Patent Application
20240246839
The present invention relates to methods for making polyaluminum sulfate (PAS) with reduced chloride content. Such methods provide a product, useful for water treatment, with about 50% basicity and reduced corrosiveness.
Polyaluminum chloride (PAC), sulfated polyaluminum chloride, and polyaluminum chlorosulfate (PACS) are inexpensive to produce, and are useful as flocculating agents for water treatment. However, these compounds release chloride ions, which can corrode pipes, leading to the release of toxic substances, in particular lead ions. The corrosive nature of chloride was recently highlighted by the water crisis in Flint, Michigan, during which the presence of corrosive chloride ions contributed to lead levels well in excess of EPA limits.
U.S. Patent Appl. Publ. No. 2017/0113943 (the '943 publication) describes a procedure for the production of PAC and PACS, which involves combining an aluminum sulfate stream with a basic stream while maintaining a pH in the neutral range. However, in the '943 publication, the solution is decanted, after the precipitate of Al(OH)3 forms, followed by a separate digestion step with polyaluminum chloride (PAC), aluminum chloride, and/or hydrochloric acid, which brings the precipitate into solution. The decantation step adds to the complexity of the process. More importantly, the resulting formulation cannot be considered “low chloride.” Therefore, a need exists for methods of inexpensively producing flocculating agents that minimize the release of corrosive chloride ions.
In accordance with one embodiment of the invention, a method of preparing a polyaluminum sulfate (PAS) solution with reduced chloride content includes forming a basic solution of sodium carbonate and sodium aluminate and adding an aqueous solution of aluminum sulfate and the basic solution to water to form a mixture having a pH between about 5 and about 7. The method further includes allowing precipitates of aluminum hydroxide to form in the mixture and adding an additional aqueous solution of aluminum sulfate to the mixture in an amount sufficient to dissolve the precipitates of the aluminum hydroxide and to form the PAS solution.
In related embodiments, the aqueous solution of aluminum sulfate and the basic solution of sodium carbonate and sodium aluminate may be added to the water as two separate streams that do not contact one another prior to being added to the water. The basic solution may have a ratio of the sodium carbonate to the sodium aluminate of about 0.1 to 0.3. The method may further include agitating the water when the two separate streams are added to the water. The method may further include adjusting flow rates of the two separate streams in order to maintain the pH of the mixture. The PAS solution formed may have less than 20 ppm chloride. The mixture may have a pH between about 6.5 to about 6.8. The method may further include adding phosphoric acid to the PAS solution. The phosphoric acid may be added at a concentration of about 0.75% to about 1.5% by weight.
In accordance with another embodiment of the invention, a PAS solution is prepared according to any of the above described methods. The PAS solution may have a basicity of between about 40 and about 70, a weight percent sulfate between about 15 and about 22, and a weight percent alumina between about 7.5 and about 12. The PAS solution may have a basicity of between about 48% and about 52%, a weight percent sulfate between about 16.8 and about 17.2, and a weight percent alumina between about 8.0 and about 8.5. The PAS solution may be stabilized with between about 0.75% and about 1.125% phosphoric acid by weight.
According to another embodiment of the invention, a method for treating water to reduce impurities includes adding a PAS solution prepared according to any of the above described methods in order to coagulate the impurities. The method further includes separating the coagulated impurities from the water to produce a treated water reduced in the impurities without increasing a chloride to sulfate mass ratio of the treated water.
The foregoing features of embodiments will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which:
Definitions. As used in this description and the accompanying claims, the following terms shall have the meanings indicated, unless the context otherwise requires:
Expressed as a percentage, the “basicity” B of polyaluminum complexes, of the type Aln(OH)mXp(SO4)q, where X is a halogen, is given by the formula B=m/3*n*100. Pure polyaluminum sulfate, with the formula Al4(OH)6(SO4)3 thus has a basicity of 50%.
“Turbidity” corresponds to the “cloudiness” of a solution. The unit used to measure turbidity is the Nephelometric Turbidity Unit (NTU). As reported here, turbidity is measured by light scattering according to USEPA Method 180.1. ISO 7027. In water testing, turbidity reflects the presence of suspended microscopic particles. Turbidity can be decreased by the use of coagulants, which cause the suspended particles to aggregate and settle.
“% TOC” refers to the total percentage of total organic carbon in a water sample. As reported here, TOC is measured according to Standard Methods for the Examination of Water and Wastewater, 18th Ed. Using the Persulfate-Ultraviolet Oxidation Method 5310 C on a Sievers 900 Portable TOC analyzer utilizing a Sievers Inorganic Carbon Remover (ICR) and a GE autosampler.
“Alum” as used herein refers to aluminum sulfate, which has the chemical formula Al2(SO4)3.
“Soda ash” is a common term for sodium carbonate, which has the chemical formula Na2CO3.
“Sodium aluminate” has the chemical formula NaAl(OH)4.
In water purification applications, solutions of polyaluminum sulfate (PAS) function effectively as coagulants, and provide an alternative to other aluminum containing coagulants such as alum, polyaluminum chloride (PAC), sulfated polyaluminum chloride, and polyaluminum chlorosulfate (PACS). PAS solutions can be formulated with ultra-low chloride content. However, current methods of producing PAS are not cost-effective when compared to other aluminum containing coagulants.
According to embodiments of the present invention, methods are described for manufacturing an ultra-low chloride, PAS coagulant useful for removing suspended colloidal matter and organic carbon in potable water, industrial process water and wastewater. The PAS solution has reduced corrosivity compared to other aluminum based coagulants and is particularly suitable for drinking water applications. The simplified methods used to produce the PAS coagulant provide a high quality product that is less expensive to produce than current methods.
The manufacture of polyaluminum sulfate (PAS) is completed in two steps in embodiments of the present invention. The first two steps involve the mixing of two solutions in a single reaction vessel, followed by an optional third step that stabilizes the resulting PAS solution. The first step involves the precipitation of aluminum hydroxide and the second step involves the digestion of the precipitated aluminum hydroxide. The method begins by forming a basic solution of sodium carbonate and sodium aluminate. Preferably, the basic solution has a sodium carbonate to sodium aluminate ratio of about 0.1 to 0.3. The method further includes simultaneously adding separate streams of an aqueous solution of aluminum sulfate and the basic solution to water to form a mixture having a pH between about 5 and about 7. The two separate streams are added into the reaction vessel such that they do not come into direct contact with one another before being dispersed into the water, under constant agitation, in the reaction vessel. The method further includes allowing precipitates of aluminum hydroxide to form in the mixture. Precipitation occurs as the pH reaches a range of about 5 to about 7 which is the range of minimum solubility for aluminum hydroxide. A key feature of the method is that the addition rates of the two streams into the reaction vessel are adjusted to maintain this pH range so that precipitation continues. The optimal range is a pH of about 6.5 to about 6.8. Sodium sulfate and carbon dioxide are produced as side reaction products. The precipitation step is typically completed in about 1 hour.
The use of a mixture of sodium aluminate solution with dissolved sodium carbonate is required to form a precipitate that will dissolve to form the final product. If only sodium aluminate solution is used, the resulting precipitate will not dissolve in a reasonable time period. If only sodium carbonate solution is used, the resulting product will be too dilute for commercial use due to its limited solubility in water.
The second step involves the digestion of the precipitated aluminum hydroxide. The method further includes adding an additional aqueous solution of aluminum sulfate to the mixture in an amount sufficient to dissolve the precipitates of the aluminum hydroxide and to form a liquid PAS solution at the desired basicity. This second step is completed as soon as the first step is finished. The time to digest the precipitated aluminum hydroxide in the additional amount of aluminum sulfate solution ranges from about 6 to about 20 hours when the proper proportion of sodium aluminate solution and sodium carbonate are used during the precipitation step.
The method may further include adding phosphoric acid to the PAS solution. This optional third and final step to form a stabilized final PAS product may include the addition of about 0.75 to about 1.5% phosphoric acid by weight to the mixture just after the final aluminum sulfate solution is added. The phosphoric acid increases the shelf life from about 2 months to about 6-12 months or more depending on the amount of phosphoric acid added.
The reactions that occur during the precipitation step and the digestion step are as follows.
Solutions of the PAS coagulant produced according to embodiments of the present invention, as described in the following working examples, are clear to slightly hazy liquids, colorless to yellow in appearance, having a percentage of alumina within the range of about 7.8% to about 8.7%, ideally within the range of about 8.0% to about 8.5%. The PAS solutions have basicities that are within the range of about 45% to about 55%, ideally within the range of about 48% to about 52%, and a percentage of sulfate within the range of about 16.5% to about 17.4%, ideally within the range of about 16.8% to about 17.2%. Here, the test methods used to determine the percentage of alumina, basicities, and the percentage of sulfate are provided in the “Test Method” column of Table I.
Table I further provides a summary of experimental results as described below compared to expected values for solutions of Al4(OH)6(SO4)3.
8-8.5
The laboratory reactor used for the following working examples is described below and shown in
A second beaker of variable size is used to make up and feed the water/sodium carbonate/sodium aluminate mixture. The beaker was placed on a VWR programmable hotplate fitted with an infrared thermocouple for controlling the mixture temperature. Agitation was provided using a second Caframo model BOC digital vertical shaft overhead mixer fitted with a 7¾ inch long shaft and a single 1½ or 2¼-inch diameter, 45° pitched, four-blade agitator (depending on the volume of the mixture). The agitator is positioned so the blades are covered by the solution water and operated at 350 rpm initially to dissolve the sodium carbonate and combine the sodium aluminate. The speed is reduced as necessary during process to eliminate splashing. The thermocouple is positioned so that the infrared probe end is near to, but not touching the bottom of the beaker and such that it does not contact the agitator impeller. The hotplate is set to maintain the exotherm temperature of the solution process of the mixture which is usually around 35° C. A glass serological pipet is clamped such that the tip reaches to the bottom of the beaker but on a slight angle so as to not interfere with the flow of the mixture and is positioned so that the piped does not contact the agitator impeller. One end of one of the #13 Masterflex tubing from the precipitator/digestor set up is fed into the serological pipet to feed the reactor with the mixture.
A plastic Boston Round bottle of appropriate volume was used to feed the aluminum sulfate solution. A hole of the same diameter as the #13 Masterflex tubing was cut into the cap to seal the bottle after the aluminum sulfate solution is added to eliminate evaporating during the process. The end of the other #13 tubing is fitted into the bottle such that the end reaches to the bottom but does not interfere with the flow of the solution.
Weigh and record the precipitator/digestor tare weight. Add the required amount of start-up water to the vessel. Place the agitator and serological pipets as described above. Set the agitator to 250 rpm.
Insert a calibrated pH probe with an automatic temperature compensator and digital pH and temperature readout midway between the serological pipets and downstream of the intended aluminum sulfate feed point. Place the probe such that the tip is covered by the start-up water and will not contact the agitator impeller.
Weigh and record the tare weigh of the beaker used to make up and feed the water/sodium carbonate/sodium aluminate mixture. Tare the balance with the vessel on it and add a slight excess of the required solution water and record the amount. Place the beaker on the programable hotplate as described above. Set the agitator at 350 rpm.
In another tared beaker, add a slight excess of the required amount of sodium carbonate and record the amount. Slowly add this to the mixture beaker. Reweigh the addition beaker to confirm all the sodium carbonate has been added. Record any difference. Allow the sodium carbonate to completely dissolve. Once dissolved set the hotplate to match the exotherm temperature of about 35° C.
In another tared plastic beaker add a 1-2 g excess of the amount of sodium aluminate required. This material is very viscous and careful addition is necessary to add the desired slight excess of material to the mixture. Re-tare the beaker with the sodium aluminate. Slowly added this to the mixture beaker until a slight excess is added. Do this by measuring the weight loss of the beaker containing the sodium aluminate. Record the total amount added by difference. From the recorded weight in steps 3, 4 and 5 the gross weight of mixture beaker can be calculated.
Tare the plastic Boston Round bottle to be used to feed the aluminum sulfate solution. Add 0.5-1.0 grams excess of the required amount of aluminum sulfate solution. Remove the bottle and tare the balance. Weigh and record the gross weight of the bottle and the aluminum sulfate solution. Insert the Masterflex tubing as described above.
Start the Masterflex pump feeding the aluminum sulfate solution on its lowest setting. When the solution reaches the reactor, and the pH drops to below 4 turn off the pump and record the temperature and pH using the meter readouts.
Start the Masterflex pump feeding the mixture on its lowest setting. When the mixture just reaches the bulk solution in the reactor restart the pump feeding the aluminum sulfate solution and start a timer.
Monitor and record the resulting bulk slurry temperature and pH using the meter readouts at one-minute intervals. Adjust the pump rates as necessary to achieve a bulk solution pH of 6.5-6.8 in as timely a manner as possible. Once the pH has stabilized increase the pump feed rates proportionally so that the water/sodium carbonate/sodium aluminate mixture feed is complete in about one hour. Monitor and record the bulk solution temperature and pH at convenient intervals.
When the water/sodium carbonate/sodium aluminate mixture feed is complete, record the bulk slurry temperature, pH and time and immediately weigh and record the final gross weight of the mixture beaker and any residual. Flush a small amount of water through the feed tubing.
Continue feeding the aluminum sulfate solution at the same rate until the bulk slurry drops below pH 5.0. Increase the feed rate to a maximum until all the aluminum sulfate solution is added. Record the bulk slurry temperature, pH and the time and immediately weigh and record the final gross weight of the bottle and residual. If a solid is being added as a stabilizer flush a small amount of water through the tubing. Remove and rinse the serological pipets used to feed the mixture and flush the tubing with water into a waste container.
In another tared beaker add a slight excess of the amount of stabilizer required. Tare and record the gross weight of the beaker and stabilizer. If a solid is being used add it directly to the top of the bulk slurry. Weigh and record the final gross weight of the beaker and any residual. If a solution is being used add it using the tubing used to feed the aluminum sulfate solution on the lowest pump setting. When all the stabilizer is added record the bulk slurry temperature, pH and the time and immediately weigh and record the final gross weight the beaker and any residual. Flush a small amount of water through the tubing.
Remove and rinse the serological pipet and flush the tubing with water into a waste container. Allow the slurry to mix until the pH has stabilized and record the final value. Remove the probe and allow as much solution to drain back into the reactor as practical. Rinse and store the probe as required. Allow the slurry to mix until all the solids have dissolved, overnight if necessary.
Once all the solids have dissolve turn off the agitator and remove the agitator. Allow as much solution to drain back into the reactor as practical. Record the time and weigh and record the final gross weight of the reactor and the final product.
Filter the solution through a #41 or #541 Whatman filter paper under vacuum. Store the final product in an appropriately sized plastic Boston Round sample bottle. Fill a 40 ml glass TOC vial for stability studies.
The product analysis and specifications obtained upon application of this procedure are shown above in Table 1.
The PAS material stabilized with 1% phosphoric acid, prepared according to embodiments of the present invention and described in the Working Examples above, was used to treat raw water samples from central Arkansas and from Anderson, South Carolina. For comparison, water samples from the same batches were treated with alum.
The lower curve of
Treatment costs in $/MG are shown in
As shown in
Table 2 shows a summary of results of numerous clarity tests that were conducted with different experimental parameters according to embodiments of the present invention. A more detailed description of the parameters and processes used for each of the clarity tests are shown in Tables 3-37 for Lot numbers MR-105, MS-100, MS-110, MS-170, MU-046, MU-064, MU-162, MU-164, MU-166, MU-168, MU-180, MV-057, MV-061, MV-064, MV-067, MV-070, MV-073, MV-076, MV-080, MV-085, MV-110, MV-115, MV-119, MV-122, MV-125, MV-128, MV-131, MV-142, MV-145, MV-155, MV-158, MV-161, MV-169, MV-173, and MV-176, respectively.
For Lot numbers MS-100, MS-110, and MS-170, Table 2 shows just the precipitation of aluminum hydroxide step. For Lot number MU-162, the sample initially gelled, but with extra alum became fluid but turbid.
The embodiments of the invention described above are intended to be merely exemplary; numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.
This patent application claims the benefit of U.S. Provisional Patent Appl. No. 63/480,216 filed Jan. 17, 2023, the disclosure of which is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63480216 | Jan 2023 | US |
