DESC:FIELD OF THE INVENTION
[001] The present invention relates to a process for increasing the efficiency of phosphoric acid extraction. More particularly, the present invention relates to a process whereby metal impurities are removed up to 95% before extraction of phosphoric acid thereby increasing the extraction efficiency of the overall process.
BACKGROUND OF THE INVENTION
[002] Phosphoric acid can be extracted from rock phosphate by two processes viz. wet process and thermal process. The thermal process normally produces a more concentrated and purer acid with huge energy consumption. The wet process involves reaction of rock phosphate with a mineral acid. This process is economic and practiced everywhere in the world. However, the phosphoric acid produced by wet process contains a variety of impurities which are removed from the acid by several techniques such as precipitation, adsorption, ion exchange and solvent extraction.
[003] In a solvent extraction process, phosphoric acid is extracted by addition of sulphuric acid in a series of well-stirred reactors followed by its extraction and concentration by addition of an organic solvent. This results in phosphoric acid and calcium sulphate (gypsum) plus other insoluble impurities. Thereafter metal impurities are removed in order to increase the concentration and purity of phosphoric acid. A typical illustration of the process of extraction known in the prior art. The known process comprises of the following sequential steps: (1) Acidulation (2) Filtration (3) Decantation (4) Extraction (19 cycles) (5) Scrubbing (8 cycles) (6) Stripping (9 cycles) (7) Purification (4 cycles) (8) Concentration (9) Decolourization.
[004] In the prior art process, the extraction step utilizes about 19 contact cycles in counter current flow to meet the phosphoric acid content in the spent brine, 8 contact cycles for scrubbing with a view to purify loaded organic layer from co-extracted metal impurities and 9 contact cycles for stripping to unload phosphoric acid from purified loaded organic layer. Therefore, the steps of extraction of phosphoric acid takes place in around 36 cycles which makes the process lengthy and inefficient. Moreover, a high grade of rock phosphate is required to be used in order to obtain high purity phosphoric acid. This makes the process expensive.
[005] Therefore, there is a need to increase the efficiency of phosphoric acid extraction which solves the problem of the prior art.
SUMMARY OF THE INVENTION
[006] According to an embodiment of the present invention, there is provided a process for increasing the efficiency of phosphoric acid extraction, the process comprising the steps of:
a) treating rock phosphate with a strong mineral acid to form a slurry;
b) filtering the slurry of step a) to separate a solid phase from an aqueous phase;
c) decanting the filtrate obtained in step b) to further remove the remaining solid phase;
d) adding a first organic solvent to the aqueous phase of step c) in two cycles to remove metal impurities;
e) adding a second organic solvent to the aqueous phase of step d) in a single cycle to further remove the metal impurities; and
f) adding a mixture of tributyl phosphate (TPB) and amyl alcohol in a ratio of 3:2 (v/v) to the aqueous phase of step e) to extract phosphoric acid,
wherein the first organic solvent is a mixture of 2-10% TBP in turpentine (w/v) and the second organic solvent is a mixture of 2-5% trialkyl amine in turpentine (w/v).
DETAILED DESCRIPTION OF THE EMBODIMENTS
[007] For the purpose of promoting an understanding of the principles of the invention, reference will now be made to embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the disclosed process and system, and such further applications of the principles of the invention therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
[008] It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.
[009] Reference throughout this specification to “one embodiment” “an embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase “in one embodiment”, “in an embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
[010] As set out in the claims, the present invention eliminates or reduces the aforementioned problems of the prior art by providing a process whereby metal impurities are removed up to 95% before extraction of phosphoric acid thereby increasing the extraction efficiency of the overall process. Additionally, any grade of rock phosphate can be used in the present process for extraction of phosphoric acid.
[011] The present process comprises of the following sequential steps: (1) Acidulation (2) Filtration (3) Decantation (4) Removal of metal impurities (5) Extraction (6) Scrubbing (7) Stripping (8) Concentration (9) Decolourization.
[012] According to the embodiments of the present invention, rock phosphate is treated with a strong mineral acid, preferably HCl, in a reactor, to form a slurry. This crude phosphoric acid slurry is then filtered to separate the solid phase from the aqueous phase. The remaining solid present in the filtrate is further separated out by decantation. This step removes around 30% of solid.
[013] Next step involves addition of a first organic solvent to the aqueous phase of step 3 in two cycles to remove metal impurities. In this step around 95% of metal impurities like Iron, Zinc, Molybdenum & copper are removed. Preferably, the first organic solvent is a mixture of 2-10% tributyl phosphate (TBP) in turpentine (w/v). Thereafter, a second organic solvent is added to the aqueous phase in a single cycle to further remove the metal impurities. Preferably, the second organic solvent is a mixture of 2-5% trialkyl amine in turpentine (w/v).
[014] Once the metal impurities are removed, a mixture of tributyl phosphate (TPB) and amyl alcohol in a ratio of 3:2 (v/v) is added to the aqueous phase to extract phosphoric acid.
[015] In an embodiment, the loaded layer of the mixture of TBP and amyl alcohol is scrubbed off with a soft acid and impure weak phosphoric acid is recycled back in the process. The phosphoric acid is then stripped from the mixture using water to obtain dilute phosphoric acid.
[016] The concentration of phosphoric acid is increased by removing the mineral acid and water by evaporating the dilute phosphoric acid. The mineral acid is recycled back in the process. Lastly, the colour of phosphoric acid is removed thereby obtaining phosphoric acid with purity greater than 86%.
[017] Preferably, both the first and second organic solvents obtained in steps d) and e) respectively are recycled back in the process for use in step (d) and step (e) respectively.
The following experimental example is illustrative of the invention but not limitative of the scope thereof:

[018] Experimental Data 1:
3000 part of rock phosphate was digested with 5500 part of 33-36% hydrochloric acid in a stirred reactor to control desire concentration of P2O5. The water quantity was adjusted to 0.5 - 0.9 part. The resultant colloidal mass was held to be settled on addition of flocculent solution. The filtration followed by decantation steps provided wet process acidic liquid. The composition of source ore and the wet process acidic liquid is provided in Table 1.
Table 1
Metal impurities level in ppm
Fe Mg Mn Zn Cu Cr Pb Ni
Rock Phosphate 5885 1216 257 276 252 137 4.0 21
Crude wet acidic liquid 1744 384 73 83 73 37 1 6

[019] Into a reactor, 250 part of crude wet process acidic liquid containing low dose of iron was mixed with a 250 part of tributyl phosphate in turpentine oil (2-10% tributyl phosphate (TBP) in turpentine (w/v)). The mass was agitated at moderate rpm which was enough time to achieve equilibrium. The mixture was transferred to a separating funnel till mixture was separated to main phases i.e. organic and aqueous. The lower acidic layer-I was taken into reactor while upper phase was collected separately. The lower acidic layer-I was again treated with tributyl phosphate in turpentine and agitated then mixture was separated to lower acidic liquid-II and upper organic layer. The lower acidic layer-II was taken with trialkyl amine in turpentine (2-5% trialkyl amine in turpentine (w/v)) and stirred then mixture was separated as lower acidic layer-III and upper organic layer. The lower acidic layers - I, II, & III were analysed for the level of metal impurities.
Table 2
Wet process acidic liquid Level of metal impurities (mg/L)
Fe Zn Mn Mg Cr Cu Ni Pb
Before treatment 954 86 87 649 37 4.9 4.6 2.3
Lower acidic layer-I 127 78 87 623 33 3.7 6.2 2
Lower acidic layer-II 34 81 87 626 32 3.5 6.3 2.5
Lower acidic layer-III 0.3 1 59 705 37 0 5.6 2
[020] It was observed that the resultant wet process acidic liquid was purified and was free from Iron, Zinc, Copper and reduced the quantity of Manganese while persisting other impurities like Magnesium, Chromium, Nickel, lead. This purified wet process acidic liquid had minimal loss i.e. 2.5% in the P2O5 content from the initial value.
[021] Experimental Data 2:
Similarly following above experimental procedure, using wet process acidic liquid having high iron impurities dose and then acidic lower layer I, II & III were analysed for the metal impurities level. More over the treatment sequence would be a validation of the process to achieve reduced impurities level.
Table 3
Wet process acidic liquid Level of metal impurities (mg/L)
Fe Zn Mn Mg Cr Cu Ni Pb
Before treatment 2409 130 98 550 56 3.7 6.7 3.5
Lower acidic layer-I 85 121 98 527 56 3.4 7.2 2.6
Lower acidic layer-II 9 118 97 540 55 3.1 6.8 2.9
Lower acidic layer-III 0.3 1.1 59 541 56 0 6.4 1.9
[022] The sequential treatment could eliminate iron, zinc, copper and reduced manganese content while keeping other ionic impurities like magnesium, chromium, nickel and lead.
[023] The purified wet process acidic liquid was taken for the P2O5 extraction using organic solvent. The organic solvent used was a combination of two or more solvent. An increase in the P2O5 extraction efficiency of solvent found revealed the importance of purification step before P2O5 extraction. Herewith in the below example illustrated in detail:
[024] Example 1 -
The purified wet process acidic liquid 50 parts from Experimental Data-1 was contacted in volume ratio of 1:2.4 with mixture of pentanol 110 parts and mineral acid 10 parts at room temperature. The separated raffinate aqueous layer was contacted further a total six times with a mixture of 110 parts pentanol and 10 parts mineral acid. The P2O5 content was 2.6g/L in the last raffinate aqueous layer and total 98.04% P2O5 was extracted in seven contact cycles.
[025] Example 2 -
In another example the purified wet process acidic liquid 50 parts from Experimental Data-1 was contacted in a volume ratio 1:2.4 with a mixture of TBP and amyl alcohol 110 parts and mineral acid 10 parts at room temperature. The obtained raffinate aqueous layer was contacted further a total three times. The P2O5 content was 2.01 g/L in the last raffinate aqueous layer and total 98.49% of P2O5 was extracted in four contact cycles.
[026] Example 3 -
In another example the purified wet process acidic liquid 50 parts from Experimental Data-2 was contacted in a volume ratio 1:2.4 with a mixture of pentanol 110 parts and mineral acid 10 parts at room temperature. The obtained raffinate aqueous layer was then contacted a total six times with a mixture of 110 parts pentanol and 10 parts mineral acid. The P2O5 content was 2.87 g/L in the final raffinate aqueous layer and total 97.65% of P2O5 was extracted in total seven contact cycles.
[027] Example 4 -
In another example the purified wet process acidic liquid 50 parts from Experimental Data-2 was contacted in a volume ratio 1:2.4 with a mixture of new formulation solvent 110 parts and mineral acid 10 parts at room temperature. The raffinate aqueous layer was then contacted a total three times with a mixture of 110 parts new formulation solvent and 10 parts mineral acid. The P2O5 content was 2.75 g/L in the final raffinate aqueous layer and total 97.76 & P2O5 was extracted in total four contact cycles.
[028] These extracted impurities were back extracted to aqueous from organic with weak aqueous Phosphoric acid. Finally, concentration of phosphoric acid up to 86% and decolourization resulted into colourless transparent syrupy liquid.
[029] The foregoing description of specific embodiments of the present invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed, and obvious modifications and variations are possible in light of the above teachings.
[030] The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, to thereby enable others, skilled in the art, to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated.
,CLAIMS:We Claim:
1. A process for increasing the efficiency of phosphoric acid extraction, the process comprising the steps of:
a) treating rock phosphate with a strong mineral acid to form a slurry;
b) filtering the slurry of step a) to separate a solid phase from an aqueous phase;
c) decanting the filtrate obtained in step b) to further remove the remaining solid phase;
d) adding a first organic solvent to the aqueous phase of step c) in two cycles to remove metal impurities;
e) adding a second organic solvent to the aqueous phase of step d) in a single cycle to further remove the metal impurities; and
f) adding a mixture of tributyl phosphate (TPB) and amyl alcohol in a ratio of 3:2 (v/v) to the aqueous phase of step e) to extract phosphoric acid,
wherein the first organic solvent is a mixture of 2-10% TBP in turpentine (w/v) and the second organic solvent is a mixture of 2-5% trialkyl amine in turpentine (w/v).
2. The process as claimed in claim 1, wherein the mixture of step f) is scrubbed with a soft acid the weak phosphoric acid of step f) is recycled back in the process for use in step (f).
3. The process as claimed in claim 2, wherein phosphoric acid is stripped from the mixture using water to obtain dilute phosphoric acid.
4. The process as claimed in claim 3, wherein the dilute phosphoric acid is evaporated to get phosphoric acid having a concentration of 86%.
5. The process as claimed in claim 1, wherein the mineral acid is preferably hydrochloric acid.
6. The process as claimed in claim 1, wherein both the first and second organic solvents obtained in steps d) and e) respectively are recycled back in the process for use in step (d) and step (e) respectively.