and a hydraulic binder specifically chosen in the form of granules, in particular by using a granulation process making it possible to obtain granules having particular dimensional and compressive strength characteristics. Thus, according to a first aspect, the subject of the present invention is a phosphate-based granular material intended to capture phosphates dissolved in wastewater, in particular usable in horizontal flow-type filters, characterized in that it presents itself In the form of granules having a size of between 2 and 10 mm, said granules comprising, expressed as a percentage by weight, based on the total weight of the granules: from 65 to 97% by weight of a natural phosphate, preferably from sedimentary origin; From 0 to 30% by weight of at least one additional component and preferably a component capable of increasing the crush strength of said granules; from 1 to 10% by weight of a hydraulic binder preferably chosen from cements; From 0 to 10% by weight of at least one granulation binder other than water; 15% by weight of water. As used herein, "natural phosphate" means any phosphate-based composition derived from natural phosphate rock commonly referred to as "phosphate rock". In the context of the present invention, preference will be given to a phosphate rock of sedimentary origin having an apatite content (Ca5 (PO4) 3 (OH, F, C1)) greater than 60%, preferably greater than 70% by weight. weight.

Advantageously, use will be made of a rock that also has a P205 content of greater than 20% by weight, and more preferably greater than 25% by weight. It has been found that such materials allow easy recycling of the granular materials according to the invention, after use, as a raw material in the manufacture of phosphate fertilizers. Among the natural phosphatic rocks that may be used in the context of the invention, mention may be made in particular of rocks originating from Morocco, Syria and Egypt.

Particularly interesting results, in particular in terms of capacity to fix phosphorus, have been obtained with a natural rock originating from Morocco having essentially the following composition: P205 total of 25 to 40%, preferably 30 to 40% P205 sr of 8 at 30%, preferably from 10 to 30% P 2 O 5 sf / P 2 O 5 tot 35 to 75%, preferably from 40 to 75% CaO from 25 to 54%, preferably from 30 to 54% Ca / P of 1.0 at 3.6, preferably from 1.2 to 3.0 CO2 of 0.5 to 20%, preferably from 0.5 to 15 F of 2 to 4 ° h, preferably from 2.5 to 4 ° h * sf: formic acid soluble In order to impart hardness and longevity to the granular material according to the invention, a specifically selected hydraulic binder is used. In general, this hydraulic binder must be insoluble or very slightly soluble in water, completely inert and must not release anything in the waters with which it is brought into contact. The researches carried out by the inventors made it possible to determine that hydraulic binders and preferably hydraulic binders chosen from cements could be used satisfactorily within the scope of the invention. Excellent results have thus been obtained with Portland cements, Portland cement compounds, melted cements or quick cements.

The granular material according to the invention is in the form of substantially spherical granules having a size of between 2 and 10 mm, preferably between 3 and 8 mm, and more preferably of the order of 5 mm, in order to offer a high specific surface, while ensuring an easy hydraulic flow. Advantageously, these granules will exhibit: a crush strength of at least 1500 g, and preferably at least 2000 g; a disintegration rate of less than 10%, and preferably less than 5% after one month in water. From the chemical point of view, these granules comprise, expressed as a percentage by weight, based on the total weight of the granules: from 65 to 97% by weight, and preferably from 75 to 91% by weight of a natural phosphate, preference of sedimentary origin; From 0 to 30% by weight, and preferably from 0 to 15% of at least one additional component, and preferably a component capable of increasing the crush strength of said granules; from 2 to 10% by weight, and preferably from 4 to 8% by weight of a hydraulic binder as defined above; From 0 to 10% by weight and preferably from 0 to 5% of at least one granulation binder other than water; from 2 to 15% by weight, and preferably from 5 to 12% by weight of water. In general, the weight ratio between the natural phosphate and the hydraulic binder will advantageously be between 98: 2 and 90:10, preferably between 96: 4 and 92: 8. Among the additional components that may be present in the granules according to the invention, mention may be made in particular of the following products: plaster, limestone, lime, pozzolan, blast furnace slag, concrete admixtures, clays, magnesium oxide sodium silicate, and more generally any other product compatible with natural phosphate, inert with respect to water and able to improve the consistency of the granules and in particular their resistance to crushing.

The granules according to the invention are crush resistant, stable over time in an aqueous environment and have a high capacity to fix the phosphorus contained in the wastewater. In general, the crush strength of the granules according to the invention will be measured by following the procedure of: - sieving the sample between 3.15 and 4 mm; - reject fine and large ones; - put a dozen grains on a scale; 10 - crush the grains one by one using a spatula by lifting the mass at the time of rupture. The resistance to crushing will be equal to: Resistance (in g / grain) t-i n with M1, M2, ..., Mn mass, in g, at the time of breaking of the grain number 1, 2, n. n: number of grains (minimum 10) The stability over time of the granules according to the invention in an aqueous environment will be measured by a disintegration test to determine the percentage of particles whose size becomes less than 1 mm after disintegration in the water. This disintegration rate can be measured by following the procedure of: measuring the moisture H of the product on a part of the sample; weigh a mass ideally of 100 g of sample, put the test sample in a 500 ml beaker and add 300 ml of water; - let the product disintegrate for 6 hours or 1 month; 30 - after these 6 hours or 1 month, quantitatively transfer the product to a sieve 1 mm mesh opening, then rinse the walls of the beaker with a trickle of water; - transfer particles larger than 1 mm into a calibrated beaker using a trickle of water; To put this fraction greater than 1 mm to dry in an oven at 100 ° C overnight; - After drying weigh the cooled beaker, deduce the mf mass of the dry product.

In this test, the disintegration rate, expressed as a percentage by weight will be: Disintegration rate [% -pds] = 1-mi x (1 - H) with mi: mass of the test sample (g) mf: mass of the product fraction greater than 1 mm after disintegration and drying (g) H: moisture content of the initial sample (%) According to a second aspect, the subject of the present invention is a process for manufacturing the granular material described above.

This process is essentially characterized in that it comprises: a) the grinding of a natural phosphate, preferably of sedimentary origin, under conditions making it possible to obtain a powder whose particle size distribution has a value D90 less than 110 μm, preferably less than 63 μm; b) mixing the powder thus obtained with a powdery hydraulic binder in predetermined proportions; c) the wet granulation of the mixture thus obtained using a granulation binder; d) drying the granules thus obtained to obtain a residual moisture content of between 2 and 15% by weight, based on the weight of the granules; e) screening the granules thus dried in order to select granules having a size of between 2 and 10 mm. Essentially, the method of manufacturing the granular material according to the invention is a wet granulation process, each step of which has been specifically designed and optimized to achieve the intended purpose. In this context, it has been determined that the natural phosphate used for the preparation of the granules must have a particular particle size in order to obtain a satisfactory granulation yield and to guarantee the solidity of the granules formed. The process according to the invention therefore comprises a step of grinding the natural phosphate, advantageously by means of a ball mill, under conditions making it possible to obtain a powder whose particle size distribution has a D90 value of less than 110 μm. preferably less than 90 μm and more preferably less than 63 μm. In other words, the natural phosphate powder resulting from the grinding must be such that 90% of the particles have a size of less than 110 μm, preferably less than 90 μm and more preferably less than 63 μm. The natural phosphate thus ground and the powdery hydraulic binder are advantageously contained in two storage and dosing devices. Each of these devices may be, for example, a storage hopper or silo equipped with dosing belts or variable speed metering screws whose respective flow rates are regulated according to the desired proportions of each of these products in the granules. The process according to the invention comprises a subsequent step in which the natural phosphate powder is intimately mixed with the pulverulent hydraulic binder which can be carried out for example in a coulter mixer. Any additional solid components of the granule may be introduced and mixed at this stage. The granulation of this mixture (and any additional components) can be initiated at this stage by introducing into the mixture of powders thus produced a portion of the granulation binder which will advantageously be water and / or steam of water. A main or complementary wet granulation step is then carried out in a wet granulator such as, for example, a rotary drum, a plate granulator, a fluidized bed granulator, or any other type of granulator known to the man of the art. 'art. In general, the granulation binder advantageously consists of water and / or water vapor and may further comprise at least one granulation binder other than the water chosen from food gums (xanthan, arabic, etc.), calcium or sodium lignosulfonates, cellulose or cellulose derivatives (HPC, HPMC, CMC, ...), starch, polyvinyl alcohol, polyvinylpyrrolidone or any other product known for its binding properties. The amount of granulation binder can be easily adapted, in a manner known per se, according to the characteristics of the granulator used. This produces substantially spherical granules. The granules thus formed are then dried preferably in a convective dryer such as, for example, a rotating drum, a fluidized bed, a flash dryer, until a residual moisture content of between 2 and 15 is obtained. % by weight, preferably between 5 and 12% by weight, based on the total weight of the granules. It has been determined that granules having a residual moisture content of less than 2% by weight are not very resistant to attrition while granules having a residual moisture of greater than 15% by weight are not sufficiently resistant to crushing. The granules thus dried are then subjected to a screening step in order to select a predetermined particle size distribution ensuring easy hydraulic flow in the bed of granular material while offering a high specific surface to optimize the exchanges between the water to be treated and the granules . Thus, the granules having a size of between 2 and 10 mm, preferably between 3 and 8 mm, will be selected. This screening operation can be performed for example on vibrating screens equipped with cutting grids to the desired dimensions. Granules larger than the maximum value retained will be milled and recycled with the raw materials at the mixing step described above. The fine particles, smaller than the desired minimum size, will be directly recycled also at the stage of the aforementioned mixture. Advantageously, the granules retained at the end of the screening step are cooled by the ambient air, for example in a fluidized bed or a rotating cylinder or directly stored at a temperature below 40 ° C. To obtain the hardness necessary to withstand the compression to which these granules will be subjected in dephosphating devices, the granules thus produced are stored in bulk for example in covered stores for a minimum period of 25 days, preferably 31 days. According to a third aspect, the present invention relates to an installation for implementing the granular material manufacturing method described above. This plant is essentially characterized in that it comprises: a grinding device making it possible to grind a natural phosphate, under conditions making it possible to obtain a powder whose particle size distribution has a D90 value of less than 110 μm; preferably less than 90 μm and more preferably less than 63 μm; a mixer, preferably a plow, fed by a ground phosphate storage device and a hydraulic binder storage device for intimately mixing said phosphate and said hydraulic binder; a wet granulator connected to one or more feeds of granulation binder (s) allowing the granulation of the phosphate and hydraulic binder mixture to be granulated; A drying device connected to the granulator and making it possible to adjust the residual moisture of the granules at a level of between 2 and 15% by weight, based on the weight of the granules. According to a fourth aspect, the present invention relates to the use of the granular material previously described in a device 25 for dephosphating wastewater. Advantageously, this device will be of horizontal flow type, preferably planted with reeds, such as that described in patent FR 2 873 678. The granular material according to the invention can also be used in any type of water dephosphatation device. used and in particular a device of vertical flow type. According to a fifth aspect, the present invention relates to a device for dephosphating wastewater preferably of the horizontal flow type comprising a bed of granular material as defined above, to be traversed by the water to be treated.

The following examples, given solely for illustrative purposes (and in no way limiting), will make it possible to better understand the invention. The description of these examples will be made with reference to the appended figures in which: - Figure 1 schematically shows a water treatment assembly; FIG. 2 represents the evolution of the phosphorus content in the water flowing through a column containing a granular material according to the invention; and FIG. 3 represents the particle size distribution of a ground phosphate powder making it possible to manufacture a granular material according to the invention. EXAMPLE 1 Manufacture of a Granular Material According to the Invention Granules according to the invention were manufactured in a continuous wet granulation pilot at a rate of 500 kg / h. The raw materials involved in this production are - for natural phosphate: - Moroccan natural phosphate, the composition of which is given in the following table, previously ground to a particle size such that 90% by weight of the particles have a smaller size. at 63 pm. The particle size distribution of the phosphate powder thus ground is given in FIG. 3. Component Content [% -pds] P2O5 total 31 P2O5 sf * 11.3 ° A) CaO 52.4 ° h CO2 8.4 ° A) F 3.6% - for the hydraulic binder: a Portland cement such as the reference compound CEMII / BM (LL-V) 42.5 R CE CP2 NF sold for example by the company Lafarge Ciment. The phosphate thus ground and the cement were respectively stored in two hoppers equipped with metering mats for feeding a mixer with a natural phosphate / cement mass ratio of 94/6, and a total raw material flow rate of 500 kg / h. A bucket elevator then conveyor belts directed these 2 components to a plow mixer where they were intimately mixed. The wet granulation phenomenon was initiated during this step by injection of 10 to 15 kg / h of steam. At the outlet of this mixer, the mixture of powders fell by gravity into a rotating cylinder 1 m in diameter and 2.5 m in length. The wet granulation was carried out by water injection. The water flow rate was continuously adjusted by the granulation operator to obtain granules having a size of 2 to 8 mm. In this example, this flow rate was between 110 and 140 kg / h. At the outlet of the granulator, the granules fell into the feed chute of the dryer. In this example, it was a co-current convective dryer supplied with hot air (2000 m3 / h) by an air-vein gas burner. The drying was carried out so that the product taken out had a residual moisture of 7 to 12% and few broken grains. For this, the air temperature at the inlet dryer was between 100 and 150 ° C and below 50 ° C output. The dried granules were then conveyed to a screening device where only the granules having a size of between 2 and 5 mm were kept. The fractions of fines (<2 mm) and large (> 5 mm) particles were recycled, after grinding if necessary, with the raw materials in the mixing step. The thus obtained and selected granules leaving the screen were passed to a fluidized bed cooler where their temperature was lowered to 35 ° C. and then packaged in bags. These finished product bags were stored for 31 days at room temperature in a covered store. At the end of the storage period, the granules produced had a humidity of 11.1%, a crush strength of 1700 g, a disintegration rate at 6 h less than 1% and a disintegration rate at 1 month of 3%. By comparison, phosphate granules obtained according to an equivalent process but in the absence of hydraulic binder had a disintegration rate of 100% from 6h. EXAMPLE 2 Demonstration of the Efficiency of the Granular Material in Accordance with the Invention The effectiveness of the granular material according to the invention for the dephosphatation of water was demonstrated in the laboratory in a dynamic assembly where The flow diagram of this assembly is given in FIG. 1 in which the reference numerals represent respectively: submerged pump, maximum theoretical flow = 970 L / h 2: beaker of 5 to 10 L constituting the phosphate water tank to be treated (5L in this example), 3: column 4.4 cm in diameter and 50 cm in height, loaded with a bed of granules ( 1 kg), and in which the water to be treated will percolate up and down, 4: cotton pad that prevents the release of granules at the bottom of the column,: pump outlet pipe feeding the column in the upper part , 6: tuy at the outlet of the column returning the 'treated' water to the water tank, 7: parafilm type cap sealing the device at the top of the column.

The water to be treated containing 25 mg / L of phosphorus, contained in the tank 2, was directed at a flow rate of the order of 100 L / h at the column head (3) previously filled with a 1 kg of granules obtained according to Example 1.

The water collected at the bottom of the column was returned to the tank 2 and was circulated in closed circuit on the column 3 packed with granules for about 2 hours. Water samples were regularly taken from the tank 2 and the phosphorus content was measured, for example, by the Briggs method using the formation of a phosphomolybdic complex and then by colorimetry or by a specifically developed ICP method. for the purposes of the study. Figure 2 shows the evolution of the phosphorus content in the water flowing through column 3 as a function of the duration of the treatment. As shown in this figure, the granules according to the invention make it possible to very effectively reduce the phosphorus content.

Claims (12)

REVENDICATIONS1. Granular phosphate material for CLAIMS1. Phosphate granular material for collecting phosphates dissolved in wastewater, particularly for use in horizontal flow type filters, characterized in that it is in the form of granules having a size of between 2 and 10 mm said granules comprising, expressed as a percentage by weight, based on the total weight of the granules: from 65 to 97% by weight of a natural phosphate, preferably of sedimentary origin; from 0 to 30% by weight of at least one additional component, preferably a component capable of increasing the crush resistance of said granules; from 1 to 10% by weight of a hydraulic binder, preferably chosen from cements; from 0 to 10% by weight of at least one specific granulation binder other than water; from 2 to 15 ° A) by weight of water. 2. Granular material according to claim 1, characterized in that it is in the form of granules having a size between 3 and 8 mm. Granular material according to Claim 1 or 2, characterized in that the abovementioned granules comprise, expressed as a percentage by weight, relative to the total weight of the granules: from 80 to 91% by weight of a natural phosphate, preferably from sedimentary origin; from 0 to 15% of at least one additional component; from 4 to 8% by weight of a hydraulic binder; from 0 to 5% by weight of at least one specific granulation binder other than water; from 5 to 12% by weight of water. 4. Granular material according to any one of claims 1 to 3, characterized in that the aforementioned granules have a disintegration rate of less than 10%, preferably less than 5% after one month in the water. 5. granular material according to any one of claims 1 to 4 characterized in that the aforementioned granules have a crush strength of at least 1500 g, preferably at least 2000 g. 6. granular material according to any one of claims 1 5 to 5, characterized in that the above-mentioned natural phosphate comes from a phosphate rock of sedimentary origin having an apatite content greater than 60% by weight, preferably greater than 70% and a P205 content greater than 20% by weight, and preferably greater than 25% by weight. 7. Granular material according to any one of claims 1 to 6, characterized in that the aforementioned hydraulic binder is selected from cements, and preferably from Portland cements. 8. A method of manufacturing a granular material according to any one of claims 1 to 7, characterized in that it comprises: a) the grinding of a natural phosphate, preferably of sedimentary origin, under conditions making it possible to obtain a powder whose particle size distribution has a D90 value of less than 110 μm, preferably less than 90 μm; b) mixing the powder thus obtained with a powdery hydraulic binder in predetermined proportions; c) the wet granulation of the mixture thus obtained using a granulation binder; d) drying the granules thus obtained to obtain a residual moisture content of between 2 and 15% by weight relative to the weight of the granules; e) screening the granules thus dried in order to select granules having a size of between 2 and 10 mm. 9. The method of claim 8, characterized in that it further comprises an additional step of storing the granules obtained for a period of at least 25 days, preferably at least 31 days. 10. Installation for the implementation of the method according to claim 8 or 9, characterized in that it comprises: - a grinding device for grinding a natural phosphate, under conditions for obtaining a powder of which the granulometric distribution has a D90 value of less than 110 μm, preferably less than 90 μm; a mixer, preferably with coulters, fed by a ground phosphate storage device and a hydraulic binder storage device, for intimately mixing said phosphate and said hydraulic binder; a wet granulator connected to a granulation binder feed source (s) allowing the granulation of the phosphate and hydraulic binder mixture to be granulated; A drying device connected to the granulator and making it possible to adjust the residual moisture of the granules at a level of between 2 and 15% by weight, based on the weight of the granules. 11. Use of the granular material according to any one of claims 1 to 7 in a wastewater dephosphorization device. 12. Device for dephosphating wastewater preferably horizontal flow type comprising a bed of granular material according to any one of claims 1 to 7, to be traversed by the water to be treated.