Description:DESCRIPTION:
Field of the invention:
[0001] The present disclosure generally relates to the technical field of neutralization of phosphogypsum, in specific relates to a method for neutralization of phosphogypsum by utilizing industrial solid waste for use in a composite base lining system with suitable permeability to meet pollution standards.
Background of the invention:
[0002] Phosphogypsum is an essential calcium sulphate, it is generated as waste from the phosphoric acid plants by the reaction of rock phosphate with sulfuric acid. It is a huge volume, gray-colored, silt-sized, acidic powdered material. The high fluorine content and presence of trace elements and heavy metals makes it harmful. Phosphogypsum, a waste product from manufacturing fertilizer, emits radon, a radioactive gas. Most of the naturally-occurring uranium, thorium and radium found in phosphate rock ends up in this waste.

[0003] For every ton of phosphoric acid produced, approximately 4-5 tons of phosphogypsum is generated. Worldwide, more than 200 million tons of phosphogypsum is generated annually. Earlier, the waste generated was disposed into the water bodies, contaminating water and effecting marine life. But with the emergence of laws and regulations, the disposal of phosphogypsum into water bodies is banned. This led to huge stockpiling of phosphogypsum on the ground surface. However, in some European countries, especially Germany, the phosphogypsum is dumped into the excavated areas or unused parts of the mines.

[0004] In India, the central pollution control board (CPCB) recommends disposal of phosphogypsum on the ground with a composite base lining system. The upper part of the base lining system consists of compacted and neutralized phosphogypsum with a coefficient of permeability, k≤10⁻⁴cm/s. Most phosphoric acid and fertilizer plants neutralize the phosphogypsum with lime in a proportion of 3-12% depending on the acidic nature of the phosphogypsum. Also, the permeability criteria of the neutralized layer have to be satisfied.

[0005] Therefore, in the present invention phospogypsum is neutralised using an alkaline industrial waste product along with lime so as to reduce the cost of neutralization.
Objectives of the invention:
[0006] The primary objective of the invention is to neutralize the phosphogypsum using an alkaline i.e.., fly ash along with lime so as to reduce the cost of neutralization.

[0007] Another objective of the invention is to satisfy the permeability requirement of the upper layer i.e., k≤10⁻⁴cm/s as suggested by the central pollution control board (CPCB).

[0008] Further, yet another objective of the invention is to minimize the neutralization cost by less usage of lime.
Summary of the invention:
[0009] The present disclosure proposes a method to neutralize the phosphogypsum using fly ash and lime. The following presents a simplified summary in order to provide a basic understanding of some aspects of the claimed subject matter. This summary is not an extensive overview. It is not intended to identify key/critical elements or to delineate the scope of the claimed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later.

[0010] In order to overcome the above deficiencies of the prior art, the present disclosure is to solve the technical problem to provide a method of neutralizing the phosphogypsum by utilizing another solid waste.

[0011] Neutralizing phosphogypsum involved mixing 10g (phosphogypsum and neutralizing material) of the sample in 100 ml of distilled water. The sample is thoroughly mixed and left undisturbed for 24 hours. The pH of the supernatant is measured after 24 hours using a pH meter. Lime mixed phosphogypsum is mixed with varying proportions of fly ash to study the increase in pH values. The amount of lime added to phosphogypsum is varied from 0.5% to 2% by weight. The values of pH of lime treated phosphogypsum with addition of fly ash are presented in Table 1.

[0012] The phosphogypsum waste is neutralized by adding the 10 to 50 weight percentage of a fly ash and 0.5 to 2 weight percentage of lime. By the consideration of pH values the fly ash ranges from 11.23 to 12.65, the lime ranges from 7 or above and the phosphogypsum ranges from 2.47 to 5. The neutralization process has accomplished in the upper part of base lining system by using the dumping yards and the system suggested by central pollution control board of India (CPCB).

[0013] Further, objects and advantages of the present invention will be apparent from a study of the following portion of the specification, the claims, and the attached drawings.
Detailed description of drawings:
[0014] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, explain the principles of the invention.

[0015] FIG. 1 illustrates a schematic view of a base lining system for phosphogypsum stack suggested by central pollution control board (CPCB) in accordance to an exemplary embodiment on the invention.

[0016] FIG 2 illustrates a plot made to fix the amount of lime and utilize the fly ash to the maximum extent possible.
Detailed invention disclosure:
[0017] Various embodiments of the present invention will be described in reference to the accompanying drawings. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps.

[0018] The present disclosure has been made with a view towards solving the problem with the prior art described above, and it is an object of the present invention to prepare a method for neutralization of phosphogypsum waste by utilizing industrial solid waste.

[0019] According to an exemplary embodiment of the invention, FIG. 1 refer to a single composite liner system 100 suggested by the central pollution control board (CPCB). Phosphogypsum is the calcium sulphate hydrate formed as a by-product of the production of fertilizer from phosphate rock. It is mainly composed of gypsum. Although gypsum is a widely used material in the construction industry, phosphogypsum is usually not used, but is stored indefinitely because of its weak radioactivity caused by the presence of naturally occurring uranium and thorium.

[0020] Phosphogypsum waste is a grey coloured, fine grained powder, acidic powdered material and silt sand material with a maximum size range of 0.5mm to 0.075mm. The agriculture sector uses large amount of chemical fertilisers to replenish and supplement the nutrients that growing plants takes from the soil. The high fluorine content and presence of trace elements and heavy metals categorized phosphogypsum as a harmful material.

[0021] Majority of the phosphoric acid plants in the country are based on wet process (di-hydrate) for production of phosphoric acid. Neutralize the phosphogypsum waste by added to the lime and fly ash, which is protect from the environment and humans. The collection of the lime and the fly ash is waste product of industrial alkaline, it contains the high base structure to stabilise the acidic structure.

[0022] In one embodiment herein, different proportions of 10 to 50 weight percent of the fly ash and 0.5-2% weight percent of lime are added to the phosphogypsum waste to neutralize the phosphogypsum. The pH values of phosphogypsum waste is varies between 2.47 and 5, the pH value of the lime varies from 7 or above and the pH value of the fly ash varies from 11.23 to 12.65.

[0023] Point ctc the recommends a composite base lining system for disposal of phospogypsum as most of the industries directly dump the waste into the water bodies or ground surface without any base lining which has negative assets on human health and the environment especial during the rainy season as the rate infiltration is even more.

[0024] Table 1
Fly ash
(%) Lime content (%)
0.5 1 1.5 2
10 3.1 4.52 5.61 6.88
20 4.1 5.79 5.82 6.9
30 4.3 5.88 6.14 7.22
40 4.4 6.32 6.43 7.7
50 4.7 6.41 6.58 7.72

Table 1 represents the values of pH of lime treated phosphogypsum with addition of fly ash

[0025] From the table, it can be stated that for a given percentage of lime, as the percentage of fly ash increases, the pH also increases. Also, for a given percentage of fly ash, the pH value of the sample increased with the increase in the percentage of lime. A plot 200 is made to fix the amount of lime and utilize the fly ash to the maximum extent possible as presented in FIG. 2. The permeability of neutralized phosphogypsum in compacted state is determined to assess its suitability for use in upper part of the composite liner system. The compaction characteristics are determined using IS Heavy Compaction test as per IS 2720 Part 8, 1983 (IS 2720: Part 8, 1983). The coefficient of permeability of the phosphogypsum is found from the variable head permeability test (IS 2720 Part 17, 1986). The Compaction and Permeability characteristics of phosphogypsum neutralized with fly ash and lime are presented in Table 2.

[0026] Table 2 represents compaction and permeability characteristics of phosphogypsum neutralized with 50% Fly ash and 1.6% lime. From the Table 2, it can be noted that phosphogypsum neutralized with 50% fly ash and 1.6% lime is has permeability greater that 10-6 m/s. Hence, it can be used in the upper part of the composite liner system.
Property Value
Compaction characteristics
(i) Optimum moisture content (%)
(ii) Maximum unit weight (g/cc)
23
1.32
Permeability(m/s) 3.99x10-7

[0027] The user found optimal usages of fly ash and lime for neutralizing the phosphogypsum waste are 50% and 1.6% respectively. Based on the graphical representation, the pH value of phosphogypsum is neutralized with the lime content to get a permeability of 3.99x10⁻⁵cm/s to satisfy the complete neutralization by the CPCB, India.

[0028] Numerous advantages of the present disclosure may be apparent from the discussion above. In accordance with the present disclosure, a method of neutralization of phosphogypsum waste with 50% fly ash and 1.6% lime of industrial alkaline waste. The purposed method is using the wet process to reduce the cost efficient compared to the other process. The purposed method is neutralization process can be held in the upper part of the base lining system.

[0029] It will readily be apparent that numerous modifications and alterations can be made to the processes described in the foregoing examples without departing from the principles underlying the invention, and all such modifications and alterations are intended to be embraced by this application.
 
, C , C , Claims:CLAIMS:
I / We Claim:
1. A method of neutralization of phosphogypsum for use in a base-lining composite system, comprising:
adding 50 percent by weight of fly ash to the phosphogypsum and 1.6 percent by weight of lime to the phosphogypsum;
whereby the phosphogypsum is neutralized by adding the industrial alkaline waste by-product and the lime and has a permeability of 3.99 x 10-5 cm/s.
2. The method of neutralization of the phosphogypsum as claimed in claim 1, wherein the industrial alkaline waste by-product is fly ash.
3. The method of neutralization of the phosphogypsum as claimed in claim 1, wherein the weight percent of the lime used is reduced by adding 10 to 50 weight percent of the industrial alkaline waste by-product to the phosphogypsum.
4. The method of neutralization of the phosphogypsum as claimed in claim 1, wherein the 1.6 weight percent of the lime is added to the phosphogypsum to provide optimum neutralization.
5. The method of neutralization of the phosphogypsum as claimed in claim 1, wherein the neutralization of the phosphogypsum is achieved in an upper part of the base-lining composite system of phosphogypsum dumping yards.