Method

Abstract

A method for recycling contaminated phosphoric acid, includes providing a solution of phosphoric acid containing carbon and calcium contaminants; heating the solution to between about 45 to about 60 °C; adding sulfuric acid; adding a flocculant which is an anionic polyacrylamide emulsion; and passing the resultant mixture through a filter. The solution may be cooled to below 30°C before adding the flocculant. The pore size of the filter can be about 5 microns. The resultant mixture can be filtered under a pressure of about 6 bar. The filtering step may involve passing the resultant mixture through a filter press. A recycled phosphoric acid obtainable by the method, and a use of the recycled phosphoric acid in the manufacture of fertiliser are also described.

Description

Field of Invention The present invention relates to a method of recycling contaminated phosphoric acid. Background to the Invention Phosphoric acid (H3PO4) is commonly used in fertilizers, soaps, detergents and household cleaning products. Food-grade phosphoric acid is used to acidify foods and beverages and can also be used as a preservative. The industrial production of phosphoric acid is usually performed via either a wet or a dry process. The phosphorus required for such processes is commonly obtained by mining naturally occurring phosphate rock. Phosphate rock is a finite, non-renewable resource and there are currently no substitutes (1. A. R. Jupp et al. Chem. Soc. Rev., 2021, 50, 87-101.). Therefore, there is a need to develop alternative, more sustainable processes to manufacture phosphoric acid. Summary of the Invention Phosphoric acid containing organic and inorganic contaminants is often referred to as "black phosphoric acid". Use of phosphoric acid in certain industries typically produces this black phosphoric acid as waste material, which is mainly contaminated with carbon and calcium contaminants. Black phosphoric acid is unsuitable for use in certain agricultural and industrial processes, such as the manufacture of clear fertilizers. If properly exploited, black phosphoric acid and other phosphoric acid-containing waste streams have the potential to replace phosphate rock as a phosphorus source in a significant proportion of phosphoric acid production. This type of circular chemistry will become increasingly more important as natural resources become more depleted. The present invention provides a method of recycling contaminated phosphoric acid waste-streams, for example from the food industry, to provide phosphoric acid which can be used in the manufacture of other products, such as fertilizers. The method of the invention comprises the steps of: a) providing a solution of phosphoric acid containing carbon and calcium contaminants; b) heating the solution to between about 45 °C to about 60 °C; c) adding sulfuric acid; d) adding a flocculant, which is an anionic polyacrylamide emulsion; and e) passing the resultant mixture through a filter. The present method provides a sustainable way to produce phosphoric acid which can be used to manufacture other products such as fertilizers. The present method achieves this by reducing the amount of carbon and calcium contaminants in the contaminated phosphoric acid to a level which makes the phosphoric acid suitable for use in the manufacture of, for example, fertilizers. The method also reduces the costs of disposing of waste phosphoric acid. Description Step a) In step a) of the method described herein, a solution of phosphoric acid containing carbon and calcium contaminants, also known as "black phosphoric acid" is provided. The solution of phosphoric acid containing contaminants may be obtained from an industrial waste-stream, such as a waste-stream from a process performed in the food industry. Typical calcium contaminants include, but are not limited to calcium phosphate Ca3(PO4)2. Typical carbon contaminants include, but are not limited to activated carbon. The solution of phosphoric acid may be an aqueous solution and may have a concentration of about 50% (w / w) to about 80% (w / w), or about 55% (w / w) to about 75% (w / w), preferably about 60% (w / w) to about 70% (w / w). More preferably the concentration is about 60% (w / w) to about 65(w / w)%. The solution of phosphoric acid may be an aqueous solution and may have a concentration of about 55% (w / w), or about 56% (w / w), or about 57% (w / w), or about 58% (w / w), or about 59% (w / w), or about 60% (w / w), or about 61% (w / w), or about 62% (w / w), or about 63% (w / w), or about 64% (w / w), or about 65% (w / w), or about 66% (w / w), or about 67% (w / w), or about 68% (w / w), or about 69% (w / w), or about 70% (w / w). Preferably the concentration is about 60% (w / w), or about 61% (w / w), or about 62% (w / w), or about 63% (w / w), or about 64% (w / w), or about 65% (w / w). Step b) In step b) of the method described herein, the solution is heated to between about 45 °C to about 60 °C. Without being bound by theory, it is thought that heating the solution between about 45 °C to about 60 °C ensures that an optimum crystal size of calcium sulphate dihydrate (gypsum) is produced in the next step of the method. Below about 45 °C, anhydrous calcium sulphate is formed (which is often a fine particulate), whilst between about 45 °C and about 60 °C, crystalline calcium sulphate dihydrate is formed. Above 60 °C, calcium sulphate hemihydrate is formed, which is crystalline but the crystals that form are generally smaller than the desired calcium sulphate dihydrate. Larger crystals are preferred because they are easier to separate from a solution. Heating the solution to between about 45 °C to about 60 °C therefore helps the present process to achieve optimal removal of calcium contaminants from the black phosphoric acid solution. The solution may be heated to between 45 °C to about 55 °C, or between about 50 °C to about 57 °C, or between about 54 °C to about 59 °C, or between about 55 °C to about 60 °C. The solution may be heated to about 45 °C, or about 46 °C, or about 47 °C, or about 48 °C , or about 49 °C, or about 50 °C, or about 51 °C, or about 52 °C, or about 53 °C, or about 54 °C, or about 55 °C, or about 56 °C, or about 57°C, or about 58 °C, or about 59 °C, or about 60 °C. The solution is stirred whilst it is heating, for example at a speed of about 5 rpm to about 30 rpm, preferably 10 to about 20 rpm. The skilled person will be able to select an appropriate stirring speed based on the reaction volume and level of contamination in the phosphoric acid solution. Step c) In step c) of the method described herein, sulfuric acid is added to the contaminated phosphoric acid solution. Without being bound by theory, it is thought that sulfuric acid reacts with the calcium contaminants in the phosphoric acid solution to form calcium sulphate and associated hydrates. As set out above, when this reaction occurs between about 45 °C to about 60 °C, desired calcium sulphate dihydrate is formed in solution. The concentration of sulfuric acid may be between about 50% (w / w) to about 98% (w / w), or between about 60% (w / w) to about 85% (w / w), or between about 75% (w / w) to about 80% (w / w), or about 70% (w / w), or about 75% (w / w). Preferably, the concentration of sulfuric acid may be about 77% (w / w). At this concentration there is a balance between acid strength and the amount of excess water introduced into the reaction. 77% (w / w) provides the optimal point between providing an acid which is as weak as possible and not introducing excess water. The weaker the acid that is added, the slower the addition rate becomes. Use of a weaker acid concentration, for example 77% (w / w) sulfuric acid reduces the reaction rate which allows for a slower crystal growth. A slower crystal growth generally leads to larger crystals. Having larger crystals is preferred because this aids the filtering process and means that an increased filtration rate can be used. The amount of sulfuric acid required can be calculated by the skilled person using methods known in the art. Suitable methods may include but are not limited to manual titration of the phosphoric acid solution, for example with sodium hydroxide, or automated titration, for example using Method A5 of the Mettler Toledo T70 Autotitrator®. The amount of sulfuric acid added will typically be dependent on the level of calcium contaminant present in the black phosphoric acid solution. The amount of sulfuric acid added may be any amount sufficient to convert substantially all, or all, of the calcium contaminant to calcium sulphate and associated hydrates. Step d) In step d) of the method described herein, a flocculant is added to the reaction mixture, wherein the flocculant is an anionic polyacrylamide emulsion. As used herein, the term "flocculant" is intended to take its normal meaning in the art and refers to a substance that promotes agglomeration of particles in a solution. An example of a suitable anionic polyacrylamide is Superfloc A1883-RS ex Kemira. The skilled person will be able to select alternative suitable anionic polyacrylamides, such as an emulsion containing one or more anionic polyacrylamides, one or more hydrocarbons and one or more surfactants. It has been discovered that anionic polyacrylamides are a particularly effective flocculant for the carbon and calcium contaminants in the present process. Flocculants other than anionic polyacrylamides have been found to be not so effective in binding the calcium and carbon contaminants together in the present process. The reaction mixture is stirred when the flocculant is added. The reaction mixture may be stirred at a speed of about 5 rpm to about 30 rpm, preferably about 10 rpm to about 20 rpm. The skilled person will be able to select an appropriate stirring speed based on the reaction volume and the amount of flocculant added to the solution. The reaction mixture may be cooled to below about 35 °C, or below about 30 °C, or below about 25 °C, or below about 20°C before the flocculant is added. Preferably, the mixture may be cooled to below about 30 °C before the flocculant is added. Without being bound by theory, it is thought that cooling the reaction mixture allows further calcium to precipitate, which encourages the flocculant to bind the calcium and carbon together, leading to larger particles that are easier to remove from the reaction mixture. The skilled person will be able to determine the amount of flocculant required, for example via laboratory testing. A suitable method is provided below: (1) A sample of the contaminated phosphoric acid solution may be taken and stirred until a homogenous mixture is obtained. (2) An amount of flocculant is added to the solution and stirred. (3) The stirring speed is reduced such that the skilled person can observe any evidence of flocculation i.e., agglomeration of particles in the solution. The time required for this reduced stirring and observing step may vary depending on sample size. For example, for a 500 g sample, about 10 minutes to about 30 minutes may be required. For larger batches, longer periods of time may be required, which the skilled person will be able to determine. (4) Steps 2 and 3 may be repeated with additional amounts of flocculant until desired flocculation is observed. (5) Once flocculation has been achieved, the skilled person can calculate the amount of flocculant required for the entire contaminated phosphoric acid solution. Step e) In step e) of the method described herein, the resultant mixture is passed through a filter to remove the carbon contaminants and the calcium sulphate and associated hydrate crystals formed in earlier steps of the method. The pore size of the filter may be about 0.5 microns to about 7 microns. Preferably, the pore size of the filter may be about 2 to about 6 microns. More preferably, the pore size of the filter may be about 5 microns. A pore size of about 5 microns provides the best fit to filter carbon contaminants and calcium sulphate dihydrate crystals out the resultant mixture. The skilled person would be able to select the correct pore size for filtration. The resultant mixture may be filtered by any suitable method known in the art. The skilled person will be able to select the best method based on the volume of the resultant mixture and the amount of flocculant to be filtered. Particularly effective filters have been found to include a filter press or a pressurised filter. An example of a suitable pressurised filter system is the BHS Candle Filter®. For pressurised filters, a pressure of about 4 bar to about 8 bar, preferably about 5 bar to about 7 bar, more preferably about 6 bar can be used. The resultant mixture is filtered at a temperature of about 20 °C to about 30 °C, preferably about 20 °C to about 25 °C, more preferably about 22 °C. A filter aid may be added to the resultant mixture before it is filtered. Addition of a filter aid may be required if the carbon contaminants have a particle size that is smaller than the pore size selected for the filtration. The filter aid also prevents the filter from binding to the contaminants that are being filtered and blocking the pores in the filter. The filtrate obtained from the filter is a recycled phosphoric acid with a reduced carbon and calcium contaminant concentration. The recycled phosphoric acid obtained from the filter may be substantially or completely free of carbon and calcium contaminants. The recycled phosphoric acid obtained from the filtering step may be directly suitable for use e.g., without requiring any further processing steps. This may include, but not be limited to, use in the manufacture of fertilizers, such as clear fertilizers, as well as in the manufacture of soaps, detergents and household cleaning products. Alternatively, the recycled phosphoric acid may require further processing following filtering before being suitable for such further uses. The recycled phosphoric acid may be used in methods of manufacturing phosphoric acid for use in any kind of industry where phosphoric acid is typically used. For example, a portion of the recycled phosphoric acid may be added to phosphoric acid obtained from other sources in order to prepare a phosphoric acid suitable for use in industry. Examples 25 tonnes of black phosphoric acid was added to vessel MV3 (21,000 litre 316L stainless steel vessel with an anchor blade, heating and cooling coil). The black phosphoric acid was stirred for 1 hour at 20 rpm. The vessel was heated to between 45 °C and 60 °C and sulfuric acid added. The amount of sulfuric required was calculated as set out below. A 500 g sample of the black phosphoric acid was used to calculate the sulfuric acid addition level to precipitate calcium sulphates by manual titration or using Method A5 using the Mettler Toledo T70 Autotitrator. Automated Titration Method A5 (Mettler Toledo T70 Autotitrator) 1. Weigh approximately 0.4 - 0.5 g to 4 decimal places of the filtered sample into a beaker and add 40 ml of de-ionised water. 2. Titrate using the Autotitrator, using Method A5. The titrator will display a % figure at the end of the titration on the screen. 3. Repeat steps 1-2 and take an average of the two results. This is the result to be quoted on the QC sheet. Automated Titration Calculation Quantity of 77% (w / w) -.. if,. raniiirarl = CBS U It from Step 3 X suirunc acid required to be added to storage tank (kg) weight of black phosphoric acid in tank (kg) Manual Titration Method 1. Fill a burette with 0.5 M sodium hydroxide. 2. Weigh in a maximum of 5 g of the filtrate into a beaker and add 40 ml of de-ionised water. 3. Titrate with 0.5 M sodium hydroxide the contents of the conical flask to an end point of pH 4.45 and record the volume of 0.5 M sodium hydroxide used = VEQ1. 4. Titrate with 0.5M sodium hydroxide the contents of the conical flask from 4.45 an end point of pH 9.22 and record the volume of 0.5 M sodium hydroxide used = VEQ2. Manual Titration Calculation Percentage addition of sulfuric acid 77% (w / w) = (VEO1 - VEQ2) x 3.18 weight of sample (g) Scale up to determine quantity of 77% (w / w) sulfuric acid to be added to the black phosphoric acid storage tank: 20 Quantity of 77% (w / w) sulfuric acid required to = % result from Step 1 x weight of black phosphoric be added to storage 100 acid in tank (kg) tank (kg) The calculated sulfuric acid addition was pumped into MV3 slowly at a rate of one IBC (Intermediate Bulk Container at 1500 kg) every 2 hours After the addition of sulfuric acid, MV3 vessel was maintained between 45 °C and 60 °C for 2 hours to achieve optimum calcium sulphate crystal growth (calcium sulphate dihydrate) The reaction mixture was cooled to approximately 30 °C. Flocculant was then added to the reaction mixture. The amount of flocculant required was calculated as set out below. The 500 g sample of black phosphoric acid was poured into a beaker and stirred vigorously to obtain a homogeneous mixture with a magnetic stirrer. 0.2 g of anionic polyacrylamide emulsion was added and stirred to achieve a vortex for 2 minutes. The speed of the stirrer was reduced only to achieve movement and observed after 10 minutes. The laboratory sample (used to determine sulfuric acid addition) was filtered using a traditional laboratory method using glass fibre filter papers (125 mm diameter), removing the particles to collect the filtrate. Based on the sample laboratory test, the level of flocculant required for the 25 tonne batch was calculated and added to vessel MV3 whilst stirring on high speed of 20 rpm. After 15 minutes the stirrer speed was reduced to 10 rpm to enable the flocculants to agglomerate over a period of 2 hours. MV3 vessel contents were then cooled to 22 °C using the chiller for approximately 8 hours. The treated 'black phosphoric acid' was then transferred to a storage tank (316L stainless steel with paddle blades), whilst stirring at 40 rpm. The filtration process was then initiated using a BHS Candle filter system. The Candle filter system contained banks of filters (1 micron) with sleeves which allow the phosphoric acid to pass but prevent the carbon and calcium sulphate from passing through. During the filtration process the product passes through a 'turbidity meter'. If the turbidity is low the liquid is pumped into a clean tank. Alternatively, if the turbidity is high the liquid is pumped back in to the 'black phos tank' to repeat the filtration process. The carbon and calcium sulphate form a 'filter cake' on the outsides of the sleeves which aids filtration further. The threshold for determining 'low turbidity' can be set by the user based on their commercial or technical needs. In the Example above, in situ monitoring was carried out by assessing turbidity relative to reference values. Black phosphoric acid was allocated a turbidity level of 100% and 0% was allocated to clear phosphoric acid, with the product of filtration passed to the clean tank when the turbidity meter value is 50% or below. It can also be convenient to calibrate the in situ percentage values with a laboratory turbidity meter for cross-checking and quality control. The percentage values above were calibrated with a Lab HACH Lange Meter DR3900 operating at 860nm, obtainable from Hach Company (P.O. Box 389, Loveland, Colorado, 80539-0389) as follows: Reclaimed phosphoric acid turbidity meter Lab HACH Lange Meter DR3900 78% 37 NTU 46% 9 NTU 19% 5 NTU In the Table above, a relative turbidity value of 46% would be below the threshold of 50% used to grade the phosphoric acid as acceptable under this Example. Once flow rate drops the filtration stops and the cake is air dried (taking 15-30 minutes). Air is also blown into the filter sleeves so they expand, allowing the cake to drop off. Filter cake is collected into an IBC located beneath the candle filter or filter press.

Claims

1. A method for recycling contaminated phosphoric acid, said method comprising:a) providing a solution of phosphoric acid containing carbon and calcium contaminants;b) heating the solution after step a) to between 45 to 60 °C;c) adding sulfuric acid after step b);d) cooling the solution after step c) to below about 35 °C before adding a flocculant which is an anionic polyacrylamide emulsion; ande) passing the resultant mixture after step d) through a filter.

2. The method of claim 1, wherein the solution is cooled to below about 30 °C before adding a flocculant in step d).

3. The method of any preceding claim, wherein the concentration of the sulfuric LO acid in step c) is between 75-80% (w / w).CMCM 4. The method of claim 3, wherein the concentration of the sulfuric acid in step c)is about 77% (w / w).CM 5. The method of any preceding claim, wherein for the duration of step c), thesolution is maintained at a temperature between 45 to 60 °C.

6. The method of any preceding claim, wherein the pore size of the filter in step e) is about 5 microns.

7. The method of any preceding claim, wherein in step e), the resultant mixture is filtered under a pressure of about 6 bar.

8. The method of any of claims 1-6, wherein in step e), the resultant mixture is passed through a filter press.

9. A method for making phosphoric acid, in which at least a portion of the phosphoric acid is obtained by the method of any of claims 1 to 8.