AU709404B2

AU709404B2 - Process for the recovery of sulphuric acid from spent acids containing metal sulphates

Info

Publication number: AU709404B2
Application number: AU27142/95A
Authority: AU
Inventors: Guenter Dr Lailach; Jurgen Dr Wiese
Original assignee: Kerr Mcgee Pigments & Co GmbH
Current assignee: Kerr-Mcgee Pigments & Co KG GmbH
Priority date: 1995-07-24
Filing date: 1995-07-24
Publication date: 1999-08-26
Anticipated expiration: 2015-07-24
Also published as: AU2714295A

Description

-I-

Process for the recovery of sulphuric acid from spent acids containing metal sulphates The invention relates to a process for the recovery of sulphuric acid from spent acids containing metal sulphates by increasing the sulphuric acid concentration in these spent acids to 50 to 75%, separating the solid metal sulphates from the reusable 50 to 70% acid and thermally decomposing the metal sulphates to form metal oxides and sulphur dioxide, from which technically pure sulphuric acid is produced.

Although the recovery of sulphuric acid from spent acids containing metal suli" 10 phates, such as for example the so-called waste acid obtained in the produce•••o tion of titanium dioxide by the sulphate process, or sulphuric pickling acids, is ecologically advantageous, it poses a major economic burden on the production processes themselves due to the high amount of energy and apparatus ~required. The usual procedure is to evaporate the dilute sulphuric acid containing metal sulphates (referred to in the following as waste acid) to a concentration in the range of 50 to 75% H 2

SO

4 in the liquid phase, at which :eeee the metal sulphates have solubility minima (EP-A 133 505). Special conditions are employed during the concentration (DE-A 2 618 121) and the crystallisation of the sulphates (EP-A 133 505) to enable the sulphuric acid eeoc separated from the solid metal sulphates to be recycled to the process producing the spent acid. In order to reduce the amount of energy required for concentration it is possible not only to use the heat of the vapours formed in multi-stage vacuum evaporation processes but also to use low-grade process waste heat for evaporating the first portion of water from the waste acid. For this purpose it is for example possible to use the waste heat of the calciner off-gases formed during the calcination of TiO 2 (EP-A 313 715) or the process heat from the SO absorber in the production of sulphuric acid from

SO

2 (DE-A 2 529 708). The steps of concentrating the waste acids and separating the metal sulphates (EP-A 194 544, EP-A 362 428) have been in- Le A 30142- FC -2tensely investigated and optimised in recent years. Problems do however frequently occur during the evaporation of the waste acid, since CaSO 4 TiO 2 and/or SiO 2 are deposited in the form of extremely poorly soluble layers on the walls of the apparatus and in particular the heat transfer surfaces, during the evaporation process. This necessitates interrupting the process and conducting complicated cleaning procedures. DE-A 28 07 380 therefore proposes that the evaporation process always be carried out in the presence of at least 2% by weight of solid iron sulphate monohydrate. Although this does not allow the formation of deposits to be prevented, the deposits formed do however contain soluble iron sulphate in addition to the insoluble constituents, so that the insoluble particles can be washed out in the form of a slurry when the unit is rinsed. The crucial disadvantage of this process is ooooo however the fact that solid iron sulphate monohydrate is only present in the system at relatively high sulphuric acid concentrations and relatively high Fe 2 contents, which can mean that it is impossible to carry out an energetically advantageous multi-stage evaporation process or is at least only possible with unfavourable boundary conditions.

The thermal decomposition of the separated metal sulphates, the cooling, purification and drying of the decomposition gases and the production of sulphuric acid from the relatively highly diluted S0 2 -containing gases are processes which require a particularly high amount of energy and particularly complicated apparatus and which greatly reduce the economic efficiency of the production processes concerned. Depending on the composition and moisture content of the metal sulphate mixture, 3.3 to 4.4 GJ/t are required as decomposition energy for the thermal decomposition in a temperature range between 900 and 1100°C. The steam produced during the cooling of the decomposition products in waste heat boilers can be used for the concentration of the waste acid. The disadvantage of this method is that the decomposition gases can only be cooled to temperatures in the range from Le A 30 142 -3- 270 320 0 C in the waste heat boiler, in order above all to prevent any corrosive damage caused by the condensation of sulphuric acid during the subsequent electrostatic gas purification (EGP) process. The decomposition gases dedusted under dry conditions have to be cooled in one or more washing operations and freed from residual dust. Water or recycled condensate can be used as the washing liquid. This has to be separated from the solids removed from the decomposition gas and disposed of. In order to prevent the mass of catalyst, on which the reaction from SO 2 to SO3 takes place, from being contaminated by non-separated solids and to prevent corrosive damage from occurring in the region of the converter unit as a result of sulphuric acid mist, the residual dusts and acid mists are removed in wet EGP processes from the decomposition gases, which have been washed and cooled to a temperature in the range from 25-50 0 C, before being dried using 95-98% sulphuric acid and introduced into the conversion process.

The object of the present invention was to improve the economic efficiency of previously known processes for the recycling of sulphuric acid from spent sulphuric acids containing metal sulphates.

This object is achieved by utilising the thermal energy contained in the :i decomposition gases which have been dedusted under dry conditions and have been cooled to about 300 0 C for evaporating the spent sulphuric acid containing metal sulphates. In this process the "waste acid" is only evaporated to a concentration in the range from 28 32% H 2 S0 4 so that iron sulphate does not crystallise during the evaporation process. The evaporation process is carried out in a quencher by directly contacting the decomposition gases with the "waste acid", preferably in scrubbers of the Venturi, ejector Venturi or countercurrent type. By allowing a partial stream of the circulating "waste acid" to trickle down the walls the encrustation of the latter with metal sulphates is prevented. At the same time, the superfine dust portion of the Le A 30 142 -4oxide mixture formed during the thermal decomposition of the metal sulphates, referred to in the present context as the roasting residue, which dust is still contained in the decomposition gases, is washed out of the latter. In order to avoid the complicated separation of these solids by filtration, the roasting residue which is almost insoluble in waste acid is passed through the subsequent multi-stage vacuum evaporation unit together with the waste acid and removed with the metal sulphates crystallised in this unit. It has surprisingly been found that the presence of roasting residue in the waste acid in the evaporation stage, in which no solid FeSO 4

.H

2 0 is present, prevents the formation of deposits of insoluble compounds such as CaSO 4 TiO 2 and/or SiO 2 on the walls of the apparatus and the heat transfer surfaces. This represents a substantial improvement of the process as a whole, since the o:ooo formation of such deposits in the evaporation units has frequently necessitato. ted interrupting the processes concerned and conducting complicated cleaning operations.

°°to o The present invention therefore relates to a process for recovering sulphuric acid from spent acids containing metal sulphates, in which the spent sulphuric oooe acid containing metal sulphates is evaporated in several stages to a sulphuric acid concentration of 50 to 75% by weight and optionally slowly cooled, the :i-20 metal sulphates thereby crystallised are separated from the reusable 50 to t 75% by weight sulphuric acid and thermally decomposed at 800 to 11500C to form metal oxides and S0 2 -containing decomposition gases, the decomposition products having temperatures of 800 to 1150°C are cooled to 270 to 350 0 C in waste heat boilers and then freed from dust in an electrostatic gas purification unit (EGP), the dedusted gases are cooled adiabatically in a quencher and the quenched gases are cooled to temperatures of 25 to 50 0

C,

demisted and dried and then processed to form sulphuric acid, characterised in that the dedusted decomposition gases are quenched with spent sulphuric acid containing metal sulphates and no solid iron(ll) sulphate, the spent Le A 30 142 sulphuric acid is introduced into the quencher and a corresponding quantity of spent sulphuric acid of a higher concentration of a maximum of 28 to 32% by weight of H 2

SO

4 is discharged therefrom, while simultaneously washing out the superfine dust removed from the decomposition gases, and the concentrated spent sulphuric acid is passed on for further evaporation to a concentration of 50 to 75% by weight of H 2

SO

4 without removing the superfine roasting residue dust suspended therein.

The process according to the invention has several advantages over the prior art. The thermal energy of the decomposition gases which is liberated on 10 cooling the gases from about 300 0 C to about 80 0 C, is not discharged into S:r cooling water without being utilised, but is used for evaporating water from oeeo° the waste acid. The removal of the superfine roasting residue dust from the washing liquid normally used for quenching the decomposition gases is not necessary. The formation of insoluble deposits in the multi-stage evaporation unit in which the subsequent evaporation of the waste acid takes place, is avoided. As a result the availability of the unit is increased while at the same time reducing maintenance costs.

The present invention is illustrated in more detail in the following example.

Unless otherwise indicated, all the percentages are percentages by weight.

SS

Comparison example 100 t/h of waste acid from the production of titanium dioxide containing 23% of H 2

SO

4 7.7% of FeSO 4 2.4% of MgSO 4 1.9% of AI 2

(SO

4 3 and 1.7% of other metal sulphates, and also containing between 0.04 and 0.1% of CaSO 4 between 0.7 and 1.4% of TiOSO 4 and between 0.001 and 0.01% of SiO 2 were worked up in a process. In a 3-stage vacuum evaporator system the waste acid was evaporated to a content of 66% H 2

SO

4 (in the liquid phase), Le A 30 142 -6and the resulting suspension was cooled to 55°C in a 7-stage crystallisation cascade and filtered. During this operation 57.7 t of water were evaporated per hour using 35 t of steam of a pressure of 5 bars and 1575 m 3 of cooling water (18 0 In the filtration process 18.65 t/h of filter cake and 28.6 t/h of 66% reusable sulphuric acid were obtained. The filter cake was introduced, together with pyrites and coal, into a fluidised bed reactor of the decomposition unit. At 950 0 C the pyrites and the coal were burnt and the metal sulphates and the adherent sulphuric acid were thermally decomposed. The decomposition gases and the roasting residue were cooled to 300'C in a waste heat boiler, during which process 21 t/h of steam of a pressure of bars were produced. The gases issuing from the waste heat boiler were :i dedusted in a EGP unit to a residual dust content of 50 to 100 mg/m 3 •ooo° The dedusted decomposition gases (36,000 m 3 /h in dry form together with 4.8 t/h of steam) were cooled adiabatically in a Venturi scrubber from 300 to 67 0 C using a condensate from a heat exchanger arranged downstream as the washing and cooling medium. In the adiabatic cooling process ("quenching process") 6.0 t H 2 0/h were evaporated off. The decomposition gas saturated ooo o with steam was then cooled indirectly in a heat exchanger with cooling water to 33 0 C. 9.3 t of H 2 0/h were condensed in this process; the amount of cooling water required was 520 m 3 3.3 m 3 /h of condensate, in which 200 to 350 g of superfine roasting residue dust were suspended, had to be discharged from the quenching cycle and disposed of.

The decomposition gases cooled to 33 0 C were demisted in a wet EGP unit, dried and processed further by a contact process to form technically pure sulphuric acid.

The first stage of the multi-stage vacuum evaporator system had to be cleaned at irregular intervals four to six times a year to remove deposited Le A 30 142 -7insoluble substances, and in particular SiO 2 This labour-intensive operation meant that the unit had to be put out of operation for several days at a time.

Example accordinq to the invention The quantity and composition of the waste acid processed was the same as in the comparison example. 25 t/h of the 23% waste acid (of a temperature of 30'C) from the comparison example are first introduced into a countercurrent scrubber which is used for quenching the decomposition gases dedusted at high temperatures. The main quantity of the circulated waste acid is injected into this scrubber countercurrently to the decomposition gas entering from above. A smaller quantity of the circulated acid is introduced into a flow-off channel arranged at the top of the scrubber and flows downwards along the scrubber wall in the form of a film. 20.8 t of waste acid containing 27.65% of H 2

SO

4 and 200 to 350 g of superfine roasting residue dust are discharged from the scrubber per hour via a stripper in which SO 2 is blown out, at a temperature of 80 0 C and introduced into the 3-stage vacuum evaporator unit together with the remaining 75 t/h of 23% waste acid. In this unit only 48.5 t/h of H 2 0 still have to be evaporated, instead of 52.7 t/h, as a result of which the consumption of 35 t/h of steam of a pressure of 5 bars is reduced to 31.5 t/h and the consumption of cooling water is reduced from 1575 m 3 /h to 1485 m 3 /h.

The decomposition gases quenched and washed with waste acid are introduced via a mist collector into the heat exchanger, in which 455 m 3 /h of cooling water are used for the condensation of 7.5 t of H 2 0/h. The pure, solid-free condensate is discharged after the stripping of SO 2 No more deposits are detected in the first evaporation stage of the vacuum evaporation unit.

Le A 30 142 -8- The claims defining the invention are as follows: 1. A process for the recovery of sulphuric acid from spent sulphuric acid containing metal sulphates by the multi-stage evaporation of the spent sulphuric acid containing metal sulphates to a sulphuric acid concentration of 50 to 70% by weight, optionally cooling the resulting suspension, separating off the reusable 50 to 75% by weight sulphuric acid from the metal sulphates, thermally decomposing the metal sulphates containing sulphuric acid at 800 to 1150°C to form metal oxides and SO 2 -containing decomposition gases, cooling the metal oxides and SO2-containing decomposition gases in waste heat steam boilers to 270 to 350 0 C, dedusting the SO,-containing decomposition gases having a temperature of 270 to 350°C in an electrostatic gas purification unit, adiabatically cooling the dedusted SO2-containing gases in a quencher, cooling the quenched gases to temperatures of 5 25 to 50 0 C, demisting and drying the SO2-containing decomposition gases and subsequently processing them to form sulphuric acid, characterised in that the quenching of the dedusted decomposition gases is carried out using spent sulphuric acid containing metal sulphates and no solid iron sulphate, the spent sulphuric acid is introduced into the quencher and a corresponding quantity of spent sulphuric acid of a higher concentration of a maximum of 28 to 32% by weight of H 2

SO

4 is discharged therefrom, while simultaneously washing out the superfine dust removed from the decomposition gases, and the concentrated spent sulphuric acid is passed on, without removing the suspended superfine roasting residue dust, for evaporation to a H 2

SO

4 concentration of 50 to 75% by weight.

2. A process according to Claim 1, characterised in that the total quantity of spent acid to be worked up is introduced into the quencher and a Le A 30 142

Claims

3. A process according to Claim 1, characterised in that only a partial quantity of the spent acid to be worked up is introduced into the quencher and a corresponding quantity having an increased concentration is discharged therefrom, combined with the remaining partial quantity of the spent acid and passed on for further evaporation.
4. A process for the recovery of sulphuric acid substantially as hereinbefore described. o 5. Sulphuric acid when prepared according to any one of claims 1 to 3. Dated this 24th day of July 1995 C CAYER ATIE NG ELLSCIIA T- By its Patent Attorney DAVIES COLLISON CAVE US 7 p:\wpdocs\inws\560I44\kr Process for the recovery of sulphuric acid from spent acids containing metal sulphates Abstract The invention relates to a process for the recovery of sulphuric acid from spent acids containing metal sulphates by increasing the sulphuric acid con- centration in these spent acids to 50 to 75%, separating off the solid metal sulphates from the reusable 50 to 70% acid and thermally decomposing the metal sulphates to form metal oxides and sulphur dioxide, from which techni- cally pure sulphuric acid is produced. S Le A 30 142