Patent Grant
10486994
The invention relates to treatment of phosphate-containing wastewater, such as phosphogypsum pond water, and the recovery of useful phosphate compounds, such as struvite, during treatment.
Phosphogypsum is a by-product of processing phosphate rock into phosphoric acid fertilizer. The production of 1 ton of phosphoric acid generates approximately 4 to 5 tonnes of phosphogypsum. Phosphogypsum is essentially a waste product. Phosphogypsum may have low level radioactivity which prevents its use in various applications.
Phosphogypsum is typically stored by being slurried and piled into large stacks, which can be up to hundreds of feet high, in open air storage sites. Water percolating through the stacks forms ponds. In 2005, there were 24 phosphogypsum stacks in Florida alone, containing 1.2 billion tonnes of phosphogypsum and 50 billion gallons of pond water (Perpich et al, 2005).
In active phosphoric acid fertilizer plants, such ponds are typically used as reservoirs for process water for use in a closed loop. The pond water is toxic and needs to be treated before it can be discharged. Furthermore, closed stacks continue to produce a contaminant-containing leachate requiring treatment.
Pond water associated with phosphogypsum stacks is strongly acidic and contains numerous contaminants including large amounts of phosphates. Data collected from a number of sources are summarized in Table 1. The column headed “Representative Value” contains results from a composite of 18 samples from 6 different plants representing the composition of saturated fresh pond water (Kennedy et al., 1991). The reported phosphorus concentration of 6,600 ppm as P is equivalent to 20,220′ppm PO4 or 0.22 Mol/L. Pond water also contains significant amounts of ammonia (ammonia is often added to phosphoric acid in phosphoric acid plants to make di-ammonium phosphate) and magnesium.
Pond water treatment chemistry is relatively complex. Pond water may contain ten major components that can form numerous soluble species and precipitates when the pH is changed and cations are added. As indicated by the data under the column headed “Range” in Table 1, the composition of pond water can vary significantly.
A cost-effective and efficient process for treating phosphate-containing wastewater while recovering commercially useful phosphate compounds would be desirable.
The following embodiments and aspects thereof are described and illustrated in conjunction with apparatus and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.
In one aspect, a method for treating, and recovering phosphate compounds from, phosphate-containing wastewater, is provided. The method comprising: (a) removing fluoride from the wastewater; (b) recovering a phosphate compound from the wastewater by maintaining supersaturation conditions for the phosphate compound; and (c) polishing the wastewater.
Step (a) may comprise precipitating the fluoride. Precipitating the fluoride may comprise raising the pH of the wastewater to about pH 3 to 4. Raising the pH of the wastewater may comprise adding a calcium-containing base with calcium in a quantity to meet stoichiometric requirements for precipitating the fluoride. The calcium-containing base may be lime. The fluoride may be precipitated as fluorite. Raising the pH of the wastewater further may comprise adding one or more calcium-free bases with cations in a quantity to meet stoichiometric requirements for precipitating the phosphate compound. The one or more calcium-free bases may be selected from the group consisting of magnesium, oxide, magnesium hydroxide, ammonium hydroxide and anhydrous ammonia.
The phosphate compound may comprise struvite or a struvite analog such as iron ammonium phosphate.
Maintaining supersaturation conditions in step (b) may comprise one or more of maintaining a supersaturation ratio of 2 to 5; maintaining a pH of at least about pH 5; controllably introducing magnesium and/or ammonium; and maintaining a concentration of phosphate higher than concentrations of magnesium and ammonia. The struvite may be recovered in the form of crystals and aggregates ranging in size from 1 to 5 mm.
Silica may be removed from the wastewater of step (a) if the wastewater from step (a) comprises a silica concentration of greater than 100 ppm. Removing silica may comprise hydrolyzing the silica by raising the pH. Raising the pH to hydrolize the silica may comprise adding a base comprising cations in a quantity to meet stoichiometric requirements for precipitating the phosphate compound. The base may be selected from the group consisting of magnesium oxide, magnesium hydroxide, ammonium hydroxide and anhydrous ammonia.
Polishing step (e) may comprise raising the pH to about pH 8 to 10. Step (c) may comprise removing ammonia using a process selected from the group consisting of breakpoint chlorination, stripping, biological nitrification and biological denitrification.
Polishing step (c) may comprise subjecting the wastewater from step (b) to a two-stage membrane treatment comprising; (i) a first membrane treatment to obtain a first concentrate comprising divalent ions and a first permeate comprising monovalent ions; and (ii) a second membrane treatment for the first permeate to obtain a second concentrate comprising monovalent ions and a second permeate comprising effluent. The first concentrate may be recirculated to step (a). The first membrane treatment may comprise nanofiltration. The second membrane treatment may comprise reverse osmosis. Prior to the two-stage membrane treatment, the pH may be lowered to about pH 3 to 5, and suspended solids may be removed by filtration. Ammonia may be removed from the second permeate by subjecting the second permeate to ion exchange. Ammonia-containing regeneration liquid of the ion exchange may be recirculated to step (b).
Prior to step (b) the wastewater may be subjected to a first membrane treatment to obtain a first concentrate comprising divalent ions and a first permeate comprising monovalent ions, wherein the first concentrate defines feed for step (b). Wastewater from step (b) may be recirculated to step (a). The first permeate may be subjected to a second membrane treatment to obtain a second concentrate comprising monovalent ions and a second permeate comprising effluent. The first membrane treatment may comprise nanofiltration. The second, membrane treatment may comprise reverse osmosis. Prior to the two-stage membrane treatment, the pH may be lowered to about pH 3 to 5, and suspended solids may be removed by filtration. Ammonia may be, removed from the second permeate by subjecting the second permeate to ion exchange. Ammonia-containing regeneration liquid of the ion exchange may be recirculated to step (b).
In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions.
The accompanying drawings illustrate non-limiting embodiments of the invention.
Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well-known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.
Some embodiments of the invention relate to methods for treating phosphate-containing wastewater while simultaneously recovering commercially useful phosphate compounds. Bases are used to neutralize the acidity of phosphate-containing wastewater. Cations from the bases are used to remove contaminants and recover phosphate compounds. Excess cations may be recirculated to maximize contaminant removal and recovery of phosphate compounds.
Some embodiments of the invention relate to treatment processes wherein the phosphate-containing wastewater is phosphogypsum pond water and the phosphate compound is recovered in the form of granular struvite. These embodiments coincide with an aspect of the invention having significant commercial utility. The scope of the invention, however, is not limited to these embodiments.
Fluoride removal step 10 comprises raising the pH of the wastewater with one or more bases to a desired pH that promotes precipitation of contaminants such as fluoride and/or sulphates but not precipitation of phosphates. In some embodiments the pH may be raised to about pH 3.0-4.0. in some embodiments the degree to which the pH is raised may vary with the composition of the wastewater. Fluoride removal step 10 results in relatively dense precipitates that settle well. The precipitates may for example be settled and separated in a pond, a clarifier, a separation tank, or the like.
The base used in fluoride removal step 10 may be a calcium-containing base. The calcium-containing base may be added in an amount such that the calcium added to any pre-existing calcium in the wastewater results in a concentration of calcium ions sufficient to cause precipitation of compounds such as fluorite, calcium fluorosilicate, calcium sulphate, and the like while being insufficient to precipitate significant amounts of calcium phosphate. This may be achieved by adding sufficient calcium into the wastewater solution at a rate such that the product of the calcium ion concentration, the concentration of a fluorine-containing ionic species and the concentrations of any other components of a calcium salt exceeds the ksp for the calcium salt without being so high as to cause significant precipitation of calcium phosphate. The total amount of calcium added in step 10 is desirably sufficient to cause precipitation of the bulk of the fluoride in the wastewater in step 10. For example, a stoichiometric amount of calcium may be introduced during step 10. As shown in
Alternatively or additionally, one or more calcium-free bases may be added to raise the pH sufficiently to precipitate the fluoride at fluoride removal step 10. In some embodiments, the calcium-free base may be selected on the basis of a phosphate compound that is desired to be recovered at phosphate recovery step 30. For example, if the phosphate compound to be recovered is or comprises struvite, as shown in
Bases containing, magnesium and/or ammonia may be added to simultaneously raise pH of the wastewater and increase the concentration of magnesium and/or ammonia cations to facilitate struvite production in a subsequent step. For example, magnesium oxide may be used to add magnesium in a quantity sufficient to raise a concentration of magnesium ions to or toward a concentration desired to later precipitate struvite. Addition of a magnesium-containing base may also assist in removal of fluoride ions by promoting precipitation of fluoride as sellaite (MgF2).
In some embodiments a mixture of two or more calcium-free bases may be used, to raise the pH at fluoride removal step 10. Bases may be added in a sequence that accounts for pH-dependent differences in solubility of the bases. For example, the base with better dissolution at a lower pH may be added before the base with lower dissolution at the lower pH. For example, if magnesium oxide and ammonium hydroxide are used, then magnesium oxide may be added first (because its dissolution is better at lower pH), and then ammonium hydroxide added next to reach the desired pH for fluoride removal.
Following fluoride removal step 10, process 1 may include a silica removal step 20. Silica removal may be desirable in some embodiments to avoid silica gel formation, which may interfere with recovery of phosphate compounds (e.g. struvite crystallization) at phosphate recovery step 30. In some embodiments, silica may be removed by adding base to hydrolyze the silica and then allowing, the silica to settle. Step 20 may conveniently be performed in a settling tank or the like. Settled silica may be removed. In some embodiments, silica may be hydrolyzed by adding a base to adjust the to pH to a pH optimal for subsequent phosphate recovery step 30. In some embodiments, the pH may be at least about 5 prior to phosphate recovery step 30.
One or more bases that contain cations (e.g. magnesium and/or ammonia) that will enhance phosphate precipitation at phosphate recovery step 30 may be used to raise the pH for silica removal step 20. As shown in
Silica removal step 20 is unnecessary in some embodiments. Since silica gel formation tends to occur only at higher silica concentrations (e.g. Si>100 ppm), embodiments of the invention for processing wastewater with low silica concentrations may not require the silica removal step. Even if the silica concentration is high enough for gel formation, the hydraulic retention time of the gel formation is typically on the order of hours. In contrast, the hydraulic retention time for phosphate precipitation at phosphate recovery step 30 may be shorter than this. For example, the hydraulic retention time for struvite formation is typically less than 1 hour, although with a high concentration feed the hydraulic retention time may be significantly longer in embodiments incorporating recirculation as described below. Silica gel formation and the need for silica removal prior to phosphate recovery step 30 may therefore be avoided even in some embodiments that process wastewater with higher silica concentrations. In some embodiments where silica is not removed prior to phosphate recovery step 30, silica is hydrolyzed during phosphorus recovery step 30 and eventually removed downstream.
Silica removal step 20 is followed by phosphate recovery step 30. As shown in
Supersaturation conditions for the phosphate compound are maintained to recover desired phosphate compounds during phosphate recovery step 30. Maintaining supersaturation conditions may for example include: maintaining a supersaturation ratio of 2 to 5 for struvite; maintaining a suitable pH for example by controllably introducing a base and/or air stripping; maintaining phosphate concentration higher than concentrations of other components of the phosphate compound; and/or controllably introducing compounds comprising at least one of the other components of the desired phosphate compound.
Supersaturation conditions for struvite may be determined in relation to the struvite solubility product Ksp given by:
Ksp=[Mg2+]eq[NH4+]eq[PO43−]eq
where the activities of the different species (i.e. [Mg2+]eq, [NH4+]eq, and [PO43−]eq) are measured respectively as soluble magnesium, ammonia and orthophosphate at equilibrium. The supersaturation ratio (SSR) may be given by:
SSR=[Mg2+][NH4+][PO43−]/Ksp,
Increases in the SSR drive crystallization of struvite.
In the case of struvite recovery, the “other components” mentioned above are magnesium and ammonia. During struvite recovery in the embodiment illustrated in
The methods described in U.S. Pat. No. 7,622,047 may be modified and/or selected to optimize phosphate recovery step 30 in various ways including one or more of the following.
Following phosphate recovery step 30 the wastewater undergoes polishing step 40 before being discharged as treated effluent. In some embodiments, polishing step 40 may involve one or more chemical treatment steps.
In the embodiment shown in
First stage membrane 150 may be configured to reject divalent ions (e.g. phosphate, sulphate, magnesium) and let monovalent ions flow through (e.g. sodium, chloride, fluoride, ammonia) to second stage membrane 160. The first stage membrane may for example comprise a reverse osmosis (RO) or nanofiltration (NF) membrane. In some embodiments, the low pH concentrates (stream A) from first stage membrane 150 may be recirculated to fluoride removal step 110. As shown in
Second stage membrane 160 may be configured to reject monovalent ions (e.g. sodium, chloride, fluoride). As shown in
An ion exchange (IX) resin bed 170 may be provided to remove ammonia from the second stage membrane permeate before discharge as treated effluent. Ion exchange regeneration liquid containing the ammonia (stream C) may be recirculated to phosphate recovery step 130 to provide pH adjustment and ammonia for recovery of phosphate compounds.
Following silica removal step 220, wastewater is directed first stage membrane 250. In a mariner similar to process 100, the wastewater may be acidified and prefiltered prior to first stage membrane 250. Concentrate from the first stage membrane is fed to phosphate recovery step 230. This concentrate may contain most of the phosphate at about twice the concentration compared to the feed for the phosphate recovery steps in process 1 and 100. Concentrated phosphate may improve the conditions for the recovery of phosphate compounds in some cases. The other elements of the processes illustrated in
Recirculation of concentrate streams A, containing for example excess magnesium, and concentrate stream C, containing for example excess ammonia, to upstream steps may result in up to complete recovery of these components into recovered phosphate compounds, for example as struvite.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:
The following example provides results of laboratory scale testing of some embodiments of the invention.
Raw pond water samples were tested in three stages: 1) F removal with Ca, 2) pH increase, and 3) struvite precipitation.
In Stage 1, CaCO3 and Ca(OH)2 were added to 2 L and 3 L samples of pond water, mixed for 60 minutes, settled for 30 minutes, then filtered and supernatants analyzed to evaluate the effect of adding the bases on both pH and the concentrations of F and PO4.Ca(OH)2 was added in both solid and slurried form (results shown for slurried form only). Ca:F molar ratios of 0.5 and 0.6 for both reagents were tested, respectively representing the stoichiometric amount and a 20% excess amount.
Both CaCO3 and Ca(OH)2 reagents raised the pH to between about 2.5-3.5 after 1 hour of mixing. CaCO3 may be preferred in some embodiments. Test results showed that with CaCO3 the F removal at 0.6 Ca:F molar ratio was lower than with Ca(OH)2 at 0.6 Ca:F molar ratio but so were the PO4 and NH3 losses. For the remaining stages CaCO3 at 0.6 Ca:F molar ratio was used.
24 hours after completion of the test, more solids had precipitated in the filtered supernatant, and the SO4 concentration had decreased along with the Ca concentration, indicating gypsum formation.
In Stage 2, Mg(OH)2 was added On slurried 40 wt % form) to the 500 mL and 1250 mL samples of Stage 1 supernatant in Mg:P molar ratios of 0.8, 0.9, and 1.0, to raise the pH of the solution nearer the pH required for struvite precipitation and also to put Mg ions in solution. Also, MgCl2 was added in 1.0 Mg:P ratio to compare the effects of adding a non-basic Mg source at this stage.
The Mg compounds were added immediately after the completion of a repeated Stage 1 test, to prevent Ca loss through gypsum precipitation. The solutions were mixed for 60 minutes and settled for 15 minutes.
The Mg(OH)2 raised the pH to 4.5-5.5, and caused nearly complete removal (>90%) of both Ca and F. A substantial amount of PO4 was also removed, but the quantity remaining was still high and sufficient for struvite production downstream. A substantial amount of the added Mg was also removed, in this stage. The MgCl2 did not raise the pH but slightly lowered it, and had very little effect on either F removal or P loss. Increasing the Mg:P molar ratio from 0.8 to 1.0 increased F removal by only 2.8% but increased PO4 losses by 11.5%, 0.8 Mg:P was selected for use in Stage 3.
In Stage 3, NH4OH was added to 500 mL samples of Stage 2 supernatant in N:P molar ratios of 0.8 and 1.0, then NaOH was used to raise the pH above 7.0. As the Mg:P ratio was approximately 0.5:1 due to the Mg loss in Stage 2, a P recovery of near 50% would be expected if the P were primarily forming struvite. The Mg was 99% removed, showing that the reaction proceeded as tar as it could given the Mg limits, and the P removal was near 58%. Struvite precipitation in wastewater is Mg limited as well, and MgCl2 or other sources of soluble Mg can be added.
An overall pH test was also conducted 250 mL of the pond water sample was placed in a beaker. The blade of an overhead mixer was placed in the sample and rotated at 70 rpm 6.95 g CaCO3 was added to obtain a 0.6 Ca:F ratio. The pH was monitored every 15 minutes. The pH was recorded at 60 minutes.
3.95 g Mg(OH)2 slurried in 5.4 g water was added to obtain a 1:1 Mg:P ratio, based on previous jar test results from Example 1. The pH was monitored every 15 minutes. The pH was recorded at 60 minutes.
2.16g dry basis/7.17g 30 wt % NH4OH was slowly added to obtain a 1:1 NH4OH:P ratio based on P after the CaCO3 precipitation.
This application claims the benefit under 35 U.S.C. § 119 of U.S. provisional patent application No. 61/346,002, which is hereby incorporated herein by reference.
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| Number | Date | Country | |
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| 20190106348 A1 | Apr 2019 | US |
| Number | Date | Country | |
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| 61346002 | May 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13698129 | US | |
| Child | 16201786 | US |
