FI89875B - FOERFARANDE FOER AOTERCIRKULERING AV METALLSULFATHALTIG SVAVELSYRA - Google Patents

Description

89875

Method for recycling metal sulphate - containing sulfuric acid

The present invention relates to a process for recycling spent metal sulphate-containing sulfuric acid by evaporating dilute sulfuric acid containing dissolved metal sulphate to a content of 40-85% I 2 SO 4 based on unsalted acid, separating the metal sulphate by filtration and reusing the separated sulfuric acid.

Metal sulfate-containing dilute acids formed in TiCl 2 production or as pickling solutions can be removed and recycled by evaporating these metal sulfate-containing sulfuric acids to an acid reusable acid concentration or the concentration with the lowest metal sulfate solubility. The metal sulphates are separated, if necessary after cooling the suspensions, usually by filtration from sulfuric acid, which is then returned to the production process (DE-A 33 27 769, U.S. Patents 2,280,508 and 2,098,056).

According to the state of the art, a vacuum drum filter is used to filter these suspensions, which usually contain 10-40% by weight of solids with a particle size of mainly 5-100 micrometers. (Ullmanss Encyklopädie d. Techn. Chemie, Veri. Chemie, Weinheim, 4th edition, 1972, 25 part 2, pages 188-190). The advantage of this device can then be seen in a simple and economical, continuous use without operating costs.

The disadvantages of vacuum filtration can be seen in the fact that the filter cakes are thixotropic and prone to cracking, depending on the grain size and the relative composition of the metal sulphates belonging to the group of iron, aluminum, magnesium-30, titanium, manganese, chromium and vanadium sulphates and hydrogen sulphates.

2,89875 The residual moisture content of these filter cakes is generally between 20 and 40% by weight, which corresponds to 12 to 25% by weight of sulfuric acid. This sulfuric acid not only leaves the production process, but also greatly complicates the handling and safe removal of metal sulfate. In the ecologically optimal removal of metal sulfates by thermal decomposition, the residual moisture of the metal sulfates requires a substantial portion of the decomposition capacity. However, the process loading caused by residual moisture is also uneconomical because it requires about 80 higher specific decomposition energies than metal sulfates.

Therefore, several experiments have been performed to reduce the residual moisture by post-moisture removal of the filter cakes obtained by vacuum drum filtration. Certain advances have been made with compression belt filters, however, the technical costs for double filtration are too high.

It is therefore an object of the present invention to provide a method in which the above-mentioned drawbacks do not occur.

Surprisingly, it has now been found that unexpectedly good filtration results are possible with minimal residual moisture in the filter cakes by means of filtration in pressure filters, which allow uniform removal of moisture from the cakes by cleaning by blowing the filter cakes with compressed air.

The present invention therefore relates to a process for recycling spent metal sulphate-containing sulfuric acid by evaporating the dissolved metal sulphates. Sulfuric acid to a content of 40-85% I / SC4, calculated on the unsalted acid, by separating the metal sulphate by filtration and re-using the separated sulfuric acid, whereupon the filtration takes place under overpressure, and compressed air is blown through the obtained filter cakes to remove moisture.

3,89875

The filters used according to the invention are usually used for the separation of very fine solids and for suspensions with a low solids content due to the discontinuous use. Thus, the seller also had the preconceived notion that filter types of this type could be recommended for filtering solids-rich metal sulfate / sulfuric acid suspensions with high solids content. Thus, it was more surprising that the residual moisture of the filter cake can be reduced by as much as 30-50% compared to cakes obtained by vacuum drum filtration, thus achieving a corresponding increase in the yield of reusable sulfuric acid. , especially also because the filter cakes obtained according to the invention are not thixotropic but crumbly and apparently dry.

In contrast to commonly used filters, filtration with the filter types according to the invention takes place in batches.

It is also advantageous in the context of the invention to carry out the filtration as long as the thickness of the cake is at least 10 mm, preferably 20-60 mm.

25 Moisture is removed from the filter cakes by blowing compressed air through the cakes. In this case, the required pressure and the duration of the blow-through are determined by the thickness of the cake, the composition of the metal sulphate mixture, the sulfuric acid content of the filter cakes and the desired final moisture. The economic optimum can be easily determined by experiments. Preferably hot air with a temperature of at most 150 ° C, preferably 40-120 ° C is used. According to the invention, the purge lasts for at least 2 minutes, preferably 4 to 10 minutes with compressed air.

4 8 9 8 75

The reuse of sulfuric acid is substantially affected by the concentration of metal sulphates dissolved in it. In order to keep this salt content low, certain conditions in the crystallization must be taken into account (cf. EP 0 135 505).

5 The salinity is further affected by the temperature in the sulphate separation, which is between 40 and 70 ° C. Although the concentration of dissolved salt in the acid is lower at lower temperatures than at high temperatures, the filterability of the colder suspensions is clearly inferior, and thus considerably higher amounts of attached moisture remain in the filter cakes after the injection with compressed air. In a preferred embodiment of the process according to the invention, the disadvantages of filtration at low temperatures and the disadvantages of high temperatures are avoided by filtering the suspension at low temperature and removing moisture from the filter cakes with hot compressed air.

A particularly preferred embodiment of the process according to the invention is based on the fact that the filtration takes place at a temperature of 35-50 ° C and the temperature of the pressurized air used for purging is 70-120 ° C.

A significant improvement over conventional vacuum filtration can be achieved by filtration with membrane filtration presses. Even more advantageous results can be obtained with pressure filters of the candle or leaf filter type, which allow an unexpectedly low residual moisture content in the filter cakes. Thus, and taking into account the simple structure of such filters, they make a significant advance in eliminating waste management difficulties.

According to the invention, particularly advantageous filters are thus candle filters. Particularly good results are obtained if, in the case where a filter comprising several candles is used, the minimum distance between the two filter candles is 100-150 mm, preferably 35 120-130 mm and the minimum distance between the candles and the filter housing is 70-100 mm.

5,69875

The invention is illustrated by the following examples without limiting the invention.

Example 1 (Comparative Example) In this and the following examples, '5 metal sulfate-containing, 65% sulfuric acid obtained by evaporation of dilute acid formed in TiCl 2 production was used. The composition of this metal sulphate suspension was as follows (values in weight percent):

FeSO 4 · H 2 O 10.6 Fe 2 <SO 4> 3 'H 2 SO 4 · 8 H 2 O 1.1

Fe2H2 <SO4) 3 · H2O 11.4 A12 (SO4) 3 'H2SO4 1 8 H2O 2.7

MgSO 4 1 H 2 O 3.6 TiSO 4 2.9

MnSO 4 1 H 2 O 0.7 V0SO 4 ° '3 H 2 SO 4 43.4 H 2 O 23.3 The particle size of the sulphates, if detectable microscopically, was between 5 and 120 micrometers, mainly between 15 and 80 micrometers. The temperature was 55 ° C.

Filtration with a vacuum drum cell filter (drum diameter 2.2 m, rotation speed 0.3 min 1) gave 2q 8-10 mm thick filter cakes. The moisture content of the thixotropic filter cakes was 28.2% by weight, which corresponded to 18.3% by weight of H2SO4.

540 kg of reusable, 65% sulfuric acid and 460 kg of filter-25 cakes were obtained per ton of suspension.

Example 2

The suspension shown in Example 1 was filtered through a membrane filter press (Pressfilter-Automat P 1,

Fa. Hoesch & Sohne, Diiren) to a cake thickness of 25 millimeters.

The filter cakes were then removed by squeezing the membrane at a pressure of 16 bar and compressed air (5 bar) was blown through them for 10 minutes to remove moisture. The residual moisture of the removed cakes was 20.3% by weight, which corresponds to 13.2% by weight.

6 39875

At a compression pressure of 8 bar, a residual moisture content of 18.9% by weight was obtained, corresponding to 12.3% by weight of F 2 SO 4. The required blowing air (50 ° C) corresponded to the need of 194 m 2 (NPT) per tonne of filtered salt-5 Example 3

The suspension according to Example 1 was filtered through a plug-in filter (Fundabac® filter manufactured by Firma Mannedorf / Switzerland).

The filter was equipped with 7 candles with a total area of 2.2 m. The minimum distance between the candles was 50 mm. The three bar was pressurized with overpressure for 30 seconds and compressed air (5 ° C) was blown into the filter cakes for 5 minutes to remove residual moisture. This reduced the barometric pressure as a filter from 2.8 bar to 2.1 bar. The filter cakes (86 kg) removed from the filter had a thickness of about 20 mm and a moisture content of 18.0% by weight, corresponding to 11.7% by weight of sulfuric acid. Example 4

According to Example 3, the mixture was filtered for 30 seconds at a pressure of 20 bar and compressed air (55 ° C) was blown through the filter cakes for 5 minutes. The residual moisture was then 14.6% by weight, corresponding to 9.5% by weight of H2SO4. Per tonne of suspension, 610 kg of 65% sulfuric acid (61.9% H 2 SO 4, 33.3% H 2 O, 4.8% dissolved sulfates) were separated (compared to 540 kg / t in the vacuum filtration of Example 1). The filter cakes (390 kg / t suspension) had no thixotropic properties at all, but were a seemingly dry powder. For the drying blow for 84 kg of filter cakes, 3 3 30 40 m (NPT) of compressed air was required, which corresponded to 476 m (NPT) per ton of filter cakes.

Example 5

In order to allow for greater cake thicknesses,

6 filter candles and 35 were removed from the Fundbac® filter

Only the middle filter candle was used in the oat experiments.

7 39075 2

The suspension according to Example 1 was fed to the filter (filtration area 0.31 m) for 80 seconds at a pressure of 5 bar. After removal of turbidity, compressed air (temperature 5 ° C, pressure decreased from 4.7 ba-5 to 3.7 bar) was blown for 10 minutes to dry blow. 28 kg of filter cakes with a moisture content of 15.9% by weight, corresponding to 10.3% by weight of sulfuric acid, were removed. The average cake thickness was 45 mm.

Example 6 In a single candle filter (according to Example 5), it was filtered for 80 seconds and, after removing the turbidity, blown dry with compressed air for 5 minutes (temperature 55 ° C, pressure dropped from 4.5 bar to 3.2 bar). The residual moisture of the removed filter cakes was 14.3% by weight, corresponding to 9.3% by weight of sulfuric acid. Blowing dry 27 kg of filter cake required 3 m (NPT) of air, which corresponded to 222 m of air (NPT) per ton of filter cake.

Example 7 According to Example 6, 80 s were filtered and blown dry for 6 minutes with compressed air (temperature 75 ° C, pressure 3.3 bar). The residual moisture of the removed filter cakes (27.5 kg) was 15.3% by weight, corresponding to 8.8% by weight of 3 sulfuric acids. Air consumption was 7 m (NPT), which corresponded to 3 25 256 m (NPT) per tonne of filter cakes.

Example 8

The suspension of Example 1 was cooled from 55 ° C to 42 ° C over 5 hours and filtered through a candle filter according to Example 6. The mixture was filtered at 5 bar for 60 s and, after removing the turbidity, blown dry for 10 minutes with compressed air (40 ° C; 4.2 bar). The filtrate contained 62.9% i 2 SO 2, 33.8% H 2 O and 3.3% dissolved sulfates (cf. Example 4). 16.4 kg of filter cake with a residual moisture content of 16.7% by weight, corresponding to 10.9% by weight of 1 H 2 SO 4, were removed.

8 89875 3 3

Air consumption was 8.3 m (NPT), corresponding to 506 m (NPT) per tonne of filter cakes.

Example 9

According to Example 8, the suspension was filtered for 5 80 seconds at a temperature of 42 ° C and a pressure of 5 bar.

After removing the turbidity, it was blown dry with compressed air at a temperature of 110 ° C and a pressure of 4.2 bar. The filtrate contained 62.8% H 2 SO 4, 33.8% I 2 O and 3.4% dissolved sulfates.

20.9 kg of filter cake with a temperature of 10 55 ° C and a residual moisture of 13.8% by weight, corresponding to 9.0% by weight of sulfuric acid, were removed. Air consumption was 5.5 m (NPT), which 3 corresponded to 263 m (NPT) per tonne of filter cakes.

Claims (6)

89875 A process for the recycling of spent metal sulphate-containing sulfuric acid by evaporating dilute sulfuric acid containing 5 dissolved metal sulphates to a content of 40 to 85% H 2 SO 4, calculated on the salt-free acid, separating the metal sulphates by filtration and using the separated sulfuric acid using overpressure and blowing compressed air through the resulting filter cake to remove moisture. Method according to Claim 1, characterized in that the filter cakes have a thickness of at least 10 mm, preferably 20 to 60 mm. Method according to Claim 1 or 2, characterized in that compressed air is blown through the filter cake for at least 2 minutes, preferably 4 to 10 minutes. Method according to one or more of Claims 1 to 3, characterized in that the temperature of the compressed air blown through the filter cake is at most 150 ° C, preferably 40 to 120 ° C. Method according to one or more of Claims 1 to 4, characterized in that the filtration takes place at a temperature of 35 to 50 ° C and the temperature of the compressed air blown through the filter cake is 70 to 120 ° C. Method according to one or more of Claims 1 to 5, characterized in that the minimum distance between the filter candles is 100 to 150 mm, preferably 120 to 130 mm, and the minimum distance between the candles and the filter housing is 70 to 100 mm. ίο S 9 8 7 5