US3269926A - Process for the electrolytic production of alkali metal phosphates - Google Patents

Process for the electrolytic production of alkali metal phosphates Download PDF

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US3269926A
US3269926A US231008A US23100862A US3269926A US 3269926 A US3269926 A US 3269926A US 231008 A US231008 A US 231008A US 23100862 A US23100862 A US 23100862A US 3269926 A US3269926 A US 3269926A
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alkali metal
chloride
electrolysis
grams
phosphoric acid
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US231008A
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Cremer Joseph
Kribbe Harri
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Knapsack AG
Knapsack Griesheim AG
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Knapsack AG
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B25/00—Phosphorus; Compounds thereof
    • C01B25/16—Oxyacids of phosphorus; Salts thereof
    • C01B25/26—Phosphates
    • C01B25/30—Alkali metal phosphates
    • C01B25/301—Preparation from liquid orthophosphoric acid or from an acid solution or suspension of orthophosphates
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
    • C25B1/00—Electrolytic production of inorganic compounds or non-metals
    • C25B1/01—Products
    • C25B1/14—Alkali metal compounds

Definitions

  • the present invention is concerned with a process for the electrolytic production of alkali metal phosphates from phosphoric acid and alkali metal chlorides.
  • Alkali metal phosphates are produced in most cases, and most simply, by neutralizing phosphoric acid with alkali metal hydroxide or alkali metal carbonate. Since these two alkali metal compounds are relatively expensive, attempts have been made to replace them with alkali metal chlorides.
  • the reaction between phosphoric acid and the alkali metal chloride is, however, rather diflicult to carry out.
  • the reaction mixture must be heated with evaporation of water and the mixture becoming increasingly more viscous. Finally, a melt of low' water content is obtained, which is very viscous in the temperature range up to about 350 C. and from which the resultant hydrogen chloride escapes only with difficulty.
  • the reaction mixture therefore must either be heated to a still higher temperature or a long reaction time must be tolerated.
  • the monophosphates which are primarily produced are converted into condensed phosphates which then may possibly be obtained in insoluble form.
  • the starting material is an aqueous solultion which contains about 20% by weight H PO and about 12% by weight NaCl or about 15% by weight KCl.
  • the starting material used in the present process is phosphoric acid of at least 50%, preferably 5070%, strength, in which the entire required amount of alkali metal chloride is suspended. This fact ensures a high chloride concentration over a prolonged period of time during the electrolysis, the formation of oxygen at the anode which leads to the destruction of the electrode and contamination of the chlorine developed, being substantially suppressed. Suspensions containing alkali metal chloride in a concentration of 350 to 650 grams per liter have proved advantageous.
  • the solid phase of alkali metal chloride disappears only when the chloride content of the electrolyte is less than ice about 5% by weight.
  • the electrolysis is interrupted when the chloride content has dropped to 2.0 to 2.5% by weight; at that time, about 85 to 88% by weight of the alkali metal chloride introduced has been converted into monoalkali metal orthophosphate.
  • the previously low oxygen content in the chlorine developed increases suddenly to values above 10% by volume. As a result of this, the current yield of course also drops suddenly. From the start of the electrolysis, up to this point, the oxygen content in the chlorine does not rise above 1% by volume, it being most of the time substantially below this value.
  • the electrolysis is carried out preferably at a temperature of 40 to 100 C., and preferably 60 to C., so that plastics, such as the relatively inexpensive polyvinyl chloride can be suitable used as material for the vessel.
  • the temperature is reached by the jouleant heat itself so that no special heating devices are necessary.
  • the electrolysis leads to higher yields of to 88% by weight, the separation of the resultant alkali metal phosphate from the rest of the alkali metal chlorides still present in the solution is simpler to effect; i.e., alkali metal phosphates of lower chloride contents can be obtain, as a result of which the possible uses of the phosphates obtained by the electrolytic process are increased.
  • the mother liquor which remains after the crystallization and contains phosphoric acid and alkali metal chloride in addition to alkali metal phosphate is used again in a new batch.
  • the electrolysis batches are so adjusted that there is obtained a P O /Me 0 molecular ratio (Me alkali metal) of about 1:1. It is not necessary to use an excess of P 0 as is preferably done in the known processes. An excess of P 0 i.e., of phosphoric acid, would reduce the chloride concentration, whereby there would again be the danger of increased oxygen formation.
  • the operation without excess acid offers the further advantage that there is no increased danger of corrosion caused by the electrolysis solution, particularly in the vessels and apparatus serving for the separation of the crystals.
  • Corrosive attack on the apparatus parts by the monoalkali metal phosphate solution which still has an acid reaction can also be avoided by neutralizing the solutions coming from the electrolysis, before separation of the crystals, with alkali metal hydroxides and/or alkali metal carbonates to the stage of dialkali metal phosphate.
  • the further treatment can then take place in vessels of ordinary iron. This feature otters the further advantage that greater yields, due to the lesser solubility of the dialkali metal phosphates, are obtained.
  • the electrolysis can be carried out in the known cells, in which connection the material of which the vessel is made must be resistant to acid and chlorine. If the chlorine and the hydrogen are to be collected separately, a cell with diaphragms or a partition must be employed.
  • electrodes there can be used the graphite electrodes customarily employed in alkali metal chloride electrolysis. The current density should be between about 5 and 50 amperes per square dccimeter.
  • the electrolyzcd solutions can be worked up in accordance with the customary known methods.
  • alkali metal chlorides of lesser purity can also be used, for incorresponding to a current yield of 86%.
  • stance products which contain sulfate ions or alkaline earth metals.
  • Such starting substances cannot be used for instance in the skull metal chloride electrolysis with mercury cathode without costly prior purification, since they lead to contamination of the mercury and thus to diflicultics in the carrying out of the process.
  • the impurities do not at all affect the course of the process, but merely thepurity of the resultant phosphate, since they are deposited also upon the crystallization. If for one specific reason or other, no particularly high demands are madeas to the purity of the product to be produced, such start ng materials can be readily employed.
  • Example I In a polyvinyl chloride vessel a suspension of 750 grams NaCl in 2500 grams of 50.3% H PO was electrolyzed, .while stirring, with a current of 25 amperes (current density about 10 aJdm?) using two graphite electrodes of a size of 175 x 140 mm., until the chloride content of the solution had dropped to 2.3%. The tempernture of the electrolyte was about 70 C. 637 grams or 85 of the NaCl was converted to monosodinrn phosphate. The electrolysis lasted for 14 hours, i.e., 350 ampere hours were consumed, corresponding to a current yield of 83.5%. The voltage was 3.8 volts.
  • Example 2 The apparatus was the same as that used in Example 1. A suspension of 970 grams of KCl in 2500 grams of 51% phosphoric acid was 'electrolyzed with a current of 25 amperes at about 65 C., until a chloride content of 2.0% was reached. 834 grams KCl (86%) were converted to KH PO 350 ampere hours were consumed, The voltage was 3.7 volts. 1370 grams KH3P04 containing 0.2%, chloride crystallized from the solution while about 150 grams KH3P04 remained in the mother liquor.
  • Example 3 In a bell-type cell, which permitted the collection of the chlorine formed, a suspension of 244 grams NaCl in 750 grams H POI, (53.3%) was elcctrolyzed with a current of amperes (current density about 12 amperes per square decimeter) until the chloride content had dropped to 2.5%. 208 grams of NaCl corresponding to 85% were converted. The time of electrolysis was 22 hours, corresponding to 110 ampere hours (current yield 87% The oxygen content of the chlorine was immeasurably small at the start of the electrolysis; after 11 hours, it had risen to 0.5% and at the end of the electrolysis, was 1.0%. 300 grams of NaH PO with a chloride content of 0.4% were crystallized from the solution while 128 grams remained in the mother liquor.
  • Me represents at least one substance selected item the group consisting of sodium and potassium.

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Inorganic Chemistry (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
  • Electrolytic Production Of Metals (AREA)

Description

United States Patent 12 Claims. (cl. zoo-90 The present invention is concerned with a process for the electrolytic production of alkali metal phosphates from phosphoric acid and alkali metal chlorides.
Alkali metal phosphates are produced in most cases, and most simply, by neutralizing phosphoric acid with alkali metal hydroxide or alkali metal carbonate. Since these two alkali metal compounds are relatively expensive, attempts have been made to replace them with alkali metal chlorides.
The reaction between phosphoric acid and the alkali metal chloride is, however, rather diflicult to carry out. The reaction mixture must be heated with evaporation of water and the mixture becoming increasingly more viscous. Finally, a melt of low' water content is obtained, which is very viscous in the temperature range up to about 350 C. and from which the resultant hydrogen chloride escapes only with difficulty. The reaction mixture, therefore must either be heated to a still higher temperature or a long reaction time must be tolerated.
At these high temperatures, it is then particularly difficult to find a suitable material for making the reaction vessel.
Furthermore, the monophosphates which are primarily produced are converted into condensed phosphates which then may possibly be obtained in insoluble form.
It has also been proposed to spray suspensions of finely ground alkali metal chloride in phosphoric acid in a heated tower. Apart from the high investment costs for this process, the separation of the resultant product from the ot'fgas presents considerable difficulties. Still further, it has already been proposed to prepare alkali metal phosphates from phosphoric acid and alkali metal chlorides by electrolysis. In this process, a solution of H PO and alkali metal chloride in water is electrolyzed between an anode of graphite and a cathode of nickel. When 25 to 50% by weight of the theoretically possible amount of alkali metal dihydrogen phosphate has been formed, the electrolysis is interrupted and the resultant product crystallized, the mother liquor being used again for a new batch.
It has now unexpectedly been found that higher yields in the electrolysis can be obtained by using substantially more concentrated solutions as the starting material. In the known process, the starting material is an aqueous solultion which contains about 20% by weight H PO and about 12% by weight NaCl or about 15% by weight KCl.
ln contradistinction thereto, the starting material used in the present process is phosphoric acid of at least 50%, preferably 5070%, strength, in which the entire required amount of alkali metal chloride is suspended. This fact ensures a high chloride concentration over a prolonged period of time during the electrolysis, the formation of oxygen at the anode which leads to the destruction of the electrode and contamination of the chlorine developed, being substantially suppressed. Suspensions containing alkali metal chloride in a concentration of 350 to 650 grams per liter have proved advantageous.
The solid phase of alkali metal chloride disappears only when the chloride content of the electrolyte is less than ice about 5% by weight. The electrolysis is interrupted when the chloride content has dropped to 2.0 to 2.5% by weight; at that time, about 85 to 88% by weight of the alkali metal chloride introduced has been converted into monoalkali metal orthophosphate. Below this chloride concentration, the previously low oxygen content in the chlorine developed increases suddenly to values above 10% by volume. As a result of this, the current yield of course also drops suddenly. From the start of the electrolysis, up to this point, the oxygen content in the chlorine does not rise above 1% by volume, it being most of the time substantially below this value.
The electrolysis is carried out preferably at a temperature of 40 to 100 C., and preferably 60 to C., so that plastics, such as the relatively inexpensive polyvinyl chloride can be suitable used as material for the vessel. The temperature is reached by the jouleant heat itself so that no special heating devices are necessary.
In order to recover the alkali metal phosphate formed, it is merely necessary to cool the solution with the resultant formation of crystallized salt; evaporation is out necessary, in contradistinction to the known method, where considerably more dilute solutions are used. The energy saved for evaporation represents another great advantage of the process of the present invention.
Due to the fact that in the present invention the electrolysis leads to higher yields of to 88% by weight, the separation of the resultant alkali metal phosphate from the rest of the alkali metal chlorides still present in the solution is simpler to effect; i.e., alkali metal phosphates of lower chloride contents can be obtain, as a result of which the possible uses of the phosphates obtained by the electrolytic process are increased.
The mother liquor which remains after the crystallization and contains phosphoric acid and alkali metal chloride in addition to alkali metal phosphate is used again in a new batch. The electrolysis batches are so adjusted that there is obtained a P O /Me 0 molecular ratio (Me alkali metal) of about 1:1. It is not necessary to use an excess of P 0 as is preferably done in the known processes. An excess of P 0 i.e., of phosphoric acid, would reduce the chloride concentration, whereby there would again be the danger of increased oxygen formation. The operation without excess acid offers the further advantage that there is no increased danger of corrosion caused by the electrolysis solution, particularly in the vessels and apparatus serving for the separation of the crystals.
Corrosive attack on the apparatus parts by the monoalkali metal phosphate solution which still has an acid reaction can also be avoided by neutralizing the solutions coming from the electrolysis, before separation of the crystals, with alkali metal hydroxides and/or alkali metal carbonates to the stage of dialkali metal phosphate. The further treatment can then take place in vessels of ordinary iron. This feature otters the further advantage that greater yields, due to the lesser solubility of the dialkali metal phosphates, are obtained.
The electrolysis can be carried out in the known cells, in which connection the material of which the vessel is made must be resistant to acid and chlorine. If the chlorine and the hydrogen are to be collected separately, a cell with diaphragms or a partition must be employed. As electrodes, there can be used the graphite electrodes customarily employed in alkali metal chloride electrolysis. The current density should be between about 5 and 50 amperes per square dccimeter. The electrolyzcd solutions can be worked up in accordance with the customary known methods.
Another advantage of the present method is that alkali metal chlorides of lesser purity can also be used, for incorresponding to a current yield of 86%.
stance products which contain sulfate ions or alkaline earth metals. Such starting substances cannot be used for instance in the skull metal chloride electrolysis with mercury cathode without costly prior purification, since they lead to contamination of the mercury and thus to diflicultics in the carrying out of the process. In this electrolysis carried out in accordance with the process of the present invention, the impurities do not at all affect the course of the process, but merely thepurity of the resultant phosphate, since they are deposited also upon the crystallization. If for one specific reason or other, no particularly high demands are madeas to the purity of the product to be produced, such start ng materials can be readily employed.
The 'following examples illustrate the invention:
Example I In a polyvinyl chloride vessel a suspension of 750 grams NaCl in 2500 grams of 50.3% H PO was electrolyzed, .while stirring, with a current of 25 amperes (current density about 10 aJdm?) using two graphite electrodes of a size of 175 x 140 mm., until the chloride content of the solution had dropped to 2.3%. The tempernture of the electrolyte was about 70 C. 637 grams or 85 of the NaCl was converted to monosodinrn phosphate. The electrolysis lasted for 14 hours, i.e., 350 ampere hours were consumed, corresponding to a current yield of 83.5%. The voltage was 3.8 volts. Half of the solution obtained was crystallized by cooling, 425 grams of NaH PO having a chloride content of 1.1% being obtained. The other half was adjusted to a pH of 9 with sodium hydroxide solution. Upon cooling it, 739 grams of Na HPO containing 1.3% chloride were obtained.
Example 2 The apparatus was the same as that used in Example 1. A suspension of 970 grams of KCl in 2500 grams of 51% phosphoric acid was 'electrolyzed with a current of 25 amperes at about 65 C., until a chloride content of 2.0% was reached. 834 grams KCl (86%) were converted to KH PO 350 ampere hours were consumed, The voltage was 3.7 volts. 1370 grams KH3P04 containing 0.2%, chloride crystallized from the solution while about 150 grams KH3P04 remained in the mother liquor.
Example 3 In a bell-type cell, which permitted the collection of the chlorine formed, a suspension of 244 grams NaCl in 750 grams H POI, (53.3%) was elcctrolyzed with a current of amperes (current density about 12 amperes per square decimeter) until the chloride content had dropped to 2.5%. 208 grams of NaCl corresponding to 85% were converted. The time of electrolysis was 22 hours, corresponding to 110 ampere hours (current yield 87% The oxygen content of the chlorine was immeasurably small at the start of the electrolysis; after 11 hours, it had risen to 0.5% and at the end of the electrolysis, was 1.0%. 300 grams of NaH PO with a chloride content of 0.4% were crystallized from the solution while 128 grams remained in the mother liquor.
We claim:
1. In a process for the electrolytic production of alkali metal phosphates from phosphoric acid and alkali metal chlorides, the improvement which comprises electrolyzing a suspension of an alkali metal chloride in aqueous phosphoric acid having an initia' concentration of at least 50% by weight, crystallizing the resulting phosphate, and separating mother liquid and phosphate from .one another.
2. The process of claim 1, wherein the phosphoric acid has an initial concentration within the range of 50 to by weight.
3. The process of claim 1, wherein the initial concentration of alkali metal chloride in the electrolyte is within the range of 300 to 650 grams per liter of electrolyte. I
4. The process of claim 1, wherein the electrolysis is interrupted when the chloride content of the electrolyte has dropped to a value within the range of 2.0 to 2.5% by weight.
5. The process of claim 1, wherein the electrulysis is carried out at temperatures with the range of 40 to 100 C.
6. The process of claim 1, wherein the electrolysis is carried out at temperatures within the range ot 6 0 to C.
7. The process of claim 1, wherein the ratio of P 0 :Me 0, in which Me is an alkali metal, is approximately 1:1 in the electrolyte.
8. The process of claim 7, wherein Me represents at least one substance selected item the group consisting of sodium and potassium.
9. The process of claim 1, wherein current having a density within the range of about Ste 50 amperes per square decimeter is used for carrying out the electrolysis.
10. The process of claim 1, wherein the solution obtained after completion of the reaction is neutralized with at least one substance selected from the group consisting of alkali metal hydroxides and alkali metalcarbonates, the solution being neutralized to the stage of dialkali metal phosphates. Y
11. The process of claim 1, wherein the mother liquor obtained on processing the phosphates is usedagain for anew batch. I
. 12. The process of claim 11, wherein the phosphates are processed by crystallization by cooling.
References Cited by the Examiner UNITED STATES PATENTS 572,512 12/1s96 Albert 204- FOREIGN PATENTS 854,145 1 l/ 1960 Great Britain.
IOHN H. MACK, Primary Examiner. MURRAY TILLMAN, Examiner. L.G. WISE, Assistant Examiner.

Claims (1)

1. IN A PROCESS FOR THE ELECTROLYTIC PRODUCTION OF ALKALI METAL PHOPHATES FROM PHOSPHORIC ACID AND ALKALI METAL CHLORIDES, THE IMPROVEMENT WHICH COMPRISES ELECTROLYZING SUSPENSION OF AN ALKALI METAL CHLORIDE IN AQUEOUS PHOSPHORIC ACID HAVING AN INITIAL CONCENTRATION OF AT LEAST 50% BY WEIGHT, CRYSTALLIZING THE RESULTING PHOSPHATE, AND SEPARATING MOTHER LIQUID AND PHOSPHATE FROM ONE ANOTHER.
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Publication number Priority date Publication date Assignee Title
US572512A (en) * 1896-12-08 Phosphates of alkalies
GB854145A (en) * 1958-05-13 1960-11-16 United States Borax Chem Electrolytic production of alkali metal phosphates

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US572512A (en) * 1896-12-08 Phosphates of alkalies
GB854145A (en) * 1958-05-13 1960-11-16 United States Borax Chem Electrolytic production of alkali metal phosphates

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