IL29253A

IL29253A - Purification and neutralisation of raw or crude gases obtained in the production of phosphorus or phosphoric acid

Info

Publication number: IL29253A
Application number: IL2925368A
Authority: IL
Original assignee: Ruhrchemie Ag
Priority date: 1967-01-26
Filing date: 1968-01-01
Publication date: 1971-08-25
Also published as: NL6800999A; FR1555936A; DE1667694B1; CH500751A; NL136380C; BE709530A; ES349572A1; GB1213158A; AT283269B

Abstract

1,213,158. Gas scrubbing process. RUHRCHEMIE A.G. 28 Dec., 1967 [26 Jan., 1967], No. 58863/67. Heading B1R. Gases from the sintering or wet-processing of phosphate rock enter a U-shaped gas scrubber at 11 and pass downwardly through limb 1 containing axially disposed spray nozzles 2 and then pass upwardly through limb 3 and are given a rotary motion 'by vanes (not shown) on the surface of an axially disposed body 4 above which a liquid spray 5 is arranged. Scrubbed gases are withdrawn at 12 after passing through a droplet eliminator 13 and sludge-containing liquid is withdrawn at 10. Liquid may also be introduced into limb 1 at 16 to prevent accumilation of dust adjacent the inlet. Calcium hydroxide may be added to the scrubbing liquid for the removal of fluorine compounds from the gases and the amount of added hydroxide is such that the pH of the liquid separated in eliminator 13 is about 8.

Description

29253/2 Purification and neutralisation of raw or crude gases obtained in the production of phosphorus or phosphoric acid ROHRCHME AKTIENGISELLSCH&FT The invention relates to the purification and neutralisation of raw or crude gases obtained in the production of phosphorus or phosphoric acid and provides both a process and an apparatus for carrying out the process.

The invention involves a novel use of the device, a so-called "spin-tube washer", for washing and wet-dedusting of gases which is described in Belgian Patent Specification No. 577 , 454. This spin-tube washer consists of or comprises a vertical cylindrical tube having centrally disposed therein a cylindrical body, closed at each end by a conical member and provided with helically arranged guide vanes, and having a lower gas inlet and an upper gas outlet and a device for the introduction of a washing liquid at its upper or lower end.

The rate of flow of the gases which are passed upwardly through the eaid device is chosen so high that the wash liquid is dammed up and maintained in a suspended state in the annular space between the inner cylindrical body and the inner wall of the vertical cylindrical tube whereby it is given a turbulent rotary motion.

Due to the high relative velocity between the liquid and the gaseous phases and the high turbulence which may thereby be obtained, extremely intimate contact between the gas and the liquid is achieved, so that the impurities to be removed from the gas are taken up by the liquid with high efficiency.

It has now been found according to the invention that the said device can be used with particularly advantageous results for the purification of dust-and fluorine-containing gases liberated during the sintering of phosphate rock in or for the electrothermic production of elemental phosphorus as well as in the production of phosphoric acid by the wet process from phosphate rock.

Hitherto, appreciable difficulties have been encountered in carrying out the purification of the said gases containing fluorine in the form of hydrogen fluoride and silicon tetrafluoride.

In accordance with the invention, satisfactory purification of such gases has for the first time been made possible by means of the spin-tube washer. The fluorine content of the starting raw gas may be as much as 400 mg/m^.

The invention further provides for the precipitation of at least part of the dust entrained in the raw gas to be purified, by feeding a wetting liquid into the said gas with the aid of at least one nozzle or like member, suitably while the gas is passing through a tube arranged before the spin-tube washer and connected in series therewith. Neutralising agents, for example calcium hydroxide (Ca(0H)2) in the form of milk of lime, may be added to the washing liquid introduced into the spin^-tube washer as well as to the wetting liquid introduced into the said tube connected in series with the spin-tube washer. This step of neutralisation is suitably adjusted in such manner that in the following separator a pH-value of about 8 is achieved. It is then unnecessary to construct the washing device from acid-resistant material and it can be manufactured from commercially available carbon steel.

Advantageously, the washing liquid exiting from the spin-tube washer can be recirculated and passed to the spray nozzles arranged in the tube, suitably a downcomer, disposed before and in series with the washer. Furthermore t a part of the washing liquid or sludge water respectively drained off downstream of the downcomer may be recirculated to the said nozzles. Due to this measure, the added amount of lime water necessary for the neutralisation is better utilized „ In the device according to the invention used for the performance of the novel process, a vertically arranged downcomer equipped with a number of axially superposed spray nozzles is provided for the precipitation of a part of the dust contents of the gas. The downcomer is provided upstream of the spin-tube washer, the latter serving for the washing and neutralisation of the gas itself. The raw gas is passed downwardly through the downcomer and upwardly through the spin-tube washer. The wetting liquid is coourrently sprayed into the inflowing gas through the nozzles. Caking of dust in the zone between the gas outlet from the raw gas conduit and the gas inlet in the downcomer may be prevented or hindered by a (further) addition of water, for example via a tangential inlet, by spraying or by means of an overflow baffle. A drain for the discharge of sludge water is provided at a position below the downcomer an the spin-tube washer.

One construction of apparatus according to the invention is illustrated by way of example and in longitudinal, schematic section in the accompanying drawing.

The dust-laden raw gas is introduced" into downcomer 1 through a conduit 11 provided with a rinsing or purging equipment 16. A number of spray nozzles 2 are superposed along the longitudinal axis of the downcomer 1 and lime water (milk of lime) is fed as wetting liquid to the nozzles 2 from a conduit 8. The amount of wetting liquid added per unit of time is controlled by a measuring device 6 (rotameter), Downcomer 1 is connected to a spin-tube washer 3 by a conduit 15 which at its lowest point is provided with an outlet 10 for sludge water. Disposed axially in known manner within the elongated spin-tube washer 3 is a cylindrical body 4 having conical covers at its upper and lower ends. For the sake of clarity, the body 4 is reproduced without the guide vanes (see Belgian Patent Specification No. 577,454 or G-erman Auslegeschrift No. 1,193,477). Washing liquid also containing milk of lime is passed through a conduit 9 to an inlet tube 5 which opens centrally above the apex of the upper cone cover of the insert or body 4. The amount of washing liquid fed in per unit of time is controlled by a measuring device 7 (rotameter).

Sludge water obtained in spin-tube washer- 3 is also discharged through the outlet 10. The neutralised, pure gas is drawn off through a conduit 12 after having passed water separator 13 from which accumulated water is taken off through a drain-line 14.

The process according to the invention is illustrated by the following examples. The terms given below and used in the Examples, have the following meanings; Z^p designates the pressure difference, determined over the whole system (downcomer, spin-tube washer) measured in mm water column pressure.

The fluorine content is the sum of the contents of fluorine ions in the form of hydrogen fluoride HF and silicon tetrafluoride Sii^ in the raw gas.

The gas volumes expressed in m represent the respective amounts of gas measured under the prevailing pressures and temperatures. These statements are important for the evaluation of the throughput efficiency of the device.

The dust originating in the phosphate sintering plant and separated from the gas in the device hereinbefore described, had the following composition: O,- 34.0$ by weight (in form of calcium phosphate) Si02 8.5$ by weight Pe205 1.3$ by weight A1203 1.2$ by weight GaO 47.0$ by weight Na20 0.6 by weight K20 0.4$ by weight F 3.7$ by weight gas The temperature of the raw/ charge was between about 90°C and 120°C, whilst that of the purified or treated gas was between about 50°C and 60°C.

The purification process was carried out in a single-flooded counter-current spin-tube washer, hairing an outer diameter of 800 mm and an inner diameter of 500 mm (inner diameter of outer vertical tube = 800 mm; outer diameter of the inner cylindrical body 4 = 500 mm). The angle of pitch of the guide vanes with respect to the horizontal was 30°. The annular clearance between the outer edge of the guide vanes and the inner wall of the vertical tube of spin-tube washer 3 was about 10 mm. The downcomer connected in series with the washer 3 was equipped with 5 nozzles.

The raw gas was introduced at a rate of the water was fed to the downcomer at a rate of about 1,500 litres/hr? and to the spin-tube washer te of about 1,000 litres/hr . ^p had a value of 190 mm water column Pure or tre Baw The following fluorine 363 contents were determined The dust content amounted to: 2.16 g/mr 0..09 g/nr5 Example 2 Single-flooded spin-tube washer 1000/600 mm 0, angle of pitch of the guide vanes 23°, annular clearance 15 ram, downcomer with 8 nozzles. a) Amount of raw gas = 15,500 - 17,500 nr/hr Amount of waters- downeomer 8,000 litres/hr spin-tube washer 3,000 litres/hr ^ p 180 - 195 Ma water column Raw gas Treated. as Fluorine content; 95 - 114 mg/m^ 4 - 6 mg/m^ Dust content; 0.66 - 0.90 g/m5 0.05 - 0.10 /m^ b) Amount of raw gas = 25,500 - 27,500 x / r.

Amount of water%- downcomer 8,000 litres/hr spin-tube washer 1,000 litres/hr z v 290 - 33 mm water column Raw gas Treated gas Fluorine content: 40 - 92 mg/m 5 - 6 mg/m Dust content 0.65 - 0.68 g/rn^ 0.07 - 0.08 g/m5 Example 3 Single-flooded spin-tube washer 1200/500 mm 0, angle of pitch of the guide vanes 30°, annular clearance 25 mm, dotmcomer with 8 nozzles, a) Amount of raw gas = 39,000 - 43,000 m^/hr Amount of water: - downcomer 8,000 litres/hr spin-tube washer 1,000 litres/hr p 175 - 210 mm water column Raw gas Treated gas Fluorine content 53 - 59 m /m^ 2 - 3 m /m^ Dust content 0.73 - 0.99 /m^ 0.03-0.05 g/m5 b) Amount of raw gas = 36,000 - 40,000 ir /hr Amount of water; - downcomer 4,000 litres/hr spin-tube washer 2,000 litres/hr ^p 180 - 215 mm water column Raw gas treated gas Fluorine content; 23 - 31 mg/nr 2<:.mg/nr Dust content: 0.68 g/m5 0.07 /m5 c) Amount of raw gas = 39,000 - 43,000 m5/hr Amount of water; - downcomer 4,000 litres/hr spin-tube washer 1,000 litres/hr £^ 185 - 210 mm water column Raw Gas Treated gas Fluorine content; 45 - 68 mg/m 2 - 6 m /m^ Dust content; 0.90 - 1.10 g/rn5 0.01 - 0.02 g/ In Example 2, the term 1000/600 mm 0 denotes that the inner diameter of the outer tube of the washer 3 was 1000 mm and that the outer diameter of the inner cylindrical body 4 was 600 mm. The term 1200/500 mm 0, used in relation to the washer 3 employed in Example 3 has a corresponding meaning. - 11 - 29253/2

Claims

1. A process for the purification of dust and fluorine-compounds containing waste gases of the electrothermic p o-^ duction of elementary phosphorus from phosphate rock or of the wet-process production of phosphoric acid, wherein the gases are conducted first through a downcomer prewashing tube and then through a spin-tube washer, and a ne tralizing agent, e.g. milk of lime, is admixed to the washing water in the prewashing tube and/or the spin- ube washer.

2. A process according to Claim 1, being applied to waste gases in which the fluorine compounds are hydrogen fluoride and/or silicon tetrafluoride.

3. A process according to Claims 1 or 2, being applied to the purification of waste gases whose fluorine content does not exceed 400 g/sr.

4. A process according to any of Claims 1 to 3, wherein at least part of the dust is removed from the waste gas by spraying a wetting liquid into the gas in the downcomer.

5. A process according to Claim 4, wherein the neutralizing agent is added to the wetting liquid sprayed into the downcomer tube.

6. A process according to any of the preceding claims wherein so much neutralising agent is used that the pH of the liquid leaving the spin-tube washer is about 8.

7. A process according to any of the preceding claims, wherein the gas is washedjin the downcomer tube with recir- - 12 - 29253/2

8. Δ process according to Claim 1, substantially as hereinbefore described xfith reference to the Examples.

9. A gas scrubbing apparatus for carrying out the process according to any of Claims 1 to 8, comprising a downcomer provided with a plurality of spray nozsles disposed in spaced relation on the longitudinal axis of the downcomer¾ and a spin-tube washer the inlet or lower end of which is connected to the outlet end of the downcomer.;

10. An apparatus according to Claim 9> wherein a vertically arranged downcomer tube, having a gas inlet at the top and a gas outlet at the bottom, is provided with axially superposed spray nozzles, and the outlet of the downcomer is connected to, the bottom of the spin-tube washer through which the gas flows upwardly.

11. An apparatus according to Claims 9 or 10, including means for the introduction of water, additional to tie water sprayed into the downcomer, into the zone between a waste gas feed conduit and the gas inlet of the downcomer.

12. An apparatus according to Claim 11, wherein the means for the introduction of additional water comprise a tangential water inlet, a spraying device or an overflow baffle or weir.

13. An apparatus according to any of Claims 9 to 12, including a sludge drain situated below the downcomer tube in the connection between the latter and the spin-tube washer. 13 - 29253/2

14. A gas scrubbing apparatus, substantially as hereinbefore described with reference to the accompanying drawing. For the Applicants PC/rb