A PROCESS FOR PRODUCING CONTINUOUSLY SUPER PHOSPHATE OF HIGH GRADE USING LOW GRAPE PHOSPHATE ROCK

Field of Invention:

The invention relates to a cyclic process for the continuous production of super phosphate, of commercially acceptable quality, substantially conforming to specification of product containing not less than 16% of available phosphate, expressed as P2O5 using phosphate rock of low grade containing less than 26% of phosphate expressed as P2O5.

Background of the invention:

In the conventional methods, super phosphates are generally manufactured as single super phosphate, by acidulation of phosphate rock containing generally not less than 30% of phosphate, expressed as P205 with sulphuric acid. Super phosphates are generally manufactured to use as fertilizers. The Triple super phosphates or concentrated super phosphates are also manufactured by acidulation of rock phosphate with phosphoric acid and extensively used as a fertilizers. However, for several agronomical reasons, single super phosphate is used since it also provides the much-needed sulphur to the crops when used as fertilizer. Since transportation of fertilizer produced thus, is expensive and to provide standards for the product fertilizer, it is generally stipulated in several parts of the world that the final fertilizer obtained from single super phosphate should contain generally not less than 16 to 17% of available phosphate. Therefore, in order to obtain a fertilizer containing super phosphate of grade not less than about 16 to 17 %, a rock phosphate having generally not less than about 30% of phosphate as P205 is required. In some processes, the rock phosphate having phosphate expressed in terms of Bone phosphate of lime or otherwise called BPL of 65, is preferred to produce single super phosphate.

The availability of good grade phosphate rock is diminishing day by day and most of the phosphate rocks of 65-67 BPL are also being used to produce phosphoric acid, thereby the availability of phosphate rock of acceptable grade is becoming scarce for producing super phosphate. However, it must be noted that the phosphate rock of very low grade, containing not more than 26%, or more substantially, between 24 to 26% is available in large quantities, which was not put to any use since it was considered as uneconomical to use in the production of phosphoric acid and Single Super phosphate of acceptable standards.

The reserves of phosphate rock are estimated to last for 130-140 years at present rate of consumption and the availability of high grade-rock phosphate is decreasing. Also, super phosphate is an inexpensive fertilizer and as a source of phosphate in most of the developing countries and the small and marginal farmers are dependent on super phosphate for their farming needs.

Summary of the Invention:

The objective of the present invention is to provide a process for producing continuously high grade super phosphate using low grade phosphate rock, available in large quantities for producing fertilizers.

A further objective of the present invention is to produce single super phosphate containing 16 to 17 % of available P ₂ O ₅ , using phosphate rock containing less than 26% and up to 24% of phosphate content expressed as P ₂ O ₅ .

Another objective of the present invention is to substantially conserve the available resources of phosphate rock of 65 BPL and above for producing phosphoric acid and such other products.

A further objective of the present invention is to provide a process for producing continuously high grade super phosphate, which is simple and cost effective.

Accordingly these objectives are achieved by a process for the manufacture of super phosphate from low-grade phosphate rock, comprising the step of acidulating rock phosphate with concentrated sulphuric acid diluted with a phosphate solution to produce a final product comprising the super phosphate of high grade.

In conventional methods the sulphuric acid used in acidulating step is diluted with water. According to the process of the present invention, while using a low grade phosphate rock as starting material, the water used for diluting the sulphuric acid is replaced with a solution containing phosphate. In a continuous process, the phosphate solution is continuously added together with the low-grade phosphate rock and sulphuric acid, which results in a final product with acceptable quality of super phosphate. This addition of phosphate solution offers advantages over the manufacturing processes known in the art .

Moreover, the process according to the present invention does not call for producing phosphoric acid to enhance the phosphate value of the product and therefore, it is considerably economical. While it is known to the prior art that the P ₂ O ₅ of super phosphate can be increased by addition of phosphoric acid to the reactants, the present invention obviates the necessity to produce phosphoric acid, which is very expensive and since the final product obtained by the process according to the present invention retains higher sulphur content, which is another important nutrient to the plants and therefore highly suitable to use as fertilizer.

The process according to the present invention also provides for producing acceptable quality of super phosphate without costly additions of expensive equipment.

Detailed description of the preferred embodiments:

The process according to the present invention is carried out using conventional equipment by acidulating ground rock phosphate with concentrated sulphuric acid diluted with a phosphate solution obtained by recovering mono calcium phosphate and small quantity of phosphoric acid from a part of the super phosphate produced. The phosphate solution is recovered from the super phosphate in the final product by agitating the solid super phosphate in a recycled wash solution and by filtration.

The phosphate rock containing phosphate expressed as P ₂ O ₅ of less than about 26% is ground to adequate fineness. The phosphate rocks are preferably ground to a fineness such that 90% of the ground phosphate rock passes through 100 tyler mesh. The ground rock is fed into a mixer. The mixer may be of the type, which is generally used to produce single super phosphate. In a preferred embodiment, a metering device is used to feed accurate or near accurate quantities of ground rock prepared into the mixer. A predetermined quantity of sulphuric acid, preferably of about 98% concentration is added into the mixer. A solution of phosphate containing phosphate preferably expressed as P2O5 of about 6 to 12% is added into the mixer. The solution of phosphate is also metered in predetermined quantity before it is fed into the mixer. The rock phosphate, sulphuric acid and the solution of phosphate may be added separately or simultaneously into the mixer.

The mixture containing the rock phosphate, sulphuric acid and the solution of phosphate are allowed to thoroughly and intimately mix to form a semi solid mixture. The

semi solid mixture is known as green super is dumped in to the storage for curing, after it passes through a mechanism called DEN, where it is retained for a time period of about between 25 minutes to one hour.

The steps of mixing the starting mixture, allowing the semisolid mixture to be retained in a DEN and dropping into the storage tank are carried out in a known manner.

The semi solid product, called the green super, is allowed to cure over a period of time, generally for about one week to several weeks.

A portion of the final cured product is made into as slurry. The slurry produced thus, is filtered preferably using a pressure filter or any other equipment normally used to filter slurries, to recover soluble phosphates from the slurry and the filtrate is collected in a storage tank.

The cake obtained in the above filtration is washed with fresh water or raw water to recover all soluble phosphate from the cake and the cake discarded as gypsum. The wash solution from the filtrate step is recycled to prepare the phosphate solution. ( to form wash solution) . The cake obtained is substantially gypsum, which is discarded from the process and used in the manufacture of cement, bricks, agricultural purpose etc. By removing gypsum from the total quantity of gypsum produced in the reaction between phosphate rock and sulphuric acid, the net phosphate value is thus enhanced.

The recovered super phosphate after separation is tested for quality compliance and bagged as finished super phosphate.

In a preferred embodiment, the phosphate solution to be introduced in the mixer is prepared by removing a part of the cured product. Preferably 15% to 30% by weight of the product from the storage is withdrawn and fed into an agitated vessel where it is

intimately mixed with wash solution obtained from filtrating step carried out to recover the final product. The mixed slurry is filtered and the filtrate containing the phosphate solution is recycled to the mixer together with rock phosphate and sulphuric acid as described herein above .

In a preferred embodiment, the phosphate solution may be prepared separately and introduced into the mixer containing the rock phosphate and sulphuric acid.

The process steps are easy to carry out and further more, in addition only an agitation and filtration unit need to be added to the existing plants for manufacturing the super phosphate from low grade rock phosphate according to the present invention.

The chemical formulae for the reactions are:

2Ca3(P04)2 + 6H2S04 — * 6CaSO4 + 4H3P04.

Ca3(P04)2 + 4H3P04— * 3Ca(H2P04)2

Thus:

3 Ca3(P04)2 + 6H2S04-) 6CaSO4 + 3 Ca (H2P04)2

When a solution containing a mixture of free phosphoric acid (H3P04) and mono calcium phosphate (Ca(H2P04)2) is added, the solution is immediately converted to phosphoric acid and gypsum and the action on rock phosphate in the mixer, is similar to the equations mentioned above. Thus, the excess of mono calcium phosphate added through the solution, finally after the curing is completed, adds to the total mono calcium phosphate available in the final product, thereby increasing the total available phosphate value on the product.

In a preferred embodiment, the recovery of soluble phosphates in the invention may be maintained close to about 96-98% by installing high efficiency filters, with capability to wash the filter cake effectively. The process efficiency also depends on the overall conversion achieved in the mixer operation.

With reference to the accompanying drawing, in which:

FIGURE 1 shows the flow diagram illustrating the process according to the present invention.

1. The rock phosphate containing the phosphate expressed as P205 of less than

26% is ground to required fineness such that 90% of the ground phosphate rock passes through 100 tyler mesh. The ground rock phosphate rock, sulphuric acid and a solution of phosphate are fed into a mixer to form a semi solid mixture.

2. The semi solid mixture containing ground rock phosphate rock, sulphuric acid and a solution of phosphate are passed into through a DEN mechanism, wherein it is retained for a specific period of time.

3. The mixture is then passed into a storage tank where it is allowed to cure to form the final product.

4. A part of the cured product is separated and fed into an agitator vessel.

5. In the agitator, the separated portion is mixed with a recycle solution obtained from the step of filtering the cured mixture to form a phosphate slurry.

6. The phosphate solution is filtered and recycled to the mixer.

Thus, the invention described above can be deployed with considerable ease to produce super phosphate. The quantity of product that needs to be recycled, the concentration of the phosphate solution that should be recycled depends on the quality of product desired and can be easily calculated by any person who is generally familiar with the process of manufacture of super phosphate.

By a certain variation or modification of the invention described above, it is possible to improve the recovery efficiency of the process by addition of a small quantity of

sulphuric acid together with wash solution obtained. This will reduce the P205 losses in the cake.

Thus it can be seen that phosphate rock of very low grade can be used to produce single super phosphate with considerable ease and practically using the same equipment in which super phosphate is produced, by mere addition of a few more equipment.

The present invention will now be illustrated by means of the following non-limiting example

EXAMPLE

1. 250 gms of Rock phosphate containing 25.7% of P2O5 on dry basis is ground to a fineness of 90% passing through 100 Tyler mesh.

2. 150 gms of previously produced super phosphate on dry basis, with 8.8% moisture containing 17% water soluble P2O5 on dry basis is mixed with 260 gms of water, thoroughly agitated for 30 minutes and filtered. The filtrate weighed 206 gms with 9.66 % of P2O5.

3. The wet cake is washed with lOOgms of water, and then again with 75 gms of water yielding a total of 188 gms of filtrate containing 2.445% of P2O5, thereby recovering 4.5966 gms of P2O5 through wash and 19.8996 gms of P2O5 in the primary filtration. The efficiency of the recovery is 96%.

4. The ground rock phosphate from step-1 is added with 151 gms of concentrated sulphuric acid and 81 gms of filtrate obtained from step-2. The addition is done simultaneously and the mixture intimately mixed vigorously for 30 minutes and allowed to dry.

5. The product obtained was analysed and the results obtained after elapse of 5 days is given below:

Moisture: 10.9% Free acid expressed as P2O5 on dry basis: 5.4%

Water soluble P2O5 on dry basis: 17.98%

Available P2O5 (NAC soluble) 18.91%

Total P2O5 on dry basis: 19.87%

The above results conform to the standards laid down for super phosphate fertilizers.

The product also has excellent mechanical properties, uniform powder, continued low moisture and exhibited very low free acidity.

The conversion efficiency on the basis of total P2O5 in the super phosphate and neutral ammonium citrate soluble P2O5, generally known as available P2O5 is more than 95%.

The above results conform to the standards laid down for super phosphate fertilizers. The product also has excellent mechanical properties, uniform powder, continued low moisture and exhibited very low free acidity. The conversion efficiency on the basis of total P205 in the super phosphate and neutral ammonium citrate soluble P205, generally known as available P205 is more than 95%.