PHOSPHATE ROCK SCRUBBING OF A SULFUR DIXOIDE GAS STREAM

Abstract

This application is directed to the field of scrubbing a sulfur dioxide-laden gas stream by a phosphate rock solution, and then regenerating that sulfur dioxide gas via acidulation, by contacting the phosphate rock solution with sulfuric acid to regenerate a high purity sulfur dioxide stream.

Claims

1. A phosphate rock solution is used to scrub a gas stream containing sulfur dioxide, capturing this sulfur dioxide in the aqueous stream.

2. This sulfur dioxide gas stream emanates from the tail gas stream exiting a sulfuric acid plant.

3. This sulfur dioxide gas stream emanates from a variety of sources, including a coal-fired power plant.

4. The phosphate rock solution which contains the sulfur dioxide as either sulfite or bi-sulfite ions is acidified with sulfuric acid and thus lowers the pH of solution in a controlled manner and thereby releasing sulfur dioxide as a gas stream.

5. This sulfur dioxide gas is then dried and compressed and enters the contact section of a sulfuric acid plant.

6. This highly concentrated sulfur dioxide stream can be fed to a Claus plant where elemental sulfur is produced.

7. As an alternate, this enriched sulfur dioxide gas stream can be admitted to a paper and pulp processing plant.

Description

PREFERRED EMBODIMENT

[0024] A phosphate rock solution with a pH>3 and preferably less than 10 to avoid unnecessary carbon dioxide pickup is added in a scrubber circulating system with pumps, holding tanks, level indicators and the needed equipment to run a scrubber. Fresh phosphate rock solution is continually added to maintain this pH, as well known in the scrubbing industry. Level control continually draws off the sulfur dioxide enriched solution, which is fed into a Phosphate Rock Acidifier which is used in the phosphate fertilizer presently. The pH of that contactor is maintained at 1 to ensure that other stronger acids (such as the halideschlorides. fluorides, and bromides) do not spring free. Any dissolved carbon dioxide will also be present with the sulfur dioxide stream, but this presents very little problem in contact sulfuric acid plants. The highly purified SO.sub.2 enriched which is saturated with water vapor then enters a little SO.sub.2 drying tower and blower which boosts the gas pressure to just beyond the operating pressure of the first pass of the catalytic converter system.

[0025] Actual pH of various Phosphate Rock Solution (PRS) in Brazil are displayed in Table Six shows clearly that the pH falls within this acceptable range.

TABLE-US-00006 TABLE SIX Brazilian Phosphate Rock and Conductivity after 60 Minutes Rock Source Sample ID pH Conductivity Uberaba L-0260411 8.73 94.93 us/cm L-0260412 8.49 94.70 us/cm Catalao L-0260413 9.05 128.47 us/cm L-0260414 8.18 134.50 us/cm

[0026] Figure Five shows the titration curves of these Brazilian phosphate when contacted with 0.05 Molar sulfuric acid solution. The starting pH of this solution is slightly lower than the data reported in Table Six since the solution contains less water at less contact time. However, the samples clearly show that the pH is within the acceptable range for scrubbing sulfur dioxide, since according to the Johnstone curve, there is expected to be no gaseous sulfur dioxide and only aqueous bi-sulfite (HSO.sub.3.sup.) ions or sulfite ions (SO.sub.3.sup.). Finally, this titration curve shows that with careful addition of sulfuric acid, that the PRS (Phosphate Rock Solution) pH can be controlled to precise levels, meaning that if there are any dissolved halides (chlorides, fluorides . . . ) that they can be degassed precisely from the PRS in a controlled manner.

[0027] Figure Six is a simplified block flow diagram of the new Phosphate Rock Solution system, with the new process equipment shown within the dotted lines. The aqueous phosphate rock solution is fed into the phosphate rock scrubber where the sulfur dioxide-laden gas stream is contacted with this slightly alkaline solution, removing virtually all the sulfur dioxide from the gas stream in the form of aqueous sulfite or bi-sulfite ion. It should be remembered that with the exiting gas from a sulfuric acid plant is essentially bone-dry and thus, with this proposed system water leaves the system as a vapor.

[0028] The sulfite and bi-sulfite aqueous Phosphate Rock stream is fed via level control to the phosphate rock acidifier, along with sulfuric acid. This process step of acidifying phosphate rock is a necessary condition for present phosphate fertilizer producers, such as the biggest manufacturing plants of OCP in Morocco and Mosaic in Florida. However, unlike the present acidifiers, where the rock is acidified to much higher levels, with this system much lower levels of acidification are desired to spring the sulfur dioxide loose without liberation of halides. Thus, as shown in Figure Five showing the titration curve of a Phosphate Rock Solution, the pH can be carefully controlled.

[0029] Once the sulfur dioxide is liberated from the Phosphate Rock Solution, it will contain water vapor that is in equilibrium with the aqueous solution. To avoid potential problems with corrosion of downstream equipment, this gas stream must be dried in a little sulfur dioxide drying tower and then compressed via a sulfur dioxide blower to the gas pressure that is needed for the existing sulfuric acid plant's catalytic converter system. Both these pieces of equipment (SO.sub.2 drying tower and SO.sub.2 blower) are sized on the small amount of enriched (>90 mole %) sulfur dioxide which is recycled back to the sulfuric acid plant, which typically contains 10-12 mole % SO.sub.2.

[0030] Alternately, the sulfur dioxide gas stream may come from a source other than a sulfuric acid plant. In this given example, phosphate rock could be shipped to a pollution source that is emitting unwanted sulfur dioxide to the atmosphere, such as a coal-fired electrical power plant. The Phosphate Rock Solution (PRS) that is laden with sulfur dioxide would then be shipped to a phosphate fertilizer company which would regenerate the sulfur dioxide by acidification, contacting the rock solution with sulfuric acid and that enriched sulfur dioxide gas stream would then be fed to their existing sulfuric acid plant.

[0031] Shipping phosphate rock very long distances is presently a commercial fact. For example, a little sulfuric acid plant located in Christchurch, New Zealand processing Moroccan phosphate rock in 2018. Thus, the economics of shipping sulfur dioxide-laden phosphate rock solution safely and processing PRS via the accompanying acidification as a feedstock to the phosphate fertilizer plant may dictate the financial feasibility of this process approach.

[0032] Additionally, the enriched and purified sulfur dioxide stream can also be introduced to other processes outside of sulfuric acid manufacturing facilities, such as Claus sulfur recovery plants (see for example, Michael Heisel's U.S. Pat. No. 5,439,664) or pulp-paper mills that may require this feedstock. Thus, there are many other possible processes which could use a highly purified sulfur dioxide stream, besides a sulfuric acid plant.

[0033] However, the preferred embodiment of this invention is shown in FIG. 6 with the phosphate rock scrubber solution system scrubbing the off-gas from a sulfuric acid plant of a phosphate fertilizer complex. That sulfur dioxide laden phosphate rock solution is then processed by acidification, as previously described.

REFERENCE CITATION

US Patents

[0034] U.S. Pat. No. 3,259,459 Moller et. al. [0035] U.S. Pat. No. 3,477,815 Miller et. al, [0036] U.S. Pat. No. 3,904,735 Atwood et. al. [0037] U.S. Pat. No. 4,795,620 Heisel et. al. [0038] U.S. Pat. No. 5,019,361 Hakka et. al. [0039] U.S. Pat. No. 5,108,731 Schoubye et.al. [0040] U.S. Pat. No. 5,439,664 Heisel et. al. [0041] U.S. Pat. No. 5,851,265 Burmaster et. al.

Literature Citations

[0042] (1) Not applicable. [0043] (2) Rochester, Colin: Acidity Functions; Academic Press; London; New York, 1970 [0044] (3) Schmidt, Max; 1972: Fundamental Chemistry of Sulfur Dioxide Removal and Subsequent Recovery via Aqueous Scrubbing; International Journal of Sulfur Chemistry: Part B; Volume 7; pages 11-19. [0045] (4) Johnstone, H. F. and P. W. Leppla; The Solubility of Sulfur Dioxide at Low Pressures; Journal of the American Chemical Society; Volume 56; November 1934; pages 2233-38.

DESCRIPTION OF FIGURES

[0046] Figure One: Standard Double Absorption-Contact Sulfuric Acid Process Figure Two: Equilibrium SO.sub.2 to SO.sub.3 for a Single Pass Absorption System

[0047] Figure Three: pH of Sulfur Dioxide Species in Solution

[0048] Figure Four: Typical AbsorptionRegeneration per Kosseim

[0049] Figure Five: Titration Curve of Phosphate Rock

[0050] Figure Six: Block Flow Diagram of Two Converter Pass (+PRS) System