CH627663A5 - Process for the purification of residual gases containing water-soluble metal chlorides - Google Patents

Description

This invention relates to the treatment of residual gases, and more particularly to the treatment of residual gases from the production of aluminum chloride, containing small amounts (15% or less) of HCl, COCl2 and vaporized metallic chlorides capable of hydrolyzing. in HCl and corresponding metal oxides.

Halogenated gases and vapors, such as phosgene (COCl2), HCl, silicon tetrachloride, titanium tetrachloride, iron chloride and aluminum chloride, as well as Cl2 present in the residual gas of a formation reactor aluminum chloride,

must be removed from gases before they are released into the atmosphere, even if present at low values, 15% or less. Although it is known, for example, that silicon tetrachlorides react with water to form a gelatinous precipitate commonly called silica gel, this reaction is not indicated and, in fact, should be avoided due to the clogging of pipes and containers which may occur by the formation of this gelatinous precipitate.

US Patent No. 1451399 indicates that if silicon chloride is contacted with a jet of water vapor at a temperature above the dewatering temperature of silicic acid, the silicon chloride will be hydrolyzed by forming silica as well as hydrochloric gas according to the reaction:

SiCl4 + 2 H20 - »4 HCl + Si02

However, in actual practice, it has been found that this transformation at usual temperatures of water vapor of about 100 ° C gives slow reaction rates.

R.F. Hudson, in "The Vapor Phase Hydrolysis of Non-Metallic Chlorides", published in volume 11 of the "International Congress of Pure Applied Chemistry London Proceedings" in 1947, indicates that this reaction must be carried out at markedly higher temperatures. He indicates that Daubree notes that the silicon tetrachloride of water vapor reacts to red heat in the presence of oxygen, giving highly crystallized silica. Hudson then indicates that, since oxygen and silicon tetrachloride do not react until higher temperatures are used, this means vapor phase hydrolysis at temperatures of the order of 700 ° C. Hudson then verified this experimentally by performing experiments in the temperature range of 25 to 100 ° C where apparently no vapor phase hydrolysis of silicon tetrachloride occurs without at least the presence of a small deposit of what he called silicon. Hudson then indicates that he is actually observing the reaction for obtaining highly crystallized silica at a temperature of 400 ° C.

It is also known to produce metal oxides from metal halides in fluidized beds. For example, US Patents Nos. 3043657,3043659 and 3043660 relate to the production of metal oxides, such as titanium dioxide or silicon dioxide, by reacting the corresponding chloride with oxygen or air in a fluidized bed at temperatures of 500 ° C or more. US Patent No. 3642441 describes the reaction of metal chlorides with water vapor in a fluidized bed. However, the bed again operates at an elevated temperature of 600 ° C or higher using the combustion of a gas such as propane in the fluidized bed.

Although these reactions in fact solve the problem of using metal chlorides, such as silicon chloride, without the formation of undesirable gelatinous products, the temperatures proposed for carrying out the reactions make these processes uninteresting from an economic point of view. We obviously know that these references are intended for the production of these oxides from concentrated chloride vapors rather than the purification of gases containing minor amounts or traces of chlorides. It is therefore an object of this invention to provide a process for purifying residual gases comprising halogenated gases which can be hydrolysed by transformation of the gases by forming solid metal oxides by hydrolysis without excessive use of heat. These objects and other advantages of the invention will be apparent from the description and the drawing.

According to the invention, residual gases containing 15% by weight of gaseous halogenated compounds comprising metal chlorides are treated, in order to transform these metal chlorides which can hydrolyze into oxides and HCl, by bringing the gases into contact with vapor d water at a temperature of 100 to 150 ° C while passing the gases through a bed of solid particles chosen from the category comprising carbon, metal oxides of groups IIA, IIIA, IVA, IVB and their mixtures, then bringing the gases into contact with liquid water at a temperature of 20-100 ° C to remove the HCl vapors.

In a preferred embodiment, the gases are first passed through a dust filter and then through a bed of activated carbon to remove at least some of the halogenated compounds. A purifying gas is then passed through the bed of activated charcoal to remove the halogenated compounds which have deposited there. The purifying gas containing the metal chlorides is then put in

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contact with water vapor in the presence of particulate carbon, metal oxides of groups IIA, IIIA, IVA, IVB and their mixtures, as described above. The HCl formed is absorbed in liquid water, forming an aqueous solution of hydrochloric acid.

The gases are then passed through a bed of activated charcoal in the presence of water to hydrolyze all of the COCl2 from the gases into HCl and C02. The gases are then further purified to remove the sulfur oxides and passed through an incinerator to oxidize CO to CO2.

The only drawing is a diagram illustrating the residual gas treatment system according to the invention.

Now consider the drawing. The residual gases in 2, which may be the effluent gases from a chlorination system in which metal chlorides are formed, first pass through line 4 in a dust filter 6, comprising a filter material in canvas or in suitable ceramic, for example wool, capable of removing particles as small as 10 µ. The gases then pass either through a line 10 or through a line 12, into an adsorption apparatus 20 or activated carbon. The gases pass through line 12 if the gases contain more than 1000 ppm of Cl2. The gases in line 12 pass through a chlorine transformer 14 which is not part of this invention.

The carbon adsorption apparatus 20 contains activated carbon preferably having a particle size of 2.38 to 4.76 mm. The gases pass through the adsorption apparatus 20 at a speed of 6 to 18 m / min to have a residence time in the adsorption apparatus of approximately 3 s. To ensure maximum adsorption of metal chlorides, such as for example aluminum chloride, silicon chloride, or titanium chloride, contained in the gas stream, a second carbon adsorption apparatus of similar dimensions is preferably used. 22 in series with the adsorption apparatus 20 and connected to it by a pipe 16.

Line 18 conducts the gases, stripped of their metal chloride content, to an HCl absorption device 30 which is maintained at a temperature of 20-100 ° C. Preferably, a series of identical HCl absorption apparatuses 30, 40 and 46 are used, as shown in the drawing, to remove HCl from the gas. The HCl 30 absorption device can be a packed bed made of a polypropylene liner for towers like Maspac (manufactured by Clarkson Controls and Equipment Company). The HCl absorption device 30 can also be of any other suitable design, for example tray columns, which facilitates gas-liquid contact. Water or weak hydrochloric acid (0-20% HCl) is introduced at the top of the absorption device 30 via line 32 and it circulates against the flow of gas, this which removes HCl from gases. The water or the acid entering the line 32 can be supplied by the absorption devices 40 and 46 or by an external source. HCl is removed via line 34 in the form of an aqueous HCl solution containing about 32% by weight of HCl. HCl can also be removed from the system using absorption devices 40 or 46.

The gases normally exit from the absorption device 30 via line 36 from where they go to the HCl absorption device 40. Or the gases can go directly to the absorption apparatus 46 or to the hydrolysis apparatus 50. When the absorption apparatus 40 and 46 are used, additional HCl is removed from the gases.

The gases leave the absorption device 46 via line 48, from where they pass into a hydrolysis container 50 in which COCl2 is transformed into HCl and C02 by contact of the gases with H20 or weak hydrochloric acid in the presence of activated charcoal. The container 50 contains activated charcoal preferably having a particle size of 2.38-4.76 mm. HCl is removed in aqueous form via line 54, from which a portion can be recycled to the hydrolysis device 50 via lines 56, 58 and the pump 60. Line 52 can also be connected to the device d absorption 30 or 40. Make-up water is added to the hydrolysis apparatus via line 62.

The gases leaving the hydrolysis apparatus 50 flow towards the purifier-tor 80 via line 64. NaOH, or Na2C03, and water are added to the purifier 80 via line 66 in sufficient quantity to maintain the pH at 5.6, to selectively remove S02 but not C02 from the gases flowing there. The gases leaving the purifier 80 are then brought via line 68 into the incinerator 84 where CO is burned with air or oxygen coming from line 86 to form carbon dioxide CO 2. The gases leaving the incinerator 84 via line 90 can be discharged into the atmosphere by any suitable means.

The metal chlorides designated by MxCly in the drawing, and being further defined as being able to be hydrolyzed to form HCl and the corresponding metal oxides and which have been removed from the gases by the adsorption apparatuses 20 or 22, are in turn removed from the activated carbon bed to thereby regenerate the bed, passing a purifying gas at a temperature of 100 to 400 ° C, preferably about 250 ° C, at a speed of about 3 to 15 m / min for about 4 at 4 p.m., in the adsorption apparatus 24 in the drawing, which is identical to and can be interchangeable with the adsorption apparatuses 20 and 22. The purifying gas, which can be nitrogen or any other inert or non-reactive gas, such as CO 2, leaves the adsorption apparatus 24 with the metal chloride entrained by line 26 and passes through an apparatus for hydrolysis 70 containing a bed of fluid particles which consists of carbon or metal oxides of groups IIA, IIIA, IVA, IVB or their mixtures, with a particle size between 0.044 mm and 0.30 mm. Steam at a temperature of 100-150 ° C enters the particle bed of the hydrolyzer 70 via line 72 and is mixed with the gas to hydrolyze the metal chlorides into metal oxides plus HCl, for example SiCl4 form Si02 + HCl. The metal oxides remain in the bed and the purifying gas and HCl leave the bed via line 74 which is connected to line 18 just before entering the absorption apparatus 30, in which the hydrochloric gas originating from l the hydrolysis apparatus 70 for metal chlorides is removed as described above.

The adsorption apparatuses 20, 22 and 24, as mentioned above, are interchangeable and, as illustrated in the drawing, can be arranged in cascade. In the embodiment shown, the adsorption device 24 after complete regeneration will replace the adsorption device 22 which in turn will replace the adsorption device 20. The adsorption device 20, which will contain the major part of the metallic chlorides adsorbed, will then be regenerated as described above for the adsorption device 24. In each case, the cleanest adsorption device will then be used as the second adsorption device in the series to ensure the complete adsorption of metal chlorides from the gas.

The water, which enters the hydrolysis apparatus 70 through line 72, is admitted at a temperature above its dew point. Conveniently, this can include water vapor at atmospheric pressure. The hydrolysis apparatus 70 is kept at a sufficient temperature (generally about 100 ° C.) to ensure that the hydrolysis reaction of the metal chloride and of the water vapor takes place above the dew point of the gases mixed to prevent the formation of gelatinous materials in the particle bed, which could otherwise clog the reactor. Conveniently, this can be done using an external heater, such as an external steam jacket or similar device.

It has been found that the transformation of silicon chloride is somewhat dependent on the temperature and that it is reduced when the temperature is raised significantly above 100 ° C. For example, for a given bed volume, speed and molar ratio of H 2 O to SiCl 4, it has been found that a temperature of 100 ° C. at atmospheric pressure can give a transformation of more than 95% of the silicon chloride, whereas a temperature of 125 ° C under otherwise identical conditions gives a transformation of only 70%, while a temperature of 150 ° C, the others

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conditions being the same, lower the conversion rate to less than 25%. Preferably therefore, the temperature in the hydrolyzer 70 should not exceed about 110-115 ° C. To obtain the desired hydrolysis of metal chloride to metal oxide, a large excess of water vapor is necessary. The molar ratio of H2O to MxCly should be at least about 10 and can be as high as 75.

The solid material used in the bed comprises carbon or a metallic oxide in particles, the metal being chosen from groups IIA, IIIA, IVA, IVB, or their mixtures. Current river sand can be used. When using a fluidized bed, the solid material, for example sand, should be ground to 44-297 µm and preferably 74-149 [i. However, when using materials such as river sand, it is necessary to perform a preliminary purification step to reduce the amount of iron in the sand. Typically, river sand can have an iron content of about 1 to about 2% total iron. For reasons that are not fully understood, the presence of iron seems to be detrimental to the process. Leaching of river sand with concentrated hydrochloric acid provides a convenient method of removing iron oxide from the sand. Such leaching reduces the iron content to as low as 0.3% by weight, which reduces problems during operation. Preferably, when silica is used in the particle bed, it is a quartzite pickling sand. Such sand contains less than about 0.03% by weight of iron. It has also been found that A1203 can be used in place of silica. However, the higher price of alumina may not justify its use as a substitute from an economic point of view.

The residence time of the gases in the hydrolyzer 70 can range from as low as 0.03 and preferably 0.065 min to longer periods of time of up to 1 min or more, depending on the speed of the gases in the hydrolyzer, the ratio of water vapor to MxCly gases in the reactor and the height of the bed. The relationship between length of stay and bed height and speed can be expressed by the following equation:

_. ,,. Bed height

Duration of stay =

Speed

The hydrolysis can be carried out in a static, fluidized or mobile bed. A fluidized bed is preferred. The velocity of the gases (total velocity of water vapor and gaseous metal chlorides) passing through a fluidized bed must be at least about 0.1 cm / s to allow minimal fluidization for the particular particle size range. Preferably, the speed is about 1 to about 200 cm / s for particles from 74 to 149 y .. When using fluidized beds having internal diameters greater than 25 cm, or the equivalent for non-circular beds , it has been found that water vapor should enter the bottom of the bed, while gaseous metal chlorides should be injected into the bed at least 5 to 7.5 cm above the steam inlet of water. In addition, the rate of entry of gaseous metal chlorides into beds of this size or more should be at least 42.7 cm / s to prevent the formation of aggregates from the bed material. For fluidized beds less than 25 cm, the injection point and the speed of the gaseous metal chlorides were not found to be decisive. Typically, the length of the nozzle in the bed should be less than 1/5 of the internal diameter of the container containing the fluidized bed. Conventional filter bags, for example a Teflon felt, are used to retain the particles in the container of the fluidized bed.

It will also be noted that the use of a fluidized bed seems to be particularly effective in eliminating the vapors or the smoke resulting from the hydrolysis of metal chlorides, in particular titanium tetrachloride.

The height of the bed is also important and should be a minimum of about 10 cm, preferably about 40 to 60 cm or more in height, to again provide sufficient gas residence time in the bed, for the interval particle size and the speed range of the specified gases.

The following examples illustrating the hydrolysis of silicon chloride will serve to illustrate the hydrolysis part of the metal chlorides of the process.

Example 1:

River sand having undergone acid leaching and having a particle size between 74 and 149 (jl) is packed in a tubular reactor with a diameter of 2.5 cm at a height of about 26 cm. The reactor is heated at a temperature of approximately 100 ° C. Water vapor and silicon chloride gas are admitted to the lower part of the reactor, at a speed of approximately 3.5 to 4.2 cm / s, at the atmospheric pressure plus the pressure due to the height of the bed, to obtain a total residence time in the reactor of approximately 0.1 min Table 1 illustrates the conversion rate of silicon chloride for various tests in which vary the molar ratio of H2O to silicon chloride In each case, the amount of unreacted silicon chloride is determined by passing the 2o gases from the reactor through a water trap containing a known amount of deionized water After each test, the trap is weighed and a sample of the liquid of c is titrated. each trap with a 0.1N sodium hydroxide solution. From this titration, one can calculate the amount of HCl in the trap. Using the reaction equation SiCl4 + 2 H20 -> 4 HCl + SiO2, one can then calculate the amount of silicon chloride used during the test from the amount of HCl found in the water trap. This calculation allows a double check with the quantity of silicon chloride used which is determined by weighing the silicon chloride supply flask before and after each particular test. The water trap and the outlet tube are then placed in a drying oven at 120-150 ° C to evaporate the water and the acid. The solid residue left in the trap and the outlet tube is weighed as 100% silicon dioxide (SiO2). This solid residue represents the silicon chloride which has not been hydrolyzed in the fluid bed. Using the above equation and the weight of the solids found in the trap and outlet tube, the amount of reacted silicon chloride is calculated. The percentage of transformation of silicon chloride is then calculated from the following equation:

40 Percentage of transformation of silicon chloride =

(g unreacted silicon chloride \

1 - -

g of silicon chloride introduced into reactor J

Analysis of the solids found in the water and the outlet tube after a normal test shows that 92 to 94% of the material is made of silicon dioxide. Thus, the assumption that the solids left in the water trap and the outlet tube after drying consist of 100% silica gives a default calculation of the percentage of transformed silicon chloride.

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Table 1

Transformation of SiCU as a function of the H20 / SiCl4 molar ratio

Molar ratio

% transformation

55

H20 / SiCU

from SiCl4 to Si02

15

96

20

98

28

99

60

36

100

45

100

71

100

The results indicate that when using a speed of 3.5 to 65 4.2 cm / s and a bed height sufficient to provide a residence time of about 0.1 min, conversion rates can be obtained. 95% or more using a wide range of molar ratios of H2O to silicon chloride.

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Example 2:

The same experiment is repeated, however increasing the speed to 4.5-5 cm / s and lowering the bed height to approximately 20 mm, which gives a residence time of approximately 0.065 min. As shown in Table 2, with this reduced residence time, an H 2 O / SiCl 4 molar ratio of at least about 35 to 40 is desired.

Table 2

Report

% transformation

H20 / SiCl4

from SiCl4 to Si02

16

74

36

96

62

99

78

92

Example 3:

The same reaction is repeated with an empty tubular reactor. The residence time is considerably increased by lowering the overall speed of the gas in the reactor. The results are given in Table 3.

Table 3

Duration of stay (min)

H2O / SiCl4 molar ratio

% transformation of SiCl4 into Si02

0.18

59.1

17.9

0.24

73

65.6

0.25

113

79

0.33

33

80.3

0.36

97

93

0.41

57

81.0

If we compare these results with those of Table 1, in which a residence time of 0.1 min under the same conditions gives a transformation greater than 95% using a molar ratio of H 2 O to silicon chloride of less than 20 , it can be seen that the presence of silica in the reactor bed according to the invention significantly reduces the residence time necessary to obtain a high percentage of transformation of the silicon chloride into solid silica.

Example 4:

The experiment of Example 1 is repeated using a fixed H 2 O / SiCl 4 molar ratio of 63, but varying the temperature of the bed from 100 to 125 and 150 ° C and using a speed of 4.6 cm / s and a residence time of 0.077 min.

Table 4

% transformation

SiCl4 to Si02 temperature

of the bed (° C)

98

100

70

125

24

150

The results indicate that, for a given speed, residence time and molar ratio, higher temperatures lead to a lowering of the percentage of conversion of silicon chloride into silica.

Example 5:

Several series of tests similar to that of Example 1 are carried out to show the effect of the residence time on the percentage of transformation by varying the height of the bed. In this case, the gas speed is between about 6.6 and 7.6 cm / s. When the two series of tests are carried out, the first series comprises a water / silicon chloride ratio of approximately 34 to 36, while the second series comprises a ratio of approximately 56 to 70. In each case, the temperature of the bed is maintained at about 100 ° C.

Table 5

H2O / SiCl4 molar ratio

Bed height (cm)

% transformation of SiCl4 into Si02

34-46

10

36

34-46

20

50

34-46

30

62

34-46

40

72

34-46

50

75

34-46

60

78

56-69

10

56

56-69

20

73

56-69

30

78

56-69

40

86

56-69

50

90

56-69

60

92

The results indicate that an increase in the residence time increases the percentage of transformation of SiCl4. The results also indicate that a longer residence time is required when the H2O / SiCl4 molar ratio is less than about 50.

Example 6:

Another experiment was carried out according to Example 1, using a residence time of approximately 0.04 to 0.05 min with an H 2 O / SiCl 4 molar ratio of 33 to 53, and varying the speed to show the effect of gas velocity on the percentage of transformation of silicon chloride.

Table 6

Speed

% transformation

(cm / s)

from SiCl4 to Si02

3.8

95

6.8

75

15

42

21

27

25.1

30

In this case, the height of the bed is increased as the speed increases to maintain the stay time approximately constant. Thus, it can be seen that despite the same residence time, higher gas velocities actually result in a reduced transformation percentage. Thus, the residence time must be increased when the gas speed is greater than about 6 cm / s when using particles of 74-149 [*.

Example 7:

14.2-20.5 normal cubic meters per minute of residual reactor gas are introduced into the residual gas treatment system. The composition of the gas stream is as follows: Co2 34% -49%, CO 17.5% -19.8%, HCl 5.5% -6.6%, COCl2 0.20% -0.26%, SiCl4 0.14% -0.15%, Cl2> 0.01%, the rest being nitrogen. An aqueous HCl solution is obtained containing 28 to 30% hydrochloric acid. The hydrochloric acid solution does not contain gelatinous substances. Less than 10 ppm COCl2 is detected in the gas leaving the COCl2 hydrolysis devices. Neither CO nor Cl2 is detected in the chimney. The amount of COCl2 in the stack is less than 0.05 ppm. After 30 hours of operation, the primary adsorption apparatus is regenerated with about 4.5 normal cubic meters per minute of a gas stream at 90 ° C. having the following composition:

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75% N2, 11% Cl2, 14% CO, for 4 h. The adsorption apparatus is then further heated to 115 ° C-300 ° C for 4 h with 3.2-5.6 m3 normal per minute of nitrogen. The purification gases are introduced into a device for the hydrolysis of metal chlorides containing a fluidized bed comprising one third by weight of Al 2 O 3 of metallic quality and two thirds by weight of sand of high purity (PGS Oklahoma N ° 1 sec) . The bed is approximately 2.44 m high and 1.68 m in diameter. Steam is also introduced into the chlorides hydrolysis apparatus at a rate of approximately 7.6 kg / min. The temperature of the fluidized bed is maintained at 104-115 ° C. The temperature of the regeneration gas is approximately 180 ° C; the speed of the regeneration gases in the supply nozzles is 91.5-122 m / s. The gases leaving the chlorides hydrolysis apparatus are cooled and freed of HCl in one of the HCl adsorption apparatus. No gelatinous precipitate can be detected in any of the HCl absorption towers, nor is there any indication of agglomeration of the fluidized beds. About 193 kg of SiCl 4 are regenerated from the carbon absorption device.

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1 sheet of drawings

Claims (11)

627,663 1. A process for purifying residual gases containing up to 15% by weight of gaseous metal chlorides which can hydrolyze to form HCl and the corresponding metal oxides, characterized in that the gases are brought into contact with steam water at a temperature of 100 to 150 ° C while passing the gases through a bed of solid particles chosen from the category comprising coal, oxides of metals from groups IIA, IIIA, IVA, IVB and their mixtures, to transform the said metal chlorides in HCl and corresponding metal oxides; then the gases are brought into contact with liquid water at a temperature of 20-100 ° C to remove the HCl vapors, and the gases are passed through a bed of activated carbon in the presence of water to hydrolyze COCl2 contained in gases in HCl and C02. 2. Method according to claim 1, characterized in that the gases are first passed through a bed of activated carbon to remove at least part of the halogenated compounds, then purifying gas is passed through the bed of carbon to remove the halogenated compounds deposited there, before the contact of the gases with water vapor in the bed of solid particles. 2 3. Method according to claim 1, characterized in that the purified gases are brought into contact with steam at a H20 / MxCly molar ratio of at least 10 and which can range up to 75, to ensure hydrolysis complete with metal chlorides. 4. Method according to claim 3, characterized in that the hydrolysis is carried out at a temperature of 100-115 ° C. 5. Method according to claim 3, characterized in that the particles in the bed have a particle size between 44 and 300 si. 6. Method according to claim 5, characterized in that said particles have an iron content of less than 0.3% by weight. 7. Method according to claim 3, characterized in that the residence time of the gases in said bed of solid particles is approximately 0.03 to 1 min. 8. Method according to claim 7, characterized in that the speed of the gases in said bed is from 1 to 20 cm / s. 9. Method according to claim 8, characterized in that said bed has a minimum height of at least 10 cm. 10. Method according to claim 1, characterized in that the gases are purified in a purifier maintained at a pH of approximately 5.6 by addition of sufficient NaOH or Na2CO3 to selectively remove the sulfur dioxide before the step of hydrolysis of COCl2. 11. Method according to claim 10, characterized in that the purified gases are incinerated to oxidize to C02 all the partially oxidized carbon compounds.