Process for desulfation of solid materials containing gypsum

Abstract

------ Method for desulfating solid materials containing gypsum The present invention relates to a method for desulfating solid materials containing gypsum, characterized in that said materials are treated in an aqueous medium with urea and ureolytic bacteria having enzymatic activity to obtain a solution of ammonium sulfate and the desulfated solid materials containing calcium carbonate.

Description

Title of the invention: Process for desulfating solid materials containing gypsum

[0001] The present invention relates to a method for desulfating solid materials containing gypsum, in particular construction waste.

[0002] The problem of sulfates in construction materials is very important in the world. The applicant company produces construction aggregates from construction waste. One of the important characteristics of the aggregates is a low total sulfate content. Indeed, sulfates can promote the appearance of swelling of the concrete by internal sulfate reaction, weakening concrete structures using sulfated aggregates. The same problem arises in road sub-bases, where the aggregates used must meet maximum sulfate ion contents to limit the geotechnical risk.

[0003] However, the construction waste from which the aggregates are produced has a high content of calcium sulfate, also called gypsum, which comes from the plaster contained in the construction waste.

[0004] One way to get rid of the sulfates is to wash them to dissolve the gypsum. This washing is limited by the low solubility of gypsum. It would therefore be desirable to be able to propose a solution to this problem in order to be able to eliminate the sulfates present in the waste in order to be able to offer a recycled aggregate with a sufficiently low sulfate content to be able to be used as a raw material for construction or road sub-bases.

[0005] The applicant company has therefore developed a desulfation process using ureolytic bacteria and urea.

[0006] Ureolytic bacteria have been used to accelerate the leaching of sulfates by precipitating calcium carbonate (CaCO3). These bacteria produce carbonate (CO32) and ammonium (NH4+) ions upon contact with urea. Calcite CaCO3 precipitates when carbonate ions come into contact with calcium ions (Ca2+). By precipitating calcite, Ca2+ ions are consumed and the dissolution of gypsum (CaSO4 2H2O) is accelerated, making it possible to limit the volume of water required for washing. The sulfate ions are solubilized and the water rich in ammonium sulfate NH4SO4 can then be used for agronomic purposes, thus recovering the sulfate ions SO4 from the waste.

[0007] To this end, the present invention relates to a process for desulfating solid materials containing gypsum, characterized in that said materials are treated in an aqueous medium with urea and ureolytic bacteria having an activity enzymatic to obtain ammonium sulfate solution and desulfated solid materials containing calcium carbonate.

[0008] The solid materials to be treated containing gypsum can be chosen from: - waste from building construction materials; - ordinary industrial waste polluted with gypsum; - excavated land; - natural aggregates; - artificial aggregates; and - recycled aggregates such as crushed concrete.

[0009] Aggregates are generally defined as materials that meet the NF P 18-545 standard.

[0010] According to the method of the invention, the following successive steps can be carried out, consisting of: a. grinding solid materials to obtain ground solid materials; b. impregnating the ground solid materials with a suspension of ureolytic bacteria having enzymatic activity; c. contacting an aqueous urea solution with the crushed solid materials impregnated with the ureolytic bacteria; and d. recovering an ammonium sulfate solution and the desulfated solid materials containing calcium carbonate.

[0011] In step (a), the materials to be treated may be ground to a grain size small enough to allow the aqueous urea solution to pass through them in step (c). In particular, the materials to be treated may be ground to a grain size of less than or equal to 2 mm, in particular from 100 μm to 1 mm.

[0012] In step (b), a suspension of ureolytic bacteria chosen from the bacteria Sporosarcina pasteurii, Sporosarcina ginsengisoli, Proteus vulgaris, Bacillus sphaericus, Halomonas eurihalina, Bacillus fusiformis and mixtures thereof may be used. In particular, Sporosarcina Pasteurii bacteria may be used, which are the bacteria Sporosarcina Pasteurii ATCC 11859.

[0013] In step (b), a suspension of bacteria having a total enzyme activity (TEA) greater than 400 pS / cm / min may be used.

[0014] Step (c) can be carried out continuously by passing the aqueous urea solution through the materials impregnated with the bacterial suspension. Alternatively, step (c) can be carried out in batches.

[0015] In step (c), 1 to 10 moles of urea per mole of gypsum may be used. This corresponds to a urea / gypsum mass ratio of 0.34 to 3.4. In particular, a urea / gypsum mass ratio of 0.45 may be used.

[0016] The formation of carbonates by bacterial ureolysis of urea allows calcium carbonate to be precipitated using calcium from a gypsum or plaster substrate. The precipitation of calcium causes and accelerates the dissolution of CaSO4 2H2O, which reduces the volume of water required for washing. The sulfate ions remain in solution with the ammonium ions and the CaCO3 precipitates. The reactions involved are shown below.

[0017] CaSO4(H2O)2 <-> Ca2+ + SO42 + 2 H2O

[0018] CO(NH2)2 + H2O -> CO32 + 2 NH4+ (enzyme)

[0019] Ca2+ + CO32 -> CaCO3

[0020] -----------------------------------------------------------

[0021] CaSO4(H2O)2 + CO(NH2)2 -> CaCO3 + SO42 + 2 NH4+ + H2O

[0022] The examples below illustrate the present invention without, however, limiting its scope. Examples 1 to 3 General operating procedure Measurement of enzyme activity

[0023] The total enzyme activity (TEA) of a sample of bacterial suspension is measured by conductimetry: the increase in conductance (Thermofisher Orion Star™A325 pH / Cond Multimeter) of a 9 mL solution of 1.1M urea placed in the presence of 1 mL of bacterial suspension is measured for 5 minutes. This increase is due to the emission of CO32 and NH4+ by the hydrolysis of urea by the bacteria. The result is given in [rS.cm '.min1. The literature states that 1 mS / cm / min corresponds to 11 mM of hydrolyzed urea / min.

[0024] Passage of bacterial suspension through a mass of materials

[0025] Matrices of materials to be desulfated were prepared.

[0026] Each matrix sample was placed in a 50mm diameter PVC tube. A cover with a 300qm (stainless steel) then 15qm (cellulose) sieve is present at the base of the tube, in order to allow the fluid passing through the matrix to percolate. 100g of sample were placed at the bottom of this installation. A volume of 250mL of bacterial suspension cultured with an AET of 1180+60 qS / cm / min is passed through each of the samples in order to impregnate the samples with the ureolytic bacteria.

[0027] The enzymatic activity in a matrix of materials (AEMD) was measured after the percolation of a volume of bacterial suspension in said matrix of materials according to equation 1 below. By measuring the initial and final volumes and the initial and final enzymatic activities, one can, by subtraction, conclude on the initial quantity of enzyme present within the matrix of materials for ureolysis. This This measurement allows the flow rate of urea solution to be adjusted so that all of the urea supplied in solution is hydrolyzed.

[0028] [Math.l] = ——--—--*-- 4 iS m* ' AS perçais® * ■

[0029] In this equation:

[0030] AETinitiaie is the total enzymatic activity measured before the start of treatment of the waste mass

[0031] AETpercoiée is the total enzymatic activity measured on the suspension

[0032] Vsb ini is the initial volume of the bacterial suspension

[0033] Percoated VSB is the volume of bacterial suspension passed through the waste mass

[0034] mcharge is the mass of materials in the column.

[0035] Since the AEMDs are between 2172 and 2936 qS / cm / min / g, the flow rate of the calcifying solution was based on the lowest enzyme activity, in order to ensure that all the urea used was consumed in all the samples. This therefore gives a flow rate of 40 mL / h of IM urea solution.

[0036] Percolation of urea solution (examples of the invention below) or distilled water (comparative examples below)

[0037] 40 mL of urea solution was passed through every hour and 40 mL of distilled water was passed through to establish control experiments.

[0038] The new volume of urea solution or distilled water is used to force out the previous volume by gravity. The volume recovered at each fraction is noted and the percolated solution is used to measure the concentration of sulfate and ammonium ions. The pH and conductivity of the recovered solution were also measured. Regularly, the CaCO3 content of the sample in the PVC tube is measured by puncturing 1g of material. The final CaCO3 content is measured. Measurement of calcification (CaCO3 content)

[0039] The CaCO3 content was measured by a Bernard calcimeter. This setup allows the volume of CO2 released by the action of hydrochloric acid HCl to be measured according to the equation below.

[0040] CaCO3 + 2 HCl -> CaCl2 + H2O + CO2

[0041] After drying the samples in an oven at 50°C for 24 h, a sample of approximately 1 g to be analyzed was placed in the presence of 25 mL of HCl in a confined chamber. After stirring for 5 min, the volume of CO2 released by the sample was measured.

[0042] The volume of CO2 provides access to the quantity of CaCO3 dissolved by the acid. The mass of CaCO3 in the sample can then be expressed using the following equation:

[0043] [Math.2] = 77 ~

[0044] in which:

[0045] mCaco3 is the mass of calcium carbonate

[0046] VCo2 is the volume of CO2 released

[0047] VmCo2 is the molar volume of carbon dioxide which is 24.79L under standard temperature and pressure conditions (P = 105 Pa, T = 25°C)

[0048] MCaco3 is the molar mass of calcium carbonate

[0049] The method is calibrated by a calibration line established with 10 points in the concentration ranges of the test samples using silica sand and by adding pure CaCO3. The coefficient of the calibration line is applied for the experimental measurements. The calibration line makes it possible to find the coefficient that links the percentage of CaCO3 and the volume of CO2. By measuring the CO2 released, we then obtain the CaCO3 of the experimental measurements

[0050] Measurement of sulfate ion and ammonium ion content

[0051] The measurement of the sulfate and ammonium ion content of the percolation solution was carried out according to the NF ISO 15923-1 standard by UV-visible spectrophotometry. The uncertainty in LOQ is 20% for sulfate ions and 10% for ammonium ions. The samples were prepared by taking 30 mL of percolated water diluted in 250 mL of distilled water.

[0052] Modeling of theoretical sulfate ion content

[0053] In order to compare the released sulfate ions with the theoretically obtained sulfate ions, the release of sulfate ions with and without bacteria was modeled according to the following equation:

[0054] [Math.3] mîïîüsæX

[0055] in which:

[0056] S(Caso42H2O) is the dissolution rate of gypsum at 20°C in water, namely 2.2 kg / m3

[0057] Water used is the volume of wash water reported per tonne of waste treated, expressed in m3 / T

[0058] Curée is the molar concentration of urea in the washing solution, expressed in moleurea / m3

[0059] MSo4 is the molar mass of sulfate ions, namely 0.096 kg / mol

[0060] r (in nS04 / nurea) yield of sulfate ions released per quantity of urea supplied (r is taken equal to 1 because the urea is introduced less quickly than the capacity of the ureolytic enzymes to hydrolyze it)

[0061] xS04 is the proportion of sulfate ions initially present in the waste (in kgS04 / Twaste treated)

[0062] The %SO4 re-argued is between 0 and 1. Examples 1 and 2

[0063] A medium simulating the amount of plaster in the waste was prepared with silica sand of medium grain size (d90%<315qm) and pure CaSO4 2H2O (Sordalab, >99%). Two matrices simulating the waste were produced, one with 7% by mass of CaSO4 2H2O (MS 7%) (Example 1) and the other with 30% by mass of CaSO4 2H2O (MS 30%) (Example 2). Example 3

[0064] A complex waste matrix (CWM) was used using a waste powder obtained by simple grinding. Before grinding, the waste contained 13% by mass of gypsum. The waste powder is sieved to <600qm.

[0065] The MDC crushed waste matrix allows bacteria to be retained. This retention allows the ureolysis action to be carried out within the mass of waste to be treated. Results

[0066] [Fig.l] shows the enzymatic activities in the material matrices of Examples 1, 2 and 3. It can be observed that the 7% MS matrix of Example 1 has a lower enzymatic activity within it than the 30% MS matrices of Example 2 and MDC of Example 3. This is due to a greater granularity of the sand used for Example 1 which does not allow for as effective retention of bacteria as the waste powder used to prepare the matrix of Example 3. In addition, the presence of 30% CaSO4 2H2O whose granularity is very fine in the 30% MS matrix of Example 2 allows for better retention than the 7% MS matrix of Example 1.

[0067] [Fig.2] shows the CaCO3 content measured in the samples after washing and the maximum content that can be obtained in the case of total dissolution of the CaSO4 2H2O present in the samples.

[0068] By measuring the CaCO3 level at 0, 4h and 24h, a progressive increase in the CaCO3 level can be observed in the different samples. The results appear in [Fig.3].

[0069] The initial CaCO3 content in the matrix of Example 3 is around 12.4% initially, probably due to a significant amount of inert materials in the waste such as calcite.

[0070] Initially in the 7% MS samples of Example 1 and 30% MS of Example 2, the presence of 1.78% by mass of CaCO3 is observed, probably due to slight contamination of the sand used to prepare the matrices.

[0071] After 24 hours, a significant increase in the proportion of CaCO3 can be observed in the materials treated with urea and bacteria compared to the comparative examples only in the presence of bacteria. The CaCO3 levels observed in the treated materials after 24 hours of treatment reach the CaCO3 levels predicted by knowledge of the initial CaSO4 2H2O content.

[0072] In the comparative examples, no increase in the presence of CaCO3 is observed. This therefore confirms that there is indeed a reaction of production of CaCO3 thanks to the supply of urea in the presence of ureolytic bacteria. The result is particularly interesting, because contrary to all the literature dealing with the precipitation of calcite induced by microbiology, there was no supply of Ca2+ ions. The production of CaCO3 in 24 hours therefore suggests that Ca2+ ions are supplied by the matrices.

[0073] The release of SO4 and NH4 in the wash water makes it possible to clearly distinguish between samples with and without urea ([Fig.4]).

[0074] It can be seen that without urea, the 7% MS matrices of Example 1 and MDC of Example 3 release only very few sulfate ions into the percolation water. This therefore confirms that the presence of ureolytic bacteria and urea allows the massive release of sulfate from the gypsum into the aqueous medium. If the quantity of sulfate ions released in the comparative Examples is not zero, this is due to the natural dissolution of CaSO4 2H2O in water, at a level of 2.2 g / L.

[0075] Regarding NH4, a notable difference in behavior is also observed between the Comparative Examples and the Examples of the invention. In the Comparative Examples, a maximum is observed in the first fraction harvested. This is probably due to the ammonium present in the remaining bacterial culture medium, washed by the addition of distilled water.

[0076] The results therefore show a massive release of sulfate and ammonium ions. They complement the observation made with the analysis of the results of the calcium carbonate content: the hydrolysis of urea by the bacteria indeed leads to the precipitation of CaCO3 and the dissolution of CaSO4 2H2O, allowing the sulfate ions to be solubilized and carried by the aqueous phase out of the matrix of materials to be treated. This makes it possible to reduce the sulfate ion content in a matrix of materials to be treated.

[0077] [Fig.5] shows the percentage of sulfate ions released as a function of the volume of water used.

[0078] It can be seen that the use of urea allows almost all of the sulfates to be released at a water volume of 6 m3 / tonne of materials, whereas this volume only allows 20% of the sulfate ions present in the comparative examples to be released.

[0079] The use of urea therefore makes it possible to save approximately 17 m3 of water / tonne of materials to wash 50% of the sulfate ions and 27 m3 of water / tonne of materials to wash 80% of the sulfate ions.

Claims

Claims

1. - A method for desulfating solid materials containing gypsum, characterized in that the following successive steps are carried out: a. grinding said solid materials to obtain ground solid materials; b. impregnating the ground solid materials with a suspension of ureolytic bacteria having enzymatic activity; c. bringing an aqueous urea solution into contact with the ground solid materials impregnated with the ureolytic bacteria; and d. recovering an ammonium sulfate solution and the desulfated solid materials containing calcium carbonate.

2. - Method according to claim 1, characterized in that the solid materials to be treated containing gypsum are chosen from: - waste from building construction materials; - ordinary industrial waste polluted with gypsum; - excavated earth; - natural aggregates; - artificial aggregates; and - recycled aggregates such as crushed concrete.

3. - Method according to one of claims 1 and 2, characterized in that in step (a), the materials to be treated are ground to a grain size sufficiently small to allow the aqueous urea solution to pass through them in step (c).

4. - Method according to claim 3, characterized in that the materials to be treated are ground to a grain size less than or equal to 2 mm, in particular from 100 μm to 1 mm.

5. - Method according to one of claims 1 to 4, characterized in that in step (b), a suspension of ureolytic bacteria chosen from the bacteria Sporosarcina pasteurii, Sporosarcina ginsengisoli, Proteus vulgaris, Bacillus sphaericus, Halomonas eurihalina, Bacillus fusiformis and their mixtures is used.

6. - Method according to claim 5, characterized in that in step (b), Sporosarcina Pasteurii bacteria are used which are Sporosarcina Pasteurii bacteria ATCC 11859.

7. - Method according to one of claims 1 to 6, characterized in that in step (b), a suspension of bacteria having a total enzymatic activity (TEA) greater than 400 pS / cm / min is used.

8. - Method according to one of claims 1 to 7, characterized in that step (c) is carried out continuously by passing the aqueous urea solution through the materials impregnated with the bacterial suspension.

9. - Method according to one of claims 1 to 8, characterized in that step (c) is carried out in batches.

10. - Method according to one of claims 1 to 9, characterized in that in step (c), 1 to 10 moles of urea are used per mole of gypsum.

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