Classifications

• • 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
  • C01B25/18—Phosphoric acid
  • C01B25/22—Preparation by reacting phosphate-containing material with an acid, e.g. wet process
  • C01B25/222—Preparation by reacting phosphate-containing material with an acid, e.g. wet process with sulfuric acid, a mixture of acids mainly consisting of sulfuric acid or a mixture of compounds forming it in situ, e.g. a mixture of sulfur dioxide, water and oxygen
  • C01B25/228—Preparation by reacting phosphate-containing material with an acid, e.g. wet process with sulfuric acid, a mixture of acids mainly consisting of sulfuric acid or a mixture of compounds forming it in situ, e.g. a mixture of sulfur dioxide, water and oxygen one form of calcium sulfate being formed and then converted to another form
  • C01B25/231—Dihydrate-hemihydrate process
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/24—Stationary reactors without moving elements inside
• • 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
  • C01B25/18—Phosphoric acid
  • C01B25/22—Preparation by reacting phosphate-containing material with an acid, e.g. wet process
  • C01B25/222—Preparation by reacting phosphate-containing material with an acid, e.g. wet process with sulfuric acid, a mixture of acids mainly consisting of sulfuric acid or a mixture of compounds forming it in situ, e.g. a mixture of sulfur dioxide, water and oxygen
  • C01B25/228—Preparation by reacting phosphate-containing material with an acid, e.g. wet process with sulfuric acid, a mixture of acids mainly consisting of sulfuric acid or a mixture of compounds forming it in situ, e.g. a mixture of sulfur dioxide, water and oxygen one form of calcium sulfate being formed and then converted to another form

Definitions

• the present invention relates to a process for producing phosphoric acid by phosphate rock attack using sulfuric acid.
• the conventional method of this type is to react the phosphate rock with sulfuric acid under conditions giving rise to crystallization of calcium sulfate dihydrate or gypsum (CaSO 4 .2H 2 O).
• the gypsum slurry obtained in a first reactor can then be subjected, in a second reactor, to a maturation allowing magnification of the sulphate grains formed, and this to increase the filterability.
• the slurry is then filtered to obtain a phosphoric acid having a content of free P 2 O 5 of the order of 25 to 35% by weight.
• the gypsum obtained still contains quite well P2O 5 , either untouched or co-crystallized, that is to say fixed in the crystal lattice of the gypsum. This limits the extraction yield of P2O 5 contained in the phosphate and renders the gypsum unsuitable for certain applications.
• the object of the present invention is to develop a process for the production of phosphoric acid by phosphate rock attack using sulfuric acid which improves the quality of the phosphoric acid production and the extraction yield of P2O 5 to from the rock.
• This method must also be able to easily be applied in an existing conventional installation and thus not require economically costly and indefensible transformations.
• the process according to the invention gives rise during the etching step under the specified conditions to a slurry of dihydrate crystals, while a tradesman was to expect the formation of hemihydrate.
• Crystallization dihydrate is not optimal, the crystals are small, they have a grain size having a d 5 o less than 20 ⁇ .
• Such a slurry would have a very low filtration coefficient and therefore could not be exploited industrially in a process requiring filtration. This aspect is however of no importance according to the invention since this first slurry is not intended to be filtered.
• the attack conditions are such that they provide a substantially stoichiometric reaction between the sulfuric acid introduced and the calcium contained in the phosphate rock, mainly in the form of carbonate and calcium phosphate.
• the acidic aqueous phase of this first slurry resulting from the attack contains no or extremely little free sulfuric acid and its content of free P 2 O 5 is quite high.
• the etching temperature may be between 70 and 80 ° C.
• the concentration of free P 2 O 5 resulting from the etching step may be 40 to 45% by weight.
• the free SO3 concentration of the acidic aqueous phase of the first slurry can be from 0.1 to 0.4% by weight.
• This first slurry is then directly subjected, in its entirety, to a conversion step which consists of heating it to a temperature above 90 ° C, preferably between 90 and 105 ° C.
• This heating causes, in a known manner, a solubilization of the gypsum crystals, a release of the P2O 5 co-crystallized in the gypsum during the etching step and a recrystallization of the calcium sulphate in the form of hemihydrate.
• a second slurry is thus obtained in a simple and easy manner in which the hemihydrate crystals have a spherical shape and are of standard size, for example having a d 5 o of 60 ⁇ , which gives a filter cake presenting an excellent coefficient of filtration.
• the production acid has an extremely low free SO 3 content advantageously of the order of 0.05% to less than 1% by weight, making it a quality phosphoric acid. It is advantageous to obtain a production phosphoric acid having a free P 2 O 5 content of 35 to 45% by weight.
• the step of heating the first slurry does not include any addition of sulfuric acid.
• the process comprises, during the step of converting the first slurry, an introduction into it of sulfuric acid.
• the quantity of sulfuric acid added must, however, be accurately metered so that, after the filtration of the second slurry, the phosphoric acid production contains a content of free SO 3 of less than 2% by weight, preferably of the order from 0.05 to 1.5% by weight, in particular of the order of 0.05 to less than 1% by weight. It is important not to exceed this content in order not to pollute the phosphoric acid with sulfuric acid which would make it unsuitable for use as phosphoric acid and would require a desulfation step.
• the process comprises the step of etching in a first reactor, and a transfer of the first slurry from the first reactor to a second reactor, wherein the conversion step with forming the second slurry, the separation step being performed on a filter.
• This method has the advantage of being able to be used in an existing plant for the production of phosphoric acid by conventional attack with sulfuric acid.
• the first reactor is the conventional plant reactor, in which different driving conditions are applied.
• the second reactor is the maturation reactor of the conventional plant. Since a magnification of the gypsum grains resulting from the etching is not necessary according to the invention, this ripening reactor can be used as a conversion reactor.
• the filter of the conventional installation can be used to filter hemihydrate instead of gypsum.
• This filter may be any known, suitable filtration device, for example a band filter, a filter cell device arranged in a carousel, among others.
• the residence time in the first reactor is from 2 to 4 hours and the residence time in the second reactor is from 0.5 to 1.5 hours.
• the residence times correspond to the residence times in the attack and maturation reactors of a conventional installation.
• FIG. 1 represents in the form of a flow diagram an example of an installation implementing a method according to the invention.
• Figure 2 is a graph illustrating formation regions of dihydrate and hemihydrate as a function of temperature, P 2 O 5 concentration and SO 3 concentration.
• Figure 3 is a graph illustrating the different processes applied for the production of phosphoric acid by sulfuric attack.
• the installation illustrated in FIG. 1 comprises an etching reactor 1 in which ground phosphated rock is introduced at 2 and sulfuric acid at 3, for example 98-99% by weight concentrated sulfuric acid. .
• An aqueous solution of phosphoric acid resulting from the process may advantageously be recycled to 4 in reactor 1.
• FIG. 2 The graph shown is taken from AV Slack, Phosphoric Acid, Vol. 1, Part I, ed. Marcel Dekker Inc., 1968, New York.
• the temperature is indicated in ° C on the ordinate, the P 2 O 5 and SO 3 concentrations on the abscissa.
• the curves represent the equilibrium between dihydrate and hemihydrate.
• the region of the graph below these curves corresponds to the dihydrate formation conditions, that located above these curves to that of hemihydrate formation.
• the shaded ellipse represents the area that meets the preferential conditions of the attack according to the present invention.
• a stable slurry of crystals of dihydrate, and not of hemihydrate is obtained.
• the ellipse marked with an asterisk meets the attack conditions according to the invention and the ellipse marked by two asterisks, connected to the first by an arrow, meets the conditions of the conversion according to the invention.
• the conversion reactor 7 it is possible, to further improve the conversion of the gypsum to hemihydrate, to add to the conversion reactor 7 a small quantity of sulfuric acid.
• This amount must, however, be such that the content of free SO3 in the production acid is less than 2% by weight, preferably 1%.
• the production acid can not be polluted by this addition of sulfuric acid.
• the process illustrated comprises a transfer of the slurry from the reactor 7 via the conduit 9 to a filter 10 of the current type.
• a filtrate 1 1 which is phosphoric acid production and a filter cake.
• the illustrated process comprises two cake washing steps.
• the second washing is carried out using a washing liquid which is preferably water, supplied with 12.
• the product of this wash obtained at 13 is a low phosphoric acid aqueous solution which serves as a washing liquid fed at 14 in the first washing step.
• the product of this first wash, obtained at 15, is an aqueous solution of phosphoric acid which can be recycled at 4 to the attack reactor 1, via a recycling line 16.
• the process according to the invention offers the advantage of an improved extraction yield, being the rechstallization, and the production of a high P2O 5 acid, with a content of more than 35% by weight, instead of the 25 to 30% obtained by the conventional method.
• Dry hemihydrate cake is highly valuable in the industry. Its stocking properties are excellent because in the presence of water or moisture the hemihydrate is converted back into gypsum.
• the particle size distribution of the crushed rock was close to that generally used in phosphoric acid plants (1, 1% increasing to 500 m, 48.8% to 150 m and 70% to 75 ⁇ ).
• Phosphate is introduced into the 30 liter feed reactor by a feed screw and the flow rate (3.2 kg / h) is controlled by a weight loss system.
• Two dosing pumps inject sulfuric acid (2.5 kg / h) and recycled acid (9.2 kg / h) from the washing of the filter cake.
• the temperature is kept constant at the required value by a heating system.
• the output rate of the attacking pulp is about 9 l / h.
• the gypsum slurry obtained during this first step is then heated to 95 ° C in a second reactor.
• the phosphoric acid produced contains 35.3% by weight of P2O 5 and 1.3% by weight of free SO3.
• the filter hemihydrate cake obtained contains a total P 2 O 5 content of 0.53% by weight which corresponds to a total extraction yield of P2O 5 (etching and filtration) of 97.9% by weight.
• the free water content of the hemihydrate cake is 22.5% and the water of crystallization is 6.3% which confirms that it is hemihydrate. After rehydration, the free water content drops to about 9.5%.
• a slurry formed of calcium sulphate and phosphoric acid containing 50% by weight of P2O 5 and 0.15% of SO3 is obtained by etching a North African phosphate (30% P2O 5 , 50.1% by weight). CaO, 3.6% F). It is stable as a dihydrate when the temperature is 72 ° C.
• the phosphoric acid obtained contains 48.2% of P2O 5 and 0.14% of SO 3 .
• a slurry of calcium sulphate and phosphoric acid containing, by weight, 41.6% P 2 O 5 0.4% SO 3 ; 0.24% Al2O3; 0.99% F is obtained by etching a North African phosphate.

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• Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Inorganic Chemistry (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
• Solid-Sorbent Or Filter-Aiding Compositions (AREA)
• Preparation Of Compounds By Using Micro-Organisms (AREA)

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