Abstract:
A process and system for producing phosphoric acid. Phosphate rock is dissolved in phosphoric acid in a reaction vessel to form a slurry. The slurry is then reacted in a first stage of crystallization with sulfuric acid to produce calcium sulfate hemihydrate and phosphoric acid. The product acid is separated from the hemihydrates via filtration, and the filter cake is then reacted, in a second crystallization step, with additional sulfuric acid to produce dihydrate calcium sulfate (gypsum) and recovery solution. The gypsum is separated from the recovery solution via filtration and removed as a by-product. The recovery solution is recycled back to the transformation tank and to the hemihydrate filtration step. A feed acid tank combines wash solution, recovery solution and product acid. Once adjusted to a target P 2 O 5  concentration, it is fed to the initial reactor vessel to dissolve the phosphate ore.

Description:
RELATED APPLICATION 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/775,049 filed Mar. 8, 2013, which is incorporated herein in its entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is generally directed to the production of phosphoric acid, and more particularly to a two-stage crystallization and filtration process employing a feed acid tank assembly and recovery solution tank assembly for the production of high strength phosphoric acid with a high recovery of P 2 O 5 . 
       BACKGROUND OF THE INVENTION 
       [0003]    Phosphoric acid (H 3 PO 4 ) has a number of commercial uses from its use in the production of agricultural products, such as fertilizers and animal feeds, to its incorporation into food products. Phosphoric acid concentration can be expressed in several ways including percent phosphoric acid (% H 3 PO 4 ), percent phosphorous pentoxide (% P 2 O 5 ) or percent phosphorous (% P) using the following conversion factors to convert between concentration units: 
         [0000]      % H 3 PO 4 ×0.724=% P 2 O 5 %
 
         [0000]      % H 3 PO 4 ×0.316=% P
 
         [0000]      % P 2 O 5 ×0.436=% P
 
         [0004]    For the purposes of this application going forward and for sake of consistency and simplicity, phosphoric acid concentration will be expressed as P 2 O 5  concentration (% P 2 O 5 ). The concentration or strength of phosphoric acid defines its suitability for a particular use. For example, commercial grade phosphoric acid has a P 2 O 5  concentration or purity of about 50-54% whereas food grade P 2 O 5  has a concentration or purity of about 54-62%. 
         [0005]    Phosphoric acid for use in agricultural products, such as fertilizers and animal feeds, is commonly produced by a wet process. In a wet process, mined phosphate rock or phosphate ore, such as tricalcium phosphate rock or apatite, is dissolved or acidulated by the addition of sulfuric acid to yield phosphoric acid and insoluble calcium sulfate by-product. The overall chemical reaction in simplified form is: 
         [0000]      Ca 3 (PO4) 2 +3H 2 SO 4 +6H 2 O=3CaSO 4 .2H 2 O+H 3 PO 4    (1A)
 
         [0006]    The overall reaction can be broken down into two separate steps. First, the formation of monocalcium phosphate by reaction of the phosphate rock with excess phosphoric acid, either added to the process and/or recycled, in the following reaction: 
         [0000]      Ca 3 (PO4) 2 +4H 3 PO 4 =3Ca(H 2 PO 4 ) 2    (2A)
 
         [0007]    Second, the reaction of the monocalcium phosphate with sulfuric acid to form additional phosphoric acid and calcium sulfate in the following reaction: 
         [0000]      3Ca(H 2 PO 4 ) 2 +3H 2 SO 4 =3CaSO 4 +6H 3 PO 4    (3A)
 
         [0008]    The calcium sulfate by-product can be obtained in various crystalline forms depending on the concentrations of various reactants in each of the above reactions and the temperature of the reactions. Two particular forms of calcium sulfate include calcium sulfate dihydrate (CaSO4.2H20), otherwise known as gypsum, and calcium sulfate hemihydrate (CaSO4.½H20). The former, or the dihydrate form, is a stable crystalline form which is readily filterable and washable in process. However, because of its stable crystalline form, phosphate can become trapped within its structure, resulting in a net phosphate loss to the system, decreasing the system overall efficiencies, which in turn, translates to increased production costs. Furthermore, the then phosphate-contaminated gypsum may be unsuitable for its end use, such as for cements, plasters, or the like. 
         [0009]    The hemihydrate form, on the other hand, has a much less stable crystalline structure, and, if filtering conditions are not closely monitored and controlled, has the propensity to hydrate on the filter. Hydration on the filter can result in an unfilterable mass which leads to process shutdowns, and ultimately increased production costs. 
         [0010]    Therefore, each step of the wet process and its various parameters, including reactant concentrations and reaction temperatures, must be carefully monitored, measured and controlled to reduce the process variability by maximizing filterability of the calcium sulfate by-product(s) and/or phosphate or P2O5 recovery. A variety of wet processes have been developed for the production of phosphoric acid, mainly with one or more of these goals in mind. 
         [0011]    U.S. Pat. No. 4,059,674 to Lopker, for example, is directed to a process for producing phosphoric acid and gypsum, employing three stages of crystallization to increase phosphate recovery. Per the Abstract section, “calcium phosphate rock particles [are mixed with] recycled phosphoric and sulfuric acids . . . to form additional phosphoric acid and calcium sulfate dihydrate (gypsum) . . . . A gypsum slurry is withdrawn and product phosphoric acid is separated therefrom and withdrawn from the process . . . . The gypsum is passed to a first recrystallizer wherein sulfuric acid is introduced, and the gypsum is recrystallized to hemihydrate. A slurry of the hemihydrate is passed to a second recrystallizer, wherein water obtained as described below is added and the hemihydrate is recrystallized to gypsum . . . .” 
         [0012]    U.S. Pat. No. 4,777,027 to Davister et al. is directed to a continuous process for preparing phosphoric acid and calcium sulfate. As described at column 1, lines 5-19, the method comprises “subjecting in a mixture containing calcium sulphate flowing through a reaction zone sequence, calcium phosphate to an attack by a mixture of sulfuric and phosphoric acids, while separating calcium sulphate and extracting at least part of the production phosphoric acid . . . . [The] calcium sulphate . . . by-product . . . may notably be comprised of dihydrate, α-hemihydrate, II-anhydrite or a mixture in very varying ratios of two or three said crystalline forms . . . .” As shown in  FIG. 2 , various recycle and feed lines directly feed the first reactor in the series of reactors. 
         [0013]    U.S. Pat. No. 4,853,201 to Ore et al. discloses a “process for recovering P2O5 values from hemihydrate crystals generated during the hemihydrate process for manufacturing phosphoric acid compris[ing] converting the hemihydrate crystals to dihydrate crystals by recrystallization in a crystallizer having a phosphoric acid concentration in the range of about 0.1% to about 10% on a P2O5 basis, and a free sulfate concentration in the range of about 0.1% to 10% . . . . The crystallizer operates at low P2O5 and high sulfate levels, thereby reducing the hydration time, which is a major benefit of this process . . . .” 
         [0014]    Similarly, U.S. Pat. No. 3,632,307 to Cornelis van Es et al. discloses a process in which “[p]hosphoric acid and gypsum are prepared from phosphate rock by acidulating same with sulfuric acid or a mixture of sulfuric and phosphoric acids to form a slurry of CaSO 4 .½H 2 O [calcium sulfate hemihydrate] in phosphoric acid. The CaSO 4 .½H 2 O is washed to removed adhered phosphoric acid and recrystallized from a solution containing phosphoric and sulfuric acids to form CaSO 4 .2H 2 O [gypsum].” 
         [0015]    These processes all use some form of recycle within the process, such as, for example, recycle of the recovery solution(s) from the filtrations step(s), and/or recycle of the wash solutions from the filtration step(s). These recycle lines are fed directly into the initial reaction vessel or acidulation tank and/or into the filtrations steps. Because each of these lines must be closely and accurately measured and monitored, process controllability issues can quickly arise due to the volume of lines feeding the different unit operations. For example, if there is variability in the measurement system such that the measures of one or more of the recycle lines is inaccurate, this can cause the concentration of reactants in the crystallizer to be off target (e.g. low sulfate levels), which causes the crystallizer to operate incorrectly, resulting in small crystal size, for example. In turn, this results in poor filtration or filtration variability, leading to loss of P 2 O 5 , and ultimately an increase in production costs. Similarly, unwanted co-crystallization can occur with similar effects downstream, as well as poor digestion in the acidulation tank if the incoming feeds are poorly controlled, i.e. the variability is too high. 
         [0016]    There remains a need for a process of producing phosphoric acid with high yield of P205 and high strength product acid by reducing the potential for measurement error and therefore reducing the system and process variability. 
       SUMMARY OF THE INVENTION 
       [0017]    Embodiments of the invention are directed to a two-stage crystallization and filtration process for producing phosphoric acid. In an embodiment, the process comprises a first reactor including a rock slurry tank and a dissolver in which mined wet rock is dissolved with phosphoric acid and optionally sulfuric acid to produce a rock slurry. The slurry is then fed to a first crystallizer/filtration assembly including a crystallizer and a hemihydrate filtration system. In the crystallizer, the rock slurry is reacted with sulfuric acid to produce product phosphoric acid and calcium sulfate hemihydrate. The product acid is extracted from the calcium sulfate hemihydrate in the first filtration assembly, and the product acid is fed to a product acid tank. The filter cake is then washed, and the wash solution is recycled back to a feed acid tank. 
         [0018]    The process further comprises a second crystallizer/filtration assembly including a transformation tank and a dihydrate filtration system. The filter cake from the hemihydrate filtration system is fed to the transformation tank where it is reacted with sulfuric acid and a combination of recycled dihydrate recovery solution and wash solution to precipitate calcium sulfate dihydrate or gypsum in the form of a slurry. The slurry is then fed to a dihydrate filtration system in which it is filtered and the recovery solution from this primary filtration is recycled back to the transformation tank. The filtered gypsum is then washed with water, and the gypsum is extracted from the process. The wash solution is then also recycled to the transformation tank. 
         [0019]    The process further comprises a feed acid tank assembly for combining recovery solution from the dihydrate primary filtration, wash solution from the hemihydrate wash filtration, and product acid from the product acid tank. The feed acid tank assembly includes a control system for monitoring or measuring and adjusting the P 2 O 5  concentration in the feed acid tank, as well as the temperature, flow rate, and other process parameters as needed. Feed acid at a target P 2 O 5  concentration from the feed acid tank assembly is then fed directly to the rock slurry tank. The feed acid tank assembly provides a single source of monitoring and regulating the feed acid to the system, rather than monitoring individual feed acid streams as required in the systems of the prior art. 
         [0020]    The process also comprises a recovery solution tank assembly to capture the recovery solution from the primary filtration of the calcium sulfate dihydrate before it is reintroduced into the process. Similar to the fee acid tank assembly, the recovery solution tank assembly provides a single point of adjustment and control of the concentration, temperature, and/or flow rate of this recovery solution before it is recycled to one or more of the transformation tank as a reactant, to the hemihydrate wash filtration step as a wash solution for the hemihydrates filter cake, and as an input to feed acid tank assembly. 
         [0021]    The feed acid tank assembly and the recovery solution tank assembly each provide a single source of monitoring and regulating the feed acid and the recycled recovery solution, respectively, to the system which results in better control of the process, less concentration variability in the various streams throughout the process, and less filter variability such that the process is efficient, economical, and stable, while producing high strength acid having concentrations of 39% P 2 O 5  or higher and high P 2 O 5  yields from the phosphate ore of 99% or greater. 
         [0022]    The above summary of the invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description that follow more particularly exemplify these embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The invention can be completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawing(s), in which: 
           [0024]      FIG. 1  is a process flow diagram of a process for producing phosphoric acid according to an embodiment of the invention. 
       
    
    
       [0025]    While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and is described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0026]    In embodiments of the invention, a hemihydrate/dihydrate process  10  (hereinafter “the HH-DH process  10 ”) for the production of phosphoric acid comprises two stages of crystallization and filtration to achieve high recovery of P 2 O 5  and high strength (or high concentration) product acid. In addition, process  10  includes a feed acid tank assembly  148  comprising one or more tanks to which product acid and other recycle streams are combined, measured, and optionally adjusted before being introduced as a single feed acid stream into the initial reactor or rock slurry tank, allowing for better process controllability and process efficiencies, while reducing process variability and sampling error. Process  10  also includes a recovery solution tank assembly  144  comprising one or more tanks to which recovery solution from the dihydrate filtration step  132  is fed, measured, and optionally adjusted before being introduced as one or more recycle streams back into process  10  at various points. 
         [0027]    First, in a rock slurry tank  102 , wet rock  100  and adjusted feed acid stream  150  are combined. Wet rock  100  can be pulverized or otherwise processed before entering rock slurry tank  102  to increase the surface area of the phosphate rock for better, more complete digestion. Adjusted feed acid stream  150  contains a combination of recycle and product acid streams at a target P 2 O 5  concentration from feed acid tank assembly  148 , as is described in more detail below. In the rock slurry tank  102 , the CO 2  in the rock is liberated as shown in reaction (1): 
         [0000]      CaCO 3 +2H 3 PO 4 →Ca 2+ +2H 2 PO 4   − +CO 2 +H 2 O   (1)
 
         [0000]    The removal of the CO 2  gas in the rock slurry tank allows for consistent dissolver operation. 
         [0028]    Rock slurry  104  from rock slurry tank  102  and re-circulated crystallizer slurry  116  (described below) are fed to a dissolver  106 . In the dissolver  106 , rock is dissolved by phosphoric acid into mono-calcium phosphate slurry  108  (or dissolver slurry  108 ) as shown in reaction (2): 
         [0000]      Ca 10 (PO 4 ) 6 F 2 +12H 3 PO 4 →9Ca 2+ +18H 2 PO 4   − +CaF 2    (2)
 
         [0000]    In the dissolver  106 , the P 2 O 5  concentration is maintained above  39 % and the Ca 2+  concentration is maintained around 1% in order to maximize rock dissolution. Undissolved rock losses are expected to be or comprise less than 0.5% of the rock P 2 O 5  fed. 
         [0029]    Fresh sulfuric acid (H 2 SO 4 )  112  and dissolver slurry  108  are fed to a crystallizer  110 . In the crystallizer  110 , Ca 2+  precipitates with SO 4   2−  as hemihydrate gypsum as shown in reaction (3): 
         [0000]      Ca 2+ +SO 4   2− +½H 2 O→CaSO 4 .½H 2 O   (3)
 
         [0030]    The SO 4   2−  concentration in the crystallizer  110  is maintained at about 2% for good crystal growth and good filterability in subsequent filtration. Due to the high P 2 O 5  concentration in the crystallizer  110 , co-crystallization of di-calcium phosphate also occurs as shown in reaction (4): 
         [0000]      Ca 2+ +HPO 4   2− →CaHPO 4    (4)
 
         [0000]    This reaction would otherwise account for 6% to 8% P 2 O 5  loss if the crystallizer solids were discharged without further processing. 
         [0031]    Crystallizer slurry  114 , made up of a combination of hemihydrate gypsum and di-calcium phosphate, is fed to a hemihydrate filter system  117  to separate product acid (H 3 PO 4 )  120  from the solids. Product acid  120  is sent to product acid tank  121  where it is used for commercial sale of product acid  120   a  and/or it is sent via  120   b  to feed acid tank assembly  148  before reentry or being recycled into process  10  as needed. 
         [0032]    After primary filtration at  118 , phosphoric acid remains in the filter cake. Recovery solution  146   b  (described in further detail below), including phosphoric acid and excess sulfuric acid, is used to wash the phosphoric acid from the cake at hemihydrate wash filtration step  122 , and the resulting wash solution  126  is returned to the feed acid tank assembly  148 . Soluble P 2 O 5  losses after washing would account for 2% to 4% P 2 O 5  loss if the cake were discharged after filtration without further processing. 
         [0033]    Washed hemihydrate filter cake  124 , fresh sulfuric acid  112 , dihydrate filter wash solution  142 , and adjusted recovery solution  146   a  are then mixed in transformation tank  128 . In the transformation tank  128 , hemihydrate gypsum and co-crystallized P 2 O 5  loss dissolve as shown in reactions (5) &amp; (6): 
         [0000]      CaSO 4 .½H 2 O→Ca 2+ SO 4   2− +½H 2 O   (5)
 
         [0000]      CaHPO 4 +H 2 SO 4 →Ca 2+ SO 4   2− +H 3 PO 4    (6)
 
         [0034]    Also, dihydrate gypsum crystallizes as shown in reaction (7): 
         [0000]      Ca 2+ +SO 4   2− +2H 2 O→CaSO 4 .2H 2 O   (7)
 
         [0035]    The SO 4   2−  concentration in the transformation tank is maintained above about 1%, and more particularly above about 3%, and more particularly above about 5% to prevent or inhibit crystallization of di-calcium phosphate. At this concentration, co-crystallized losses are expected to be less than 0.5% of the rock P 2 O 5  fed. 
         [0036]    Transformation tank slurry  130 , which includes dihydrate gypsum, is fed to a dihydrate filter system  131  to separate recovery solution  134  from the solids. Recovery solution  134  is then sent to recovery solution tank assembly  144 , comprising one or more tanks. Recovery solution tank assembly  144  allows for a single point of adjustment and control of the concentration, temperature, and/or flow of recovery solution  134  before it is recycled to one or more of transformation tank  128  at  126   a,  hemihydrate wash filtration step  122  at  126   b,  and/or feed acid tank assembly  148  at  126   c.    
         [0037]    After primary filtration  132 , recovery solution remains in the filter cake. Water  138 , such as, for example, fresh water from the battery limits, process water, and/or other applicable water sources, is used to wash this recovery solution from the cake in a wash filtration step  136 , and the resulting wash solution  142  is returned to the transformation tank  128 . Soluble P 2 O 5  losses after washing are expected to be less than 0.5% of the rock P 2 O 5  fed. Dihydrate gypsum  140  is extracted from the wash filtration step  136 . 
         [0038]    As mentioned above, the feed acid tank assembly  148 , comprising one or more tanks supplies adjusted feed acid stream  150  to rock slurry tank  102  at the beginning of process  10  as a single feed acid stream  150 . Adjusted feed acid stream  150  is made up of the combined streams in feed acid tank assembly  148  including hemihydrate wash solution  126  from hemihydrate wash filtration step  122 , adjusted recovery solution  146   c  from recovery solution tank assembly  144 , and product acid  120   b  from product acid tank  121  as needed. By incorporating feed acid tank assembly  148 , the acid feed stock, or feed acid stream  150 , is simplified to a single source, allowing for better control and more consistent concentration of feed acid stream  150 . In other words, the feed acid tank assembly  148  acts as a buffer for the feed acid stream  150 , allowing adjustments to acid or reactant concentrations, flow, and/or temperatures at tank assembly  148  before entry into rock slurry tank  102 . 
         [0039]    Furthermore, the use of feed acid tank assembly  148  to produce a single adjusted feed acid stream  150 , as opposed to multiple streams directly feeding into rock slurry tank  102 , requires a single measurement system for monitoring the concentration and flow of feed acid stream  150 . This eliminates the need for complex measurement and control systems needed in the processes of the prior art. In the prior art, a separate instrument or measurement system is required to measure or monitor each individual feed into the initial reactor or acidulating tank, which increases the likelihood of equipment or measurement/sampling error. By using feed acid tank assembly  148 , only a single feed acid stream (or P 2 O 5  concentration) requires monitoring and controlling, thereby reducing the likelihood of measurement error and reducing control variability, which in turn reduces variability in the concentrations of the unit operations, such as the dissolver  106 , the crystallizer  110 , and/or the transformation tank  128 , throughout process  10 . This results in better control and less variability in the filterability of both the hemihydrate and dihydrate gypsum, creating a more economic, better controlled process, and overall higher P 2 O 5  yield. 
         [0040]    Similarly, recovery solution tank assembly  144  also acts as a buffer to monitor the input stream  146   a  into transformation tank  128  as well as input stream  146   b  to hemihydrate wash filtration step  122  and input stream  146   c  to feed acid tank assembly  148 . Controlling one or more of concentration, flow, and temperature of recovery solution  134  at a single point before re-entry into process  10  allows for reduction of sources of sampling and measurement error in the system, resulting in the reduction of process variability and increase in process efficiency. 
         [0041]    According to a non-limiting embodiment of the invention, a control system includes control of flows to each of the unit operations described above. However, alternative control systems can also be contemplated, and the control system described below is for exemplary purposes only. 
       Rock Slurry Tank  102   
       [0042]    Flow of rock slurry  104  to the dissolver  106  is controlled by operator to adjust to the target plant rate. Flow of wet rock  100  to the rock slurry tank  102  is ratio controlled to the flow of rock slurry  104  to the dissolver  106 . Rock slurry tank level controls the wet rock  100  to rock slurry  104  ratio. 
         [0043]    Flow of feed acid stream  150  to the rock slurry tank  102  is ratio controlled to the flow of wet rock  100  to the rock slurry tank  102 . The rock slurry tank solids is controlled by operator adjustment of the feed acid stream  150  to wet rock  100  ratio. The crystallizer solids is controlled by adjusting the rock slurry tank solids target. 
         [0000]    Feed Acid Tank assembly  148   
         [0044]    Flow of product acid via  120   b  to feed acid tank assembly  148  is cascade controlled by the feed acid tank level. The flow of recovery solution  146   c  is ratio controlled to the flow of product acid  120   b  to tank assembly  148 . Feed acid tank P 2 O 5  concentration is then controlled by operator adjustment of the recovery solution to product acid ratio. Crystallizer P 2 O 5  concentration is then controlled by simply adjusting the feed acid P 2 O 5  concentration target—a single source as opposed to multiple sources of P 2 O 5 . 
       Dissolver  106   
       [0045]    Flow of crystallizer slurry  116  to the dissolver  106  is ratio controlled to the flow of rock slurry  104  to the dissolver  106 . Dissolver Ca 2+  concentration is controlled by operator adjustment of the crystallizer slurry  116  to rock slurry  104  ratio. 
         [0046]    Flow of dissolver slurry  108  to the crystallizer  110  is cascade controlled by the level in the dissolver  106 . 
       Crystallizer  110   
       [0047]    Flow of sulfuric acid  112  to the crystallizer  110  is ratio controlled to the flow of rock slurry  104  to the dissolver  106 . Crystallizer SO 4   2−  concentration is controlled by operator adjustment of the sulfuric acid  112  to rock slurry  104  ratio. 
         [0048]    Pressure in the crystallizer is cascade controlled by the crystallizer temperature. Crystallizer temperature is set by the operator. 
       Hemihydrate Filter  117   
       [0049]    Flow of crystallizer slurry  114  to the hemihydrate primary or HH primary filter  118  is cascade controlled by the level in the crystallizer  110 . 
         [0050]    Flow of adjusted recovery solution  146   b  to the HH wash filter  122  is ratio controlled to the flow of crystallizer slurry  114  to the HH filter  119 . HH wash solution P 2 O 5  concentration is controlled by operator adjustment of the recovery solution  146   b  to crystallizer slurry  114  ratio. Flow of HH wash solution  126  is directed to the feed acid tank assembly  148 . 
         [0051]    Speed of the HH filter is ratio controlled to the flow of crystallizer slurry  114  to the HH primary filter  118 . HH filter cake thickness is controlled by operator adjustment of the filter speed to crystallizer slurry ratio. The flow of recovery solution  126  to the feed acid tank assembly  148  is controlled by adjusting the HH filter cake thickness target. 
       Transformation Tank  128   
       [0052]    Flow of adjusted recovery solution  146   a  to the transformation tank  128  is adjusted by the operator to control transformation tank solids. 
         [0053]    Flow of sulfuric acid  112  to the transformation tank  128  is ratio controlled to the total of the flow of adjusted recovery solution  146   a  and DH wash solution  142  to the transformation tank  128 . Transformation tank SO 4   2−  concentration is controlled by operator adjustment of the sulfuric acid  112  to recovery  146   a  and DH wash  142  ratio. 
       Dihydrate Filter  131   
       [0054]    Flow of transformation slurry  130  to the dehydrate or DH primary filter  132  is cascade controlled by the level in the transformation tank  128 . 
         [0055]    Flow of water  138  to the DH filter  136  is ratio controlled to the flow of transformation slurry  130  to the DH filter  132 . Recovery solution tank assembly  144  level will control the water  138  to transformation slurry  130  ratio. 
         [0056]    Flow of DH wash solution  142  is directed to the transformation tank  128 . 
         [0057]    Speed of the DH filter is ratio controlled to the flow of transformation slurry  130  to the DH filter  132 . DH filter cake thickness is controlled by operator adjustment of the filter speed to transformation slurry ratio. The flow of adjusted recovery solution  146   a  to the transformation tank  128  is controlled by adjusting the DH filter cake thickness target. 
       Product Acid Tank  121   
       [0058]    Flow of product acid  120   a  to the battery limits is cascade controlled by the product acid tank level. 
         [0059]    By reducing the filter variability and the feed acid concentration variability, the HH-DH process  10  produces phosphoric acid having a concentration in a range of about 35% to about 45% P 2 O 5 , and more particularly of about 39% P 2 O 5  or more at &gt;99% P 2 O 5  recovery. 
         [0060]    Various embodiments of systems, devices and methods have been described herein. These embodiments are given only by way of example and are not intended to limit the scope of the invention. It should be appreciated, moreover, that the various features of the embodiments that have been described may be combined in various ways to produce numerous additional embodiments. Moreover, while various materials, dimensions, shapes, configurations and locations, etc. have been described for use with disclosed embodiments, others besides those disclosed may be utilized without exceeding the scope of the invention. 
         [0061]    Persons of ordinary skill in the relevant arts will recognize that the invention may comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features of the invention may be formed or combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the invention may comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art. 
         [0062]    Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein.