Method for etching a phosphate source using acid

Abstract

Process of acid attack with sulphuric acid of a phosphate source comprising calcium or not comprising calcium for a predetermined time period ranging from 20 to 180 minutes in the conditions wherein the molar ratio of sulphate from the sulphuric acid and possibly from the phosphate source to the calcium present in the phosphate source ranges from 0.6 to 0.8, and the content in P.sub.2O.sub.5 in the attack tank is of less than 6%.

Claims

1. A process of acid attack of a phosphate source including calcium for the production of a phosphate-based purified compound comprising the steps of: a) an acid attack using sulphuric acid of said phosphate source, during a predetermined time period ranging from 20 to 180 minutes with a formation of a first suspension containing a first solid matter and a first liquid phase in which the first solid matter is in suspension, said first solid matter comprising at least calcium phosphate and impurities, said first liquid phase comprising phosphoric acid and dissolved monocalcium phosphate, said attack being conducted in input conditions wherein the molar ratio of the sulphate from the sulphuric acid and from the phosphate source to the calcium present in the phosphate source ranges from 0.6 to 0.8 and the P.sub.2O.sub.5 content is of less than 6% by weight, b) a first filtration of said first slurry with a separation of said first solid matter from said first liquid phase, and c) a retrieval from said first liquid phase of a purified phosphate-based compound.

2. The process according to claim 1, wherein said acid attack takes place in 1, 2 or more attack tanks.

3. The process according to claim 1, wherein the predetermined time period is from 20 to 120 minutes.

4. The process according to claims 2, wherein the P.sub.2O.sub.5 content in the first liquid phase in the attack tank or tanks is less than 5% by weight.

5. The process according to claim 2, wherein said attack is performed at a temperature in the attack tank or tanks of 90° C. or less.

6. The process according to claim 2, wherein the sulphuric acid is a diluted sulphuric acid, before being added in the attack tank or tanks.

7. The process according to claim 6, wherein said diluted sulphuric acid features a H.sub.2SO.sub.4 concentration of 13% or less by weight.

8. The process according to claim 1, wherein the molar ratio of sulphate from the sulphuric acid and from the phosphate source to the calcium present in the phosphate source ranges from 0.68 and 0.78.

9. The process according to claim 1, further comprising an addition of a base to said first suspension, prior to filtration.

10. The process according to claim 1, further comprising, before said step whereby said phosphate-based purified compound is retrieved from said first liquid phase, an addition of a base to said first liquid phase after filtration with the formation of a second suspension comprising a second solid matter in suspension in a second liquid phase and a filtration of said second suspension to separate said second solid matter in suspension from said second liquid phase, said second phosphate-based purified compound thereby being retrieved from said second liquid phase from the first liquid phase with a low content of said second solid matter.

11. The process according to claim 1, wherein said phosphate source containing calcium is chosen from: traditional phosphate rocks, phosphate rocks with a low P.sub.2O.sub.5 content, ashes, sludge from wastewater processing plants, bone ash, pig manure, chicken manure, ash from the sludge of wastewater processing plants, sludge from wastewater processing plants, and of or a raw material having a phosphate content of less than 30% by weight of P.sub.2O.sub.5 with respect to a total weight of the raw material.

12. The process according to claim 1, wherein said purified phosphate-based compound is a monocalcium phosphate MCP, a dicalcium phosphate DCP or a phosphoric acid.

13. The process according to claim 10, wherein said second liquid phase is recycled by introduction in said attack tank or tanks.

Description

EXAMPLES

Example 1

Attack of a Phosphate Source at the Laboratory Scale

(1) 100 g of a phosphate source (phosphate rock) containing 30.5 g of P.sub.2O.sub.5, 49.5% of CaO equivalent, 3.95% of fluorine, 0.308% of Fe.sub.2O.sub.3 equivalent, 0.547% of Al.sub.2O.sub.3 equivalent and 0.303% of MgO, equivalent, by weight with respect to the weight of the phosphate source are brought into contact with sulphuric acid diluted to a concentration of 2% during a 30-minute attack period, at an attack temperature of 60° C. and with a SO.sub.4/Ca molar ratio of 0.8. The SO.sub.4/Ca molar ratio defines the quantity of acid required for the attack of the phosphate source containing the Ca at the inlet of the attack tank.

(2) Once the additives have been added, the composition is left to stir for half-an-hour before filtration.

(3) The suspension is then filtered in a vacuum on a Büchner funnel. The different quantities thus obtained are recorded and the calcium phosphate and liquid phase products are analysed.

(4) In the laboratory protocol, this is a batch process, without cleaning. However, the cleaning was extrapolated and the quantity of P.sub.2O.sub.5 in the impregnation liquid of the filtration cake was calculated.

(5) The attack yield is calculated with the following calculation: (Mass of P.sub.2O.sub.5 in the filtrate+mass of P.sub.2O.sub.5 in the impregnation liquid of the filtration cake)/(total mass of P.sub.2O.sub.5 in the phosphate source). The P.sub.2O.sub.5 content in the impregnation liquid corresponds with the P.sub.2O.sub.5 that is recoverable by cleaning of the cake in an industrial process.

(6) The quantity of added diluted sulphuric acid is of 3499 g for a SO.sub.4 content of 70.2 g. The SO.sub.4/Ca ratio if of 0.8 due to the calcium content of the phosphate source.

(7) The retrieved liquid phase has a volume of 3.09 litres for a weight of 3125 g, and a pH of 2.1. The P.sub.2O.sub.5 content in the liquid phase is of 0.83% and the SO.sub.3 content is of 0.16% by weight with respect to the total weight of the liquid phase. The mass of P.sub.2O.sub.5 in the impregnation liquid is of 1.2 g.

(8) The CaO/P.sub.2O.sub.5 molar ratio in the first solution is of 0.58, whereas the residual CaO content in the liquid phase is of 0.19% by weight with respect to the weight of the liquid phase.

(9) The yield of the attack in P.sub.2O.sub.5 is of 89%. As can be seen, despite the use of sulphuric acid at a low concentration of 2% and sub-stoichiometric attack conditions with a total attack period of just 30 minutes, the attack yield in P.sub.2O.sub.5 is significantly high.

Example 2

Attack of a Phosphate Source at the Laboratory Scale

(10) 150 g of a phosphate source (rock) containing 15.8 g of P.sub.2O.sub.5, 27.6% of CaO equivalent, 2.2% of fluorine, 2.37% of Fe.sub.2O.sub.3 equivalent, 2.88% of Al.sub.2O.sub.3 equivalent and 0.416% of MgO, equivalent, by weight with respect to the weight of the phosphate source are brought into contact with sulphuric acid diluted to a concentration of 5% during a 30-minute attack period, at an attack temperature of 40° C. and with a SO.sub.4/Ca molar ratio of 0.8 according to the protocol of example 1:

(11) The quantity of added diluted sulphuric acid is of 1131 g for a SO.sub.4 content of 61.0 g. The SO.sub.4/Ca ratio if of 0.8 due to the calcium content of the phosphate source.

(12) The retrieved liquid phase has a volume of 0.955 litres for a weight of 976 g, and a pH of 1.8. The P.sub.2O.sub.5 content in the liquid phase is of 1.97% and the SO.sub.3 content is of 0.27% by weight with respect to the total weight of the liquid phase. The mass of P.sub.2O.sub.5 in the impregnation liquid is of 3.24 g.

(13) The CaO/P.sub.2O.sub.5 molar ratio in the first solution is of 0.43, whereas the residual CaO content in the liquid phase is of 0.33% by weight with respect to the weight of the liquid phase.

(14) The yield of the attack in P.sub.2O.sub.5 is of 95%. As can be seen, despite the use of sulphuric acid at a low concentration of 5% and a phosphate source with a very low phosphate content, in sub-stoichiometric attack conditions with a total attack period of just 30 minutes, the attack yield in P.sub.2O.sub.5 is significantly high.

Example 3

Attack of a Phosphate Source at the Laboratory Scale

(15) 100 g of a phosphate source (phosphate rock) containing 30.5 g of P.sub.2O.sub.5, 49.5% of CaO equivalent, 3.95% of fluorine, 0.308% of Fe.sub.2O.sub.3 equivalent, 0.547% of Al.sub.2O.sub.3 equivalent and 0.303% of MgO, equivalent, by weight with respect to the weight of the phosphate source are brought into contact with sulphuric acid diluted to a concentration of 5% during a 30-minute attack period, at an attack temperature of 60° C. and with a SO.sub.4/Ca molar ratio of 0.8 according to the protocol of example 1:

(16) The quantity of added diluted sulphuric acid is of 1398 g for a SO.sub.4 content of 70.1 g. The SO.sub.4/Ca ratio if of 0.8 due to the calcium content of the phosphate source.

(17) The retrieved liquid phase has a volume of 1.13 litres for a weight of 1161 g, and a pH of 2.2. The P.sub.2O.sub.5 content in the liquid phase is of 2% and the SO.sub.3 content is of 0.20% by weight with respect to the total weight of the liquid phase. The mass of P.sub.2O.sub.5 in the impregnation liquid is of 2.6 g.

(18) The CaO/P.sub.2O.sub.5 molar ratio in the first solution is of 0.38, whereas the residual CaO content in the liquid phase is of 0.30% by weight with respect to the weight of the liquid phase.

(19) The yield of the attack in P.sub.2O.sub.5 is of 85%. As can be seen, despite the use of sulphuric acid at a low concentration of 5% and sub-stoichiometric attack conditions with a total attack period of just 30 minutes, the attack yield in P.sub.2O.sub.5 is significantly high.

Comparative Example 1

Attack of a Phosphate Source at the Laboratory Scale

(20) 100 g of a phosphate source (phosphate rock) containing 30.5 g of P.sub.2O.sub.5, 49.5% of CaO equivalent, 3.95% of fluorine, 0.308% of Fe.sub.2O.sub.3 equivalent, 0.547% of Al.sub.2O.sub.3 equivalent and 0.303% of MgO, equivalent, by weight with respect to the weight of the phosphate source are brought into contact with sulphuric acid diluted to a concentration of 5% during a 30-minute attack period, at an attack temperature of 60° C., but in this case with a SO.sub.4/Ca molar ratio of 1 according to the protocol of example 1:

(21) The quantity of added diluted sulphuric acid is of 1747 g for a SO.sub.4 content of 87.2 g. The SO.sub.4/Ca ratio if of 1 due to the calcium content of the phosphate source.

(22) The retrieved liquid phase features a volume of 1.4 litres for a weight of 1429 g, and a pH of 2.1. The P.sub.2O.sub.5 content in the liquid phase is of 1.63% and the SO.sub.3 content is of 0.61% by weight with respect to the total weight of the liquid phase. The mass of P.sub.2O.sub.5 in the impregnation liquid is of 3.5 g.

(23) The CaO/P.sub.2O.sub.5 molar ratio in the first solution is of 0.26, whereas the residual CaO content in the liquid phase is of 0.17% by weight with respect to the weight of the liquid phase. The yield of the attack in P.sub.2O.sub.5 is of 88%. As can be seen in the comparative example under stoichiometric conditions, the specific consumption of sulphuric acid is greater for similar levels of attack yield. The consumption of the calcium source required for neutralisation is also more significant.

Example 4

Sub-Stoichiometric Attack of a Phosphate Rock at the Scale of the Pilot Project

(24) The pilot project comprises 3 stirred tanks that are thermostated with oil-heated double shells. The tanks are successive overflow tanks; the first two tanks have a 20-litre capacity and the third tank features a 30-litre capacity and serves only as a buffer before filtration.

(25) 10 litres of water are poured into the first tank and are heated to working temperature. The phosphate source and the diluted sulphuric acid are added in the first reactor at flows that correspond with the required attack conditions (SO.sub.4/CaO ratio, attack duration H.sub.2SO.sub.4 concentration for the attack of the phosphate source, P.sub.2O.sub.5 content in the attack tank).

(26) The produced suspension overflows into the second reactor. The second reactor is configured to perform a neutralisation prior to filtration. Neutralisation prior to filtration is not systematically performed.

(27) The suspension finally overflows into the third reactor that supplies the filtration unit.

(28) The suspension quantity is filtered every 30 minutes. Two types of filtration are alternately performed: Filtration for recycling purposes in the attack reactor: the filtration cake is not cleaned and it is recycled in the first (attack) reactor to increase the proportion of solids in the reaction medium. The liquid phase (filtrate) is poured into a vat and stored for the neutralisation and DCP production step. This filtration step is certainly not required at the industrial scale. Naturally, it can be performed, but it is not necessary. At the pilot project scale, this step can advantageously be performed as it is preferable to increase the solid matter content in the attack reactor. Filtration for the production of calcium sulphate: in this case, the calcium sulphate cake is cleaned with a predetermined quantity of water to retrieve the P.sub.2O.sub.5 contained in the impregnation liquid. The liquid phase and the cleaning filtrate are poured into the filtrate retrieval vat. The calcium sulphate is unloaded for its evacuation.

(29) The installation is in a stable state, the calcium sulphate and liquid phase (filtrates) samples are collected for analyses and the different products are also analysed.

(30) The yield is calculated as follows: mass of P.sub.2O.sub.5 in the liquid phase (g/h)/mass of P.sub.2O.sub.5 in the phosphate source (g/h).

(31) A phosphate source in the form of a rock containing 30.3% by weight of P.sub.2O.sub.5, 47.6% of CaO equivalent, 3,68% of fluorine, 0.144% of Fe.sub.2O.sub.3 equivalent, 0.18% of Al.sub.2O.sub.3 equivalent and 0.542% of MgO equivalent, by weight with respect to the weight of the phosphate source is added to the attack tank in the presence of sulphuric acid diluted to 10% by weight with respect to the weight of the diluted acid, with a SO.sub.4/Ca molar ratio of 0.8. The attack temperature is of 60° C. and the attack duration is of approximately 1 hour. The pH in the attack tank is of 2.04. The flow of the rock is of 2.67 kg/h, and the acid flow is of 17.5 litres/h. The P.sub.2O.sub.5 content in the attack suspension is of 4.5% by weight with respect to the total weight of the suspension.

(32) During filtration, the flow of the retrieved liquid phase is of 16.13 kg/h.

(33) The attack yield is of 93%.

(34) As can be seen, the laboratory tests are confirmed by the pilot project, and the attack yield of the phosphate rock in the presence of diluted sulphuric acid and in conditions of low P.sub.2O.sub.5 content in the attack tank (<6%) and of a short attack period is particularly high when sub-stoichiometric conditions are implemented.

Example 5

Sub-Stoichiometric Attack of a Phosphate Rock at the Scale of the Pilot Project

(35) The pilot used is the same as in example 4.—and the same process as in example 4.—is implemented.

(36) The same phosphate source as in example 4.—is added to the attack tank in the presence of sulphuric acid diluted to 5% by weight with respect to the weight of the diluted acid, with a SO.sub.4/Ca molar ratio of 0.7 owing to the calcium content of the phosphate source. The attack temperature is of 60° C. and the attack duration is of approximately 1 hour. The pH in the attack tank is of 2.5. The flow of the rock is of 3 kg/h, and the acid flow is of 35.6 litres/h. The P.sub.2O.sub.5 content in the attack suspension is of 2.32% by weight with respect to the total weight of the suspension.

(37) During filtration, the flow of the retrieved liquid phase is of 35.44 kg/h.

(38) The attack yield is of 94%.

(39) As can be seen, with respect to the example 4.—, despite the presence of sulphuric acid twice as diluted, the P.sub.2O.sub.5 yield is even higher.

Example 6

Sub-Stoichiometric Attack of a Phosphate Rock at the Scale of the Pilot Project

(40) The same pilot as in example 4.—is used, the same process as in example 4.—is implemented, with the exception that in the second reactor, i.e. the neutralisation reactor before filtration, the pH has been adjusted to 2.48 by adding Ca(OH).sub.2 lime milk.

(41) A phosphate source in the form of a rock containing 34.9% by weight of P.sub.2O.sub.5, 49.8% of CaO equivalent, 3,78% of fluorine, 0.136% of Fe.sub.2O.sub.3 equivalent, 0.386% of Al.sub.2O.sub.3 equivalent and 0.156% of MgO equivalent, by weight with respect to the weight of the phosphate source is added to the attack tank in the presence of sulphuric acid diluted to 5% by weight with respect to the weight of the diluted acid, with a SO.sub.4/Ca ratio of 0.8, due to the calcium content of the phosphate source. The attack temperature is of 60° C. and the attack duration is of approximately 1 hour. The pH in the attack tank is of 2. The flow of the rock is of 2.6 kg/h, and the acid flow is of 35.7 litres/h. The P.sub.2O.sub.5 content in the attack suspension is of 2.10% by weight with respect to the total weight of the suspension.

(42) During filtration, the flow of the retrieved liquid phase is of 38.22 kg/h.

(43) The attack yield is of 92%.

Example 7

Sub-Stoichiometric Attack of a Phosphate Rock at the Scale of the Pilot Project

(44) The pilot used is the same as in example 4.—and the same process as in example 4.—is implemented.

(45) A phosphate source in the form of a rock containing 24.90% by weight of P.sub.2O.sub.5, 40.5% of CaO equivalent, 2,54% of fluorine, 3.97% of Fe.sub.2O.sub.3 equivalent, 1.13% of Al.sub.2O.sub.3 equivalent and 1.88% of MgO equivalent, by weight with respect to the weight of the phosphate source is added to the attack tank in the presence of sulphuric acid diluted to 5% by weight with respect to the weight of the diluted acid, with a SO.sub.4/Ca ratio of 0.8, due to the calcium content of the phosphate source. The attack temperature is of 60° C. and the attack duration is of approximately 1 hour. The pH in the attack tank is of 1.95. The flow of the rock is of 3.19 kg/h, and the acid flow is of 34.5 litres/h. The P.sub.2O.sub.5 content in the attack suspension is of 1.82% by weight with respect to the total weight of the suspension.

(46) During filtration, the flow of the retrieved liquid phase is of 37.91 kg/h.

(47) The attack yield is of 90%.

Example 8

Production of DCP from Phosphate Rocks at the Scale of the Pilot Project

(48) For the production of DCP, the pilot implemented to perform the attack of the rock is used in an uncoupled manner in the first attack. Items of equipment are therefore used sequentially.

(49) The pilot used is the same as in example 4.-. In this example, the liquid phase retrieved from the filtration of example 7 is treated to precipitate the DCP by neutralisation in the following manner:

(50) quicklime (or limestone) is added to the nominal flow in the reactor in which the liquid phase retrieved from the example 7 is also introduced, and the pH is regularly controlled.

(51) When the pH is equal to 5.5/6 the filtrate feed pump is started. The pH is regularly controlled and the limestone or quicklime feed flow is adapted to maintain the pH between 5.5 and 6.

(52) Filtration is conducted every thirty minutes from the buffer tank. Every second time, the filtration cake containing the calcium sulphate is recycled in the first attack reactor to increase the rate of solid matter in the reaction medium.

(53) The production cake containing the precipitated DCP is retrieved and the mother liquors are stored in a vat. Product samples (DCP and mother liquors) are collected for analysis.

(54) The neutralisation temperature is of 60° C., the pH in the first tank is of 4.4, whereas it reaches 5.55 in the second tank. The flow of quicklime is of 1.05 kg/h.

(55) The precipitation yield of the DCP is calculated with the formula (P.sub.2O.sub.5 content in the DCP/P.sub.2O.sub.5 initially present in the MCP solution and the acid) and the P.sub.2O.sub.5 balance of the operation is of 92%.

Comparative Example 2

Sub-Stoichiometric Attack of a Phosphate Rock at the Scale of the Pilot Project

(56) The pilot used is the same as in example 4.—and the same process as in example 4.—is implemented.

(57) The same phosphate source as in example 4.—is added to the attack tank in the presence of sulphuric acid at 20% by weight with respect to the weight of the acid, with a SO.sub.4/Ca ratio of 0.8 owing to the calcium content of the phosphate source. The attack temperature is of 60° C. and the attack duration is of approximately 1 hour. The pH in the attack tank is of 1.73. The flow of the rock is of 5 kg/h, and the acid flow is of 15.6 litres/h. The P.sub.2O.sub.5 content in the attack suspension is of 7.10% by weight with respect to the total weight of the suspension.

(58) During filtration, the flow of the retrieved liquid phase is of 13.3 kg/h.

(59) The attack yield is of 65%.

(60) As can be seen, with respect to example 4.—, the presence of a sulphuric acid with a greater concentration and a P.sub.2O.sub.5 content of more than 6% lower the yield to 65%.

(61) It is understood that the present invention is by no means limited to the embodiments described above and that many modifications may be made thereto without departing from the scope of the appended claims.