Claims:WE CLAIM:

1. A pre-filtration process for acid color removal and simultaneously improving filtration rate in a conventional production process of phosphoric acid involving acidulation of rock phosphate source rich in organics with a mineral acid in which pre-filtration process comprising of steps:-
a) preparing a plurality of flocculant solutions from plurality of flocculant powders at a concentration of 1-5 gpl with water in cylindrical cone with tangential inlet at specific angle of 900 for water entry,
b) diluting each of the plurality of flocculant solutions separately to 1 – 1.5 gpl,
c) feeding the flocculant solutions sequentially and separately to the filter feed slurry, and
d) mixing the mixture of step (c) for a particular duration for color removal before the slurry is allowed to enter a filter for filtration, the said method characterized in adding flocculant solutions to the reaction slurry before filtration, and more specifically adding a combination of flocculants to the slurry.
2. The pre-filtration process as claimed in claim 1, wherein atleast one of the flocculant powder is anionic powder.
3. The pre-filtration process as claimed in claim 1, wherein atleast one of the flocculant powder is cationic powder.
4. The pre-filtration process as claimed in claim 1, wherein duration of mixing is in the range 15-120 seconds.
5. The pre-filtration process as claimed in claim 1, wherein the selected flocculant have different molecular weights.
6. The pre-filtration process as claimed in claims 1 to 3, wherein the ratio of cationic and anionic flocculants may be varying from 100:0 to 0:100.
7. The pre-filtration process as claimed in claim 1, wherein the adding of flocculant solutions to the reaction slurry before the filtration is arranged after the filter feed pump discharge in the pipe line connected to the filter at spaced locations for the plurality of solutions thereby avoiding interaction of flocculant solutions before entering into reaction slurry.
8. The pre-filtration process as claimed in claim 7, wherein the pipe line angle at which the flocculant solution entering the slurry is fixed at the 450 angle to the filter feed pump discharge pipe line where the flocculant mixes with slurry for effective mixing and to reduce the back pressure on the pumps.
9. The pre-filtration process as claimed in claim 1, wherein the pumping of the flocculant solution to the filter is by means of progressive cavity flow pump to with stand back pressure of the filter feed pump without any pulsations and more particularly to avoid shearing action on the flocculant chains during pumping for its effective action in black color removal.
10. The pre-filtration process as claimed in claim 1, wherein a flap type is non return valve is fixed in horizontal position on the discharge pipe line of the flocculant dosing pump as close as possible to the mixing point with the slurry pipe line to avoid any reaction slurry back flow towards flocculant dosing pump and to consistently dose required quantity of the flocculant solution to filter without any interruptions.
11. The pre-filtration process as claimed in claim 1, wherein the flocculants are selected on the basis that the flocculants will not affect the downstream equipment material of construction, particularly have no interactions with the belt filter rubber parts.
12. The pre-filtration process as claimed in claim 1, wherein the fertiliser granules produced with the acid generated by the addition of the flocculants will not have any impact of the black color matter on its appearance in comparison to the color of the fertiliser granules produced with acid produced from low organic rock phosphates .
13. The pre-filtration process as claimed in claim 1, wherein the high organic rock phosphate is having P2O5 less than 31% but still would like to use it on 100% basis to produce product acid above or equal to 26% P2O5 concentration then combination of cationic and anionic flocculants in the ratio of 100:0 to 0:100 are proven to be effective.
14. The pre-filtration process as claimed in claim 1, wherein the produced acid is having organic carbon removed to an extent of up to 90% in comparison to the acid produced without flocculant addition.
, Description:FIELD OF INVENTION

The present field of invention is about producing phosphoric acid free from suspended colloidal organic matter by separating it along with the solid stream (gypsum solids) during reaction-slurry filtration by the addition of flocculant, which otherwise would get carried along with the acid stream producing black color acid and as a result, the fertilizer produced with this acid will be of varied black color shades, particularly during usage of rock phosphates rich in colloidal insoluble organics on 100% basis or as a blend in combination with other rock phosphates.

BACK GROUND OF THE INVENTION

The production of phosphoric acid process involves acidulation of rock phosphate source (apatite) with mineral acid to produce phosphoric acid along with corresponding salt as a by-product. In the wet process of manufacturing phosphoric acid adopting dihydrate route, sulfuric acid is the most commonly used mineral acid and as a result, calcium sulphate dihydrate (gypsum) is obtained as a byproduct adopting di-hydrate process. The output from the acidulation reactor is a slurry which is filtered to separate gypsum, producing impure dilute phosphoric acid stream containing fine suspended gypsum particles, fluoro silicates and organics along with metal salts. The composition or in other sense the quality of the acid produced is purely dependent upon the source of the rock phosphate used. This acid can be used directly for the production of the fertilizers or to be treated, concentrated in the evaporators and purified at subsequent stages to produce merchant grade acid or food grade acid depending upon the intended use.

Treatment or purification steps involve separation of the suspended solids by clarification at the different stages of the acid concentration to produce merchant grade acid, acid color removal by addition of different additives and going for solvent extraction to remove soluble impurities for the production of food grade acid.

In the dihydrate process, the concentration of the suspended solids, specifically very fine gypsum solids & fluoro silicates in the acid stream from the filter is dependent upon the type of the rock phosphate source & its composition, reactor throughput rate at which it is operated and operating conditions that decides the gypsum crystal morphology for good filtration. At the other end, the concentration of the organic matter and soluble impurities in the acid are mainly because of the source of the rock phosphate used in the production of phosphoric acid and they are least influenced by the throughput rate and operating conditions.

The choice of the rock phosphate source used for the production of the phosphoric acid is dependent upon its availability, composition, cost, throughput rate at which it can be processed, efficiency and more importantly based on the suitability of the acid produced from it for the intended use.

In the present context to explain, there are few sources of rock phosphates abundantly available in the world at low cost with certain limitations that they demand high filtration area & specific reaction volume, offer low efficiency and produce acid of black color due to the presence of high organics but with acceptable limits of other impurities suitable for the production of the fertilizers. One way to overcome the limitations with low filtration rate, high specific reaction volume and low efficiency is to use such rock phosphates in combination as a blend with better performing rock phosphates. Alternatively with existing reaction volume and filter area, certain additives can be added during the reaction or the filtration step or in the both steps of the process to enhance filtration rate, efficiency and throughput. Even after doing all this, the acid produced will be very black in color if the source of the rock phosphate used is rich in organics and the conventional process adopted to remove this organic matter along with solids in the acid is by settling in clarifiers by the addition of flocculants. There are very effective flocculants available in the market that can do this job easy but the major limitation in this process approach is that the settled solids along with organic matter has to be separated from the clarifier bottom and it is to be recycled back to the phosphoric acid plant to recover P2O5.This solids recycle may affect the performance of the reactor and the filter as well. Alternatively, the clarifier bottoms has to be utilized for the production of the fertilizers where color of the product is not an issue but this may not be a possibility for all the plants.

PRIOR ARTS

The prior arts discusses about the usage of the flocculants to improve the filtration rate as it forms flocs with the gypsum solids modifying its morphology for enhanced filtration. This was done by adding the suitable flocculant type to the reaction slurry just before pumping it to the filter. The flocculants are also added to the acid stream with solids and organic content just before the inlet to the clarifiers where they are settled quickly to obtain clarified clean acid. Though studies till date indicate the enhancement of the filtration rate by the addition of the flocculants but they are not completely successful in removing the color matter also at the filtration stage itself and as a result separate addition of the flocculant is also required at the clarification step for the removal of color matter along with the solids by settling. The extent of the color matter removal at the filtration step is varying depending upon the type of the rock phosphate source used and the concentration of the suspended organic colloidal matter in it. Given below are the few prior arts related to the usage of the flocculants for the above mentioned acid treatment process.

US patent: 4,800,071:”Filtration aids for removal of calcium solids from aqueous phosphoric acid” describes method for increasing the filtration rate, for the removal of gypsum and gangue from phosphoric acid in the phosphoric acid manufacturing process by the addition of sulfonated acrylamide polymer containing at least 1 mole percent sulfonate and having a reduced specific viscosity (RSV) of at least 10, sulfonated polymer selected from various acrylic groups. Our work is different from this prior art as the major focus in our work is on the removal of the finely suspended colloidal organic black matter to produce phosphoric acid free from black color and simultaneously achieve enhanced filtration rate by overcoming the effect of these black organics on the filtration rate.

US patent: 5,173,280: “Method of clarifying phosphoric acid using high charge sulfonic type polymer” describes a method for clarifying phosphoric acid by flocculation wherein the flocculant source is from the sulfonic functionality family with high anionic charge content preferably from high molecular acryl group. Our work is different from this prior art as we added flocculant to the filter feed slurry before filtration but not to the phosphoric acid before clarifiers and as a result the color matter is separated along with gypsum without entering into phosphoric acid.

US patent: 3,644,091: “Clarification of phosphoric acid” describes a method of introducing into the acid a small amount of clarification additive which is a sulfonated polystyrene having a molecular weight from about 1 to about 40 million and specific position & concentration of sulfonation in the benzene ring, the clarification additive being added to the acid in one or more zones of the process that includes digestion, at inlet or exit of evaporation zone, at inlet or exit of centrifugation zone or into the storage zone. Our work is different from this prior art in a way that our focus is on the removal of the black color matter and enhance filtration rate by adding flocculant before filtration.

US patent: 4,291,005: “Settling out suspended solids in the phosphoric acid product solutions” describes settling of the suspended solids in the product phosphoric acid by the addition of organic polymeric flocculant agent which is copolymer of predominant proportion of polymeric acrylic acid and minor proportion of acrylamide. Our work is different from this prior art in which flocculant is added to the phosphoric acid before clarifiers to settle solid matter that is to be recycled or re-processed but where as in our case, we added flocculant to the filter feed slurry before filtration and as a result the color matter is separated along with gypsum without entering into phosphoric acid.

US patent: 4,332,779: “Phosphoric acid filtration process” describes a process for the filtration of the phosphoric acid by adding to the pre-filtration step an effective amount of a cationic flocculant and a dispersant (the dispersants being chosen from a specific group of poly acrylates, poly sulfonates poly phosphates and sulfonated poly condensates) in the ratio of 0.1 to 1.0 to 10.0 to 1.0 for the removal of impurities and filtration rate enhanced over previous known art. Our work is different from this prior art in a way that we have not added any dispersant additionally along with the flocculant whether it is cationic or anionic, as the dispersion action is also taken care by the flocculant itself. Moreover, the extent of color removal and filtration rate achieved in our work is enhanced over above prior art.

US patent: 5,318,707: “Method of clarifying phosphoric acid” describes method for clarifying phosphoric acid by the addition of high molecular weight carboxylic acid/ sulfonic acid type polymer and settling it for the removal of the solids. Our work is different from this prior art in which flocculant is added to the phosphoric acid before clarifiers to settle solid matter that is to be recycled or re-processed but where as in our case, we added flocculant to the filter feed slurry before filtration and as a result the color matter is separated along with gypsum without entering into phosphoric acid.

OBJECT OF INVENTION

The object of the invention is to develop process methodology for the removal of the color matter through gypsum at the filtration stage itself and simultaneously targeting enhanced filtration rate by the addition of the flocculants that can be of cationic, anionic or combination in nature of different molecular weight ranges and charge density.

Another object of the invention is to study the effect of variation in the time of mixing flocculant with the reaction slurry and variation in the input dosage and combination of the flocculants type along with its sequence required with increased through put rate. This study was done with rock phosphate rich in high organic suspended colloidal matter and also on the blend combinations in which at least one of the rock phosphates is rich in high suspended colloidal organic content.

Another object of the invention is to study the influence of the variation in reaction process parameters on the slurry condition and its corresponding effect on the action of the flocculants in color removal and achieving enhanced filtration rate.

Another object of the invention is to study the influence of the product acid P2O5 concentration, particularly above 26% on the action of the flocculants in removing color matter and achieving enhanced filtration rate.

Another object of the invention is to identify suitable location where the flocculant is to be added to the reaction slurry whether it is to be added to the filter feed tank or in the filter feed pump discharge or at the exit point of the filter feed pump discharge pipe.

Another object of the invention is to evaluate the effect of the flocculants used on the material of construction of filter and downstream equipment by soaking tests.

Another object of the invention is to produce fertilizer using the acid produced by the addition of flocculant to check and ensure that there is no impact of the black color matter on fertiliser granules appearance in comparison to the fertiliser granules produced with acid generated from other rock phosphates sources low in organic black color matter.

Another object of the invention is to identify optimum concentration of the flocculant solution in the range of 0.5 to 3 gpl to be dosed, more specifically at 1 gpl concentration in order to achieve effective mixing with the slurry to maximize color removal and filtration rate.

Another object of the invention is to prepare flocculant solution free from clots or lump formation by effectively mixing the solid powder flocculant with the tangentially entering water jet at suitable angle, more specifically at 900 to the mixing cylindrical cone by providing sufficient retention time, specifically designed for this purpose.

Another object of the invention is to select suitable pump for this application whether centrifugal pump or positive displacement type and even in positive displacement type whether plunger or diaphragm or progressive cavity flow type for consistent dosage of flocculant solution, to have minimal or no shear effects on the flocculant chains for effective color removal and that which can pump continuously against back pressure of filter feed pump without failure. In specific, we chose to operate with progressive cavity flow pump as it meets all the above requirements and gives continuous flow without pulsations.

Another object of the invention is to optimize the angle in between 00 to 1800, to be specific at 450 angle at which the flocculant solution has to enter the mixing zone where it intersects with the filter feed pump discharge pipe line in order to minimize the back pressure on flocculant dosing pump and also for effective mixing with reaction slurry before it reaches the filter.

Another object of the invention is to select suitable non return valve for the application whether spring loaded type or flap type to be fixed in the flocculant pump discharge line and also to select suitable position whether to be fixed in the horizontal or vertical position. To be specific, we chose to operate with flap type non return valve fixed in the horizontal position in the pipe line as it is found to be more suitable for slurry applications.

DESCRIPTION OF THE DRAWINGS:

Figure 1

Part number Parts
1 Rock Phosphate with black color matter
2 Sulfuric acid
3 Return acid
4 Reactor
5 Filter feed tank
6 Filter feed pump
7 Filter
8 Gypsum with black color matter
9 Product acid free from black matter
10 Evaporator
11 Concentrated acid free from black matter
12 Granulation
13 Fertiliser free from black matter
14 Cationic flocculant solution
15 Anionic flocculant solution
16 Flocculant dosing pump
17 Non return valve

Figure 2

Part Number Parts
1 Product acid when no flocculant was added
2 Product acid when flocculant was added

DESCRIPTION OF THE INVENTION

In the commercial process of manufacturing phosphoric acid by di-hydrate route, rock phosphate is ground to fineness required in a grinding section, typically in a ball mill before it is fed to the reactor where it reacts with the sulfuric acid and weak phosphoric acid recycled from the filtration section. The sulfuric acid can be fed as concentrated acid at 93-98% concentration or it can be diluted with water to required concentration before feeding it to the reactor. The quantity and the concentration of the sulfuric acid & recycled weak phosphoric acid fed to the reactor is fixed based on the process conditions that are to be maintained in the reactor, mainly temperature, percentage solids, % free sulphate concentration in the reactor and product phosphoric acid strength as percentage P2O5.The specific reaction volume(m3/tpd p2o5) and the process conditions that determines the quality of gypsum crystal morphology play a key role in determining the throughput and the P2O5 efficiency that can be achieved with each rock phosphate.
The reaction slurry from the reactor flows to the filter feed tank from where it is fed to the filter, typically a belt filter or a tilting pan filter by filter feed pump. This slurry is filtered on the filter to separate gypsum from the phosphoric acid. As said earlier, for a given rock phosphate, process conditions and the residence time maintained in the reactor determine the quality of the gypsum crystals produced which in turn fixes the specific filtration rate (tpd p2o5 per square metre) that can be achieved. The gypsum cake is washed adopting 2 to 3 stage counter current washing cycle before being disposed through dry or wet disposal system.
The product acid from the filter having concentration of 25-30% P2O5 along with fine suspended gypsum solids, fluoro silicates, organic matter and soluble metal salts is pumped to the product acid storage tanks. The quality of the acid produced with each rock phosphate or from the blend of different sources of rock phosphate is dependent on the concentration of these insoluble and soluble impurities as mentioned above that will have direct or indirect effect on the downstream operations mainly acid storage, acid evaporation and fertilizer granulation.

The quality of the fertilizer in terms of its nutrient fixation in the process, granulation to get good sphericity, crushing strength and appearance is highly dependent upon the quality of the acid used for the production. The composition of the rock phosphate source used in the phosphoric acid manufacturing process play a major role in fixing the quality of the acid produced. In the present context, we chose to work with the rock phosphate sources that are rich in suspended colloidal organic matter either on 100% basis or a blend with other rock phosphates which when used for the production of the phosphoric acid produce black color acid that is not directly suitable for the production of the fertilizer as the appearance of the product is black in color and color of the granules will appear in varied color shades, particularly when produced from the acid obtained from blend rock phosphates in which at least one of the rock sources is high in suspended colloidal organic matter.

It is always advantageous to remove or eliminate this black color organic matter entry into the acid at the filtration stage itself completely so that the effect of this color matter in the downstream operations can be eliminated. The downstream effects can be felt in terms of the organic matter deposits in the storage tanks, in the tubes of evaporators during concentration and in the final fertilizer product produced. In the conventional process, this organic matter removal is done by settling along with fine suspended solids at the acid clarification stage by flocculant addition or by activated carbon treatment in acid purification process.

Drawing inputs from the prior arts, we worked in developing a process methodology to completely eliminate black color matter entry into the acid stream .In the new process, we adopted flocculant addition technique to the reaction slurry before filtration itself where in the added flocculants are chosen not just from either anionic family or cationic family but in combination of different molecular weight and charge density ranges so that both the needs of enhanced filtration along with complete black color matter free acid is met. Fortunately, the advantage with the phosphoric acid and gypsum slurry mixture containing organic colloidal matter is that it responds to both the anionic and cationic flocculants in such a way that the anionic nature of the flocculants will predominantly modify the gypsum crystal morphology to enhance the filtration rate and the cationic nature of the flocculant will predominantly form good floc with the fine suspended colloidal organic matter for its effective removal along with the gypsum, thus avoiding its entry into the acid stream.

The addition of the flocculants that we chose to add can be from either family of anionic or cationic if we chose to work with blend rock phosphates in which at least one of the rock phosphates is rich in organics and the contribution of these kind of rock phosphates as a whole not to exceed 50% in the blend. If we are using the rock phosphates rich in organic matter more than 50% in the blend or on 100% basis then the combination of flocculants from cationic and anionic family is found to give consistently good filtration rate and by ensuring removal of the black color organic matter along with gypsum in the filtration step, resulting in acid without black color.

The preparation of the flocculant solutions from the powders of anionic and cationic type must be done in the separate tanks and should not be allowed to interact with each other prior to their mixing with the filter feed slurry. The preparation of the flocculant solutions can be done at the concentration of 1 to maximum of 5 gpl in water and to be diluted with the suitable source (can be other than water) to 1 to 1.5 gpl before feeding it to the slurry for effective mixing and action. Proper care must be taken during the preparation of flocculant solution mainly when flocculant powder is added to the diluent liquid to avoid any clot or lump formations and to be mixed for sufficient time for complete hydration.

The sequence of the addition of the flocculant type, dosage mix, point of addition whether the single point or multiple points to the filter feed slurry can be altered and changed according to the rock phosphate type and the concentration of the black color matter in it. The mixing time of the flocculant solution to the slurry before it enters the filter can be from the minimum of 15 sec to 120 sec depending upon the desired filtration rate and color removal in the acid to be achieved based on the gypsum crystal morphology and concentration of the color matter in the filter feed slurry.

The inventiveness of the invention is in the method for the removal of the black color matter and achieve enhanced filtration rate simultaneously by the addition of the combination of flocculants from cationic and anionic family to the filter feed slurry.

a. The addition of the flocculants in combination from the cationic & anionic family is must if the rock phosphate is rich in suspended colloidal organic matter and when it is processed as single rock source, particularly when targeting product acid P2O5 concentration above 26%.

b. The addition of the flocculants in combination from the cationic & anionic family is must if the percentage of the sources of the rock phosphates in the blend that are rich in suspended organic colloidal matter is more than 50 percent, particularly when targeting product acid P2O5 concentration above 26%.

Further the invention, also ensures that the extent of the filtration rate enhancement can be up to 50% by the addition of flocculants that are of anionic or cationic single source or in combination when compared to that without adding flocculants and also the extent of the color reduction in the final product acid can be up to 90% in comparison to the acid produced without flocculant addition.

The other advantages of the invention is in the method of selective sequence for the addition of the flocculants that are cationic and anionic in nature as a combination, whether cationic to be added first and then anionic or the other way. It is very essential to add cationic first followed by the anionic to achieve the desired result as mentioned above.

Further the invention discloses that the point of the addition of the cationic and anionic flocculants, whether to be done at the single point or at the multiple points. It is very essential to add them at multiple points to avoid interaction between the two kinds to achieve the desired result as mentioned in above.

Further the invention discloses that the time for the mixing of the flocculants in the filter feed slurry when added in combination ranges from 15 to 120 sec.

Further the invention discloses that the ratio of the cationic to anionic flocculant vary from 100:0 to 0:100, covering in between ratios.

Pilot Study:
In order to evaluate the suitability and usage of the high organic suspended colloidal matter containing rock phosphates for the production of phosphoric acid by the addition of flocculants, detailed study was done in our pilot facility at Visakhapatnam in the state of Andhra Pradesh.
The present invention talks about the development of the process route to produce phosphoric acid free from black color matter by the addition of the flocculants.
Our pilot facility which can be operated at 60-120 kg/hr. rock feed is commissioned in the year 2007 and is well equipped with all the installations for rock grinding, reaction and filtration system to evaluate different sources of rock phosphate, simulating main commercial plant operating philosophy.
For the present invention, we chose to work with two different sources of high organic suspended colloidal matter containing rock phosphates individually on 100% basis and each in combination as a blend with two other low organic matter containing rock phosphates in our pilot plant.
During the pilot test run, slurry sample from the pilot reactor is collected for every 5 hours to conduct filtration test with and without addition of flocculants to establish the specific filtration rate that can be achieved with each source of organic rock and its different blend combinations and simultaneously checking the quality of the acid produced by analyzing the black color matter in it.

Procedure for filtration test:
Collect the reaction slurry from the sample point and measure its specific gravity. Now to get about 800ml of slurry to conduct filtration test, weigh the slurry sample based on the specific gravity. Now weigh required quantity of flocculant solution whether cationic or anionic and add it to the weighed quantity of slurry under mixing. If the flocculant solutions are used in combination of both cationic and anionic type then weigh them separately and mix them one after other at required dosage following a standard sequence to be adopted and mixing time required. Now once the slurry is conditioned with flocculant solution addition by proper mixing for sufficient time, filter the slurry on a filter cloth fixed in a Buchner funnel set up, operated under vacuum. It is important to use the same filter cloth that is being used in the main phosphoric acid plant for the filtration test to get more reliable results.

The time of filtration till the acid leaves the surface of the cake is recorded as slurry filtration time. The resultant filtrate collected in the filtrate container is checked for black color removal by physical visibility by pouring it into a measuring jar and sample analysed for total organic carbon content in the acid. During every filtration test with different combinations, varying dosage, mixing time and varying sequence of addition, it is important to conduct one blank test first without addition of any flocculant solution to the same test slurry and record slurry filtration time, check acid color by physical visibility and analyse the filtrate for total organic carbon matter.

Following the above mentioned filtration test procedure, filtration tests are conducted with slurry produced from rock phosphates that are rich in black color causing organic matter on 100% basis and in combination as a blend with other rock phosphates at different ratios during pilot trials.

The below mentioned examples gives the details of the study conducted by the addition of flocculants of cationic or anionic type in nature and in combinations to achieve enhanced filtration rate and produce acid free from black color matter.

For the entire pilot study, we used the following sources of rock phosphates

1. High P2O5 grade rock phosphate having about 36% P2O5, referred as 36% grade T
2. Medium P2O5 grade rock phosphate having 33%P205, referred as 33%grade M
3. Both the above rocks are having almost negligible black suspended organic matter in it,
4. The other two sources are low P2O5 grade rock phosphates having 30%P2O5 with high suspended organic black matter in it, these are referred as 30% grade A & 30% grade P.

We worked with flocculant samples procured from different vendors out of which results obtained from two vendors samples whose performance is found to be superior and met the intended purpose are presented below.

The two vendor samples for the reference purpose are coded as below
Vendor Type Mol weight Charge density Code
1
Cationic High low Floc A
Cationic High medium Floc B
Cationic High High Floc C
Anionic High low Floc D
Anionic High medium Floc E
Anionic High High Floc F

Vendor Type Mol weight Charge density Code
2 Anionic High low Floc G
Anionic High medium Floc H
Anionic High High Floc I

Classification of the above flocculants by molecular size and charge are as follows
Range Molecular weight (Dalton) Charge density
(mol %)
High ca.107 50-100
Medium 105-106 ca.25
Low 104-105 ca.10
Anionic CD range, meq g-1: 1.4 to 10.6
Cationic CD range, meq g-1: 1.5 to 7.3

We tried different sources of flocculants from both cationic and anionic family. Within each source, flocculants are selected from different molecular weights and different ionic concentrations or charge density in the range of low, medium, high categories.

Suitable cationic flocculants tested in the present invention include, but are not limited to the copolymers of acrylamide and its co monomers containing ammonium, sulfonium and phosphonium quaternaries. The preferable co monomers used alongside acryl amide are Metha acryloyl oxy ethyl tri methyl ammonium chloride, Acryloyl oxy ethyl tri methyl ammonium chloride, Metha acryl oxy propyl tri methoxysilane, Diallyl dimethyl ammonium chloride, Vinyl tri methoxy silane, Butyl acrylate, Acryloyl amino-2 hydroxy propyl tri methyl ammonium chloride, Epi chloro hydrin, Di methyl amine, Tri methyl amino ethyl chloride acrylate.

Suitable anionic flocculants tested in the present invention include, but are not limited to the co polymers of acrylamide and its co monomers containing sodium acrylate, acrylic acid, preferably from carboxylate and sulfonate ions as anionic functional groups.

Example 1

Pilot experiments are conducted with the combination of 36% grade T, 33% grade M and 30% grade A in the blend ratio of 30:40:30 by the addition of flocculants whose slurry filtration test results are presented below
TABLE 1
Flocculant
Type Ratio Flocculant quantity
( 1 gpl in water)
ml Slurry volume ml Mixing time sec Sequence of addition Slurry filtration time
sec TOC ppm Physical visibility for black color
Blank NA 0 800 0 NA 24 525 Black
Cationic Anionic
Floc A NA 3-5 800 30 NA 20 21 Clean acid
Floc B NA 3-5 800 30 NA 24 187 Partial black
Floc C NA 3-5 800 30 NA 28 262 Partial black
Floc H NA 3-5 800 30 NA 17 62 Clean acid
* A: Anionic, C: Cationic, NA: Not Applicable, TOC: Total organic carbon

Example 2

Pilot experiments are conducted with the 30% grade P on 100% basis by the addition of flocculants whose slurry filtration test results are presented below
TABLE 2
Flocculant
Type Ratio Flocculant quantity
( 1 gpl in water)
ml Slurry volume ml Mixing time sec Sequence of addition Slurry filtration time
sec TOC ppm Physical visibility for black color
Blank NA 0 800 0 NA 36 1575 Black
Cationic Anionic
Floc A NA 15-20 800 120 NA 26 47 Clean acid
Floc H NA 15-20 800 120 NA 18 90 Clean acid
Floc A Floc H 50:50 15-20 800 120 C,A 18 54 Clean acid
Floc A Floc H 50:50 15-20 800 120 A,C 20 78 Clean acid
* A: Anionic, C: Cationic, NA: Not Applicable, TOC: Total organic carbon

Example 3
Pilot experiments are conducted with the combination of 36% grade T, 33% grade M and 30% grade P in the blend ratio of 30:40:30 by the addition of flocculants whose slurry filtration test results are presented below
TABLE 3
Flocculant
Type Ratio Flocculant quantity
( 1 gpl in water)
ml Slurry volume ml Mixing time sec Sequence of addition Slurry filtration time
sec TOC ppm Physical visibility for black color
Blank NA 0 800 0 NA 32 645 Black
Cationic Anionic
Floc A NA 3-5 800 60 NA 30 39 Clean acid
Floc H NA 3-5 800 60 NA 28 75 Clean acid
Floc B NA 3-5 800 60 NA 32 258 Partial black
Floc G NA 3-5 800 60 NA 30 310 Partial black
* A: Anionic, C: Cationic, NA: Not Applicable, TOC: Total organic carbon

Example 4
Pilot experiments are conducted with the combination of 33% grade M and 30% grade P in the blend ratio of 50:50 by the addition of flocculants whose slurry filtration test results are presented below

TABLE 4
Flocculant
Type Ratio Flocculant quantity
( 1 gpl in water)
ml Slurry volume ml Mixing time sec Sequence of addition Slurry filtration time
sec TOC ppm Physical visibility for black color
Blank NA 0 800 0 NA 38 829 Black
Cationic Anionic
Floc A NA 10-15 800 90 NA 34 45 Clean acid
Floc H NA 10-15 800 90 NA 28 88 Clean acid
Floc A Floc H 30:70 10-15 800 90 A,C 32 68 Clean acid
Floc A Floc H 30:70 10-15 800 90 C,A 30 51 Clean acid
* A: Anionic, C: Cationic, NA: Not Applicable, TOC: Total organic carbon

Main Plant trial run:
After successfully establishing the process methodology to produce black color free acid by the addition of flocculants during the usage of high organic rock phosphates in our pilot plant study, subsequently main plant trials are conducted at Vizag phosphoric acid manufacturing unit as per the best established conditions in the pilot plant, following the process mentioned in the figure 1.Flocculant solution preparation system was installed in the plant before taking up the trial which mainly consists of tanks with agitator, cylindrical cone with tangential water inlet to feed the flocculant powder into the tanks and flocculant solution feeding pump. Sufficient agitation and residence time is to be maintained in the tanks to ensure that the flocculant is completely uncoiled and dissolved in water giving uniform homogeneous solution. The flocculant solution from the tanks is pumped using suitable flocculant dosing pump that mixes with the reaction slurry at the discharge end of the filter feed pump at specific locations to ensure that it completely mixes with the slurry before it reaches the filter. The added flocculant solution by virtue of its ability to form flocs with the black organic matter ensures that all the suspended fine colloidal organic matter in the reaction slurry is removed along with the gypsum without allowing it to enter product acid. Apart from this it also modifies the gypsum crystal morphology to enhance filtration rate.
Two trials are conducted in the plant with high organic rock phosphate source. Both the trials are conducted with blend of three sources of rock phosphate in which high organic rock phosphate is mixed at 25 percent level. The production rate, tpd P2O5 achieved with this blend having suspended colloidal black organic matter containing rock phosphate at 25 percent level is on par with the production rate of the plant using the blend of the other two rock phosphate sources in which no suspended black organic matter is present. The acid produced during the plant trial run is observed to be very clean without any presence of the black matter by physical observation and the analysis of the acid for the total organic carbon indicate that the percentage of black color matter removal is up to 90 percent in comparison with the acid produced using the same blend in which no flocculant solution is added. Figure 2 shows the comparison of the black color acid when no flocculant was added with the black color free acid produced by the addition of the flocculant.
The produced acid free from the black color matter from the storage tanks is used for the production of the fertilizer in our C-Train granulation plant. During the granulation plant run with this acid, the color of the fertilizer granules produced is observed to be very good in appearance without any black color shades. The fertilizer product samples collected at random intervals is checked for color and it is observed to be on par and sometimes better in comparison to the fertilizer product produced with the acid generated with the blend of other two rock phosphate sources having no suspended black organic matter in it.

As disclosed above, the disclosure in one aspect discloses a pre-filtration process for acid color removal and simultaneously improving filtration rate in a conventional production process of phosphoric acid involving acidulation of rock phosphate source rich in organics with a mineral acid. This pre-filtration process comprising of steps:-

a) preparing a first flocculant solution from an anionic powder at a concentration of 1-5 gpl,
b) preparing a second flocculant solution from an cationic powder, at a concentration of 1-5 gpl,
c) Preparing the flocculant solution without any lump or clot formation by effective mixing of flocculant powder with water in a specifically designed cylindrical cone with tangential inlet at specific angle of 900 for water entry.
d) diluting the first solution to 1 – 1.5 gpl,
e) diluting the second solution to 1 – 1.5 gpl,
f) feeding the cationic solution and anionic solution sequentially and separately to the filter feed slurry, and
g) Mixing the mixture of step (f) for a particular duration for color removal before the slurry is allowed to enter a filter for filtration, the said method characterized in adding flocculant selections to the reaction slurry before filtration, and more specifically adding a combination of anionic and cationic flocculants to the slurry.

In another aspect the process discloses the duration of mixing to be in the range 15-120 seconds and the selected flocculant have different molecular weights.

In another aspect the selected flocculants will be cationic and anionic flocculants and these may be varying from 100:0 to 0:100 in the ratio.

In another aspect the high organic rock phosphate used in the process is having P2O5 less than 31% but still would like to use it on 100% basis to produce product acid above or equal to 26% P2O5 concentration then combination of cationic and anionic flocculants in the ratio of 100:0 to 0:100 are proven to be effective.

In another aspect the process can enhance the filtration rate (tpd P2O5 per square metre) by up to 50% in comparison to the filtration rate achieved without flocculant addition, particularly when processing high organic rock phosphate on 100% basis.

In another aspect the pre-filtration process can produce acid with organic carbon matter removed to an extent of up to 90% in comparison to the acid produced without flocculant addition.

In another aspect the flocculant solution addition is to be done only after the filter feed pump discharge in the pipe line going to the filter at different locations for the cationic and anionic solutions in order to avoid interaction before it mixes with slurry.

In another aspect in the process of pre-filtration, in the pipe line angle at which the flocculant solution enters the slurry must be fixed at the 450 angle to the filter feed pump discharge pipe line where the flocculant mixes with slurry for effective mixing and to reduce the back pressure on the pumps.

In another aspect the flocculant solution is to be pumped using a specific type of progressive cavity flow pump to with stand back pressure of the filter feed pump without any pulsations and more particularly to avoid shearing action on the flocculant chains during pumping for its effective action in black color removal.

In another aspect a flap type non return valve is to be fixed in horizontal position on the discharge pipe line of the flocculant dosing pump as close as possible to the mixing point with the slurry pipe line to avoid any reaction slurry back flow towards flocculant dosing pump and to consistently dose required quantity of the flocculant solution to filter without any interruptions.
In another aspect the choice of the flocculants are selected in such a way that it will not affect the downstream equipment material of construction, particularly have no interactions with the belt filter rubber parts.

In another aspect the fertiliser granules produced with the acid generated by the addition of the flocculants will not have any impact of the black color matter on its appearance in comparison to the color of the fertiliser granules produced with acid produced from low organic rock phosphates.

The examples and embodiments are provided only for the purpose of understanding and none of them shall limit the scope of the invention. All variants and modifications as will be envisaged by skilled person are within the spirit and scope of the invention.