KR100377049B1 - How to recover sulfuric acid from spent acid containing metal sulfate - Google Patents

Abstract

The present invention increases the concentration of sulfuric acid in these spent acids from spent acids containing metal sulfates to 50-75%, separates solid metal sulfates from reusable 50-70% acid, and then pyrolyzes the metal sulfates, A method for recovering sulfuric acid by forming sulfur dioxide that can industrially produce pure sulfuric acid.

Description

How to recover sulfuric acid from spent acid containing metal sulfate

The present invention increases the sulfuric acid concentration in these waste acids from spent acid containing metal sulfate to 50-75%, separates solid metal sulfate from 50-70% reusable acid, and then pyrolyzes the metal sulfate. The present invention relates to a method for recovering sulfuric acid by forming a metal oxide and sulfur dioxide capable of producing industrially pure sulfuric acid.

Although recovery of sulfuric acid from waste acid containing metal oxides such as spent acid or sulfuric acid-based washing acid, which is obtained in the production of titanium dioxide by the sulfate method, requires a large amount of energy and equipment, although it is environmentally advantageous. The production process itself is a significant economic burden. A common method is to dilute dilute sulfuric acid containing metal sulphate (hereinafter referred to as "waste acid") to the concentration of sulfuric acid in the liquid phase to 50 to 75% by weight, the minimum solubility of the metal sulphate (European Patent Publication 133 505). Special conditions are used for concentration (German Patent Publication No. 2 618 121) and crystallization of sulfate (European Patent Publication No. 133 505) so that sulfuric acid can be separated from the solid metal sulphate and recycled to the process of releasing waste acid. To reduce the amount of energy required for concentration, it is possible to evaporate the first water fraction from the waste acid, as well as to use the heat of the steam formed in the step vacuum evaporation process as well as the lower process waste heat. To this end, for example, waste heat of the calciner exhaust gas formed upon calcination of TiO2 (European Patent Publication No. 313 715) or process heat from the SO3 absorber during the production of sulfuric acid from SO2 (Germany Patent Publication No. 2 529 708). ) Can be used. The concentration of waste acid and the separation of metal sulphate (European Patent Publications 194 544 and 362 428) have been intensively studied and optimized in recent years. However, the problem often occurs during the evaporation process of waste acid, as CaSO4, TiO2 and / or SiO2 precipitate in the form of a layer which is extremely insoluble in the walls of the apparatus and especially the heat transfer surface during the evaporation process. This requires stopping the process and carrying out a complex cleaning procedure. Therefore, German Patent Publication No. 28 07 380 suggests that the evaporation process always be carried out in the presence of at least 2% by weight solid iron sulfate monohydrate. This does not prevent the formation of precipitates, but because the formed precipitates contain soluble iron sulfate in addition to insoluble components, insoluble particles may be washed off in slurry form when the device is rinsed. However, the decisive disadvantage of this method is the fact that solid iron sulfate monohydrate is present in the reaction system only at relatively high sulfuric acid concentrations and relatively high Fe2 + content, which is not possible to perform an energy-efficient multistage evaporation process or only under adverse boundary conditions. It could also mean

Pyrolysis on separated metal sulphates, cooling, purifying and drying cracked gases, and sulfuric acid production from relatively dilute SO2 containing gases require particularly large amounts of energy and particularly complex equipment and greatly reduce the economic efficiency of the associated production process. Processes

Depending on the composition and moisture content of the metal sulphate mixture, 3.3 to 4.4 GJ / ton is required as the decomposition energy for pyrolysis at temperatures between 900 and 1100 ° C. The steam produced during the cooling of the decomposition products in the waste heat boiler can be used for the concentration of the waste acid. A disadvantage of this method is that the gas can only be cooled to a temperature in the range of 270 to 320 ° C. in the waste heat boiler in order to prevent corrosion damage caused by the condensation of sulfuric acid in the subsequent electrostatic gas purification (EGP) process. The cracked degassed gas under dry conditions must be cooled by one or more washing operations and also to remove residual dust. Water or recycled condensate can be used as the wash liquor. This cleaning solution must be disposed of separately from the solids removed from the cracked gas. Decomposition of residual dust and acid mists to prevent the catalyst mass from which the reaction from SO2 to SO3 occurs is contaminated by undissolved solids and to prevent corrosion damage at the converter device site due to sulfuric acid mist. After removal from the wet EGP process, the cracked gas is washed, cooled to a temperature in the range of 25-50 ° C., dehydrated with 95-98% sulfuric acid and introduced into the conversion process.

It is an object of the present invention to improve the economic efficiency of existing methods for recycling sulfuric acid from spent sulfuric acid containing metal sulfate.

This object is achieved by utilizing the thermal energy contained in a cracked gas cooled to about 300 ° C. by dust removal under dry conditions to evaporate spent sulfuric acid containing metal sulfate. In this method, "waste acid" is only evaporated to a sulfuric acid concentration in the range of 28 to 32% so that iron sulfate does not crystallize during the evaporation process. The evaporation process is carried out in a quench cooler, preferably Venturi, ejector venturi or countercurrent scrubbers by bringing the cracking gas directly into "waste acid". The wall covered with metal sulphate is prevented by causing the partial stream of "waste acid" circulated to fall off the wall. At the same time, the ultrafine dust fraction, referred to herein as roasting residue, still contained in the cracking gas in the oxide mixture formed during the thermal decomposition of the metal sulfate, is washed away from the cracking gas. To avoid complicated separation operations of these solids by filtration, almost insoluble roast residues in the spent acid are passed through the subsequent multistage evaporator with the spent acid and removed together with the crystallized metal sulphate in the device. Surprisingly, it has been found that the presence of roasting residues in waste acid without solid FeSO 4 H 2 O in the evaporation step prevents the formation of precipitates of insoluble compounds such as CaSO 4, TiO 2 and / or SiO 2 on the walls and heat transfer surfaces of the device. This means that the formation of such precipitates in the evaporation system has resulted in a significant improvement in the overall process, taking into account the frequent necessity of interrupting the relevant processes and performing complex cleaning operations.

Thus, the present invention provides a multi-step evaporation of spent sulfuric acid containing metal sulphate to bring the sulfuric acid concentration to 50 to 75% by weight, optionally slowly cooling and thus from the reusable 50 to 75% by weight of sulphate And then pyrolyzed at 800 to 1150 ° C. to produce cracked gas containing metal oxides and SO 2, and the decomposition product having a temperature of 800 to 1150 ° C. was cooled to 270 to 350 ° C. in a waste heat boiler, and then an electrostatic gas purification device (EGP ) To remove dust and heat the dust-free gas in a quenching machine to cool the quenched gas to a temperature of 25-50 ° C., and then process it after mist removal and drying to produce sulfuric acid. A method for recovering sulfuric acid from a waste acid, the method comprising a metal sulfate, but no solid ferrous sulfate Sulfuric acid is used to quench the degassed cracked gas, feed the spent sulfuric acid into the quench and simultaneously release the corresponding amount of waste sulfuric acid having a sulfuric acid concentration of up to 28 to 32% by weight from the quench while simultaneously Ultrafine dust removed from the washing is removed, and the concentrated spent sulfuric acid is continuously treated to further evaporate the sulfuric acid concentration of 50 to 75% by weight without removing the dust.

The method according to the invention has several advantages over the prior art. When the cracked gas is cooled from about 300 ° C. to about 80 ° C., the thermal energy of the gas emitted is not used in the cooling water without being utilized, but is used to evaporate water from the waste acid. There is no need to remove ultrafine roasted residue dust from wash liquids normally used to quench cracked gases. The formation of insoluble precipitate in a multistage evaporation apparatus where subsequent evaporation of the spent acid is also avoided. As a result, maintenance costs are reduced while the utility of the device is increased.

The following describes the present invention in more detail by way of examples. All percentages are by weight unless otherwise indicated.

Comparative example

Contains 23% H2SO4, 7.7% FeSO4, 1.9% MgSO4 2.4% Al2 (SO4) 3 and 1.7% other metal sulphates released in the production of titanium dioxide, and also CaSO4 0.04 to 0.1%, TiSO4 0.7 to 1.4% and SiO2 0.001 100 tonnes / hour of spent acid containing from 0.01% to 0.01% was treated in one process. The waste acid was evaporated to 66% sulfuric acid content (in liquid) in a three stage vacuum evaporator system and the resulting suspension was cooled to 55 ° C. in a seven stage crystallization cascade and filtered. This operation consumed 35 tons of steam with a pressure of 5 bar and 1575 m3 of coolant (18 ° C) and evaporated 57.7 tons of water per hour. The filtration process yielded 18.65 tonnes of filter cake and 28.6 tonnes of reusable 66% sulfuric acid per hour. The filter cake was put together with pyrite and coal into the fluidized bed reactor of the cracker. Pyrite and coal were burned at 950 ° C. to pyrolyze metal sulfate and attached sulfuric acid. The cracked gas and roasted residues were cooled to 300 ° C. in a waste heat boiler, producing 21 tons of steam with a pressure of 40 bar per hour. In the EGP unit, dust was removed from the gas exiting the waste heat boiler so that the residual dust content was between 50 and 100 mg / m3.

In a venturi scrubber, condensate from a downstream heat exchanger was used as a washing and cooling medium to remove the dedusted cracked gas (36.000 m3 / hr in dry form, with 4.8 tons / h steam) from 300 ° C to 67 ° C. Adiabatic cooling. In the adiabatic cooling process (“quenching process”) 6.0 tonnes of water were evaporated per hour. The cracked gas saturated with steam was then indirectly cooled to 33 ° C. in a heat exchanger employing cooling water. In the process, 9.3 tonnes of water were condensed per hour, and the required amount of cooling water was 520 m3 per hour. Condensate 3.3 m3 / hr suspended from 200 to 350 g of ultrafine roasted residue dust had to be discharged in a quench cycle and disposed of.

The mist of cracked gas cooled to 33 ° C. was removed in a wet EGP apparatus, dried and further treated by contact to produce industrially pure sulfuric acid.

The first stage of a multistage vacuum evaporator system requires four to six cleanings at irregular intervals annually to remove precipitated insoluble and especially SiO 2. This labor-intensive task means that the device will not run for several days at a time.

Example

The amount and composition of the treated waste acid were the same as in the comparative example. 25% tonnes / hour of 23% spent acid (temperature 30 ° C.) as in the comparative example was first introduced into a countercurrent scrubber used to quench the degassed gases at high temperature. Most of the circulated waste acid was injected into this scrubber in the direction of countercurrent to the cracking gas flowing in from the top. A small amount of circulated acid was introduced into the outlet channel installed at the top of the scrubber to flow down the membrane along the scrubber wall. Per hour, 20.8 tonnes of spent acid containing 27.65% sulfuric acid and 200 to 350 g of ultrafine roasted residue dust was discharged from the scrubber at 80 ° C through a stripper from which SO2 was discharged and the remaining 75 tonnes / hour of 23% spent acid And into a three stage vacuum evaporator apparatus. The water to be further evaporated in this unit was 48.5 tons per hour instead of 52.7 tons per hour, resulting in a steam consumption of 5 bar pressure reduced from 35 to 31.5 tons per hour and cooling water consumption to 1575 m3 per hour. Reduced to 1485 m3.

The cracked gas, which was quenched and washed with waste acid, was introduced through a mist collector into a heat exchanger, where 455 m3 of cooling water per hour was used to condense 7.5 tons of water per hour. After stripping the SO 2, the pure, solid free condensate was discharged.

The precipitate was no longer detected in the first evaporation step of the vacuum evaporation unit.

Claims (3)

Spent sulphuric acid containing metal sulphate is evaporated in multiple stages to bring the sulfuric acid concentration to 50 to 70% by weight, optionally cooling the resulting suspension and separating the reusable 50 to 75% by weight sulfuric acid from the metal sulphate. Pyrolysis of the metal sulfate containing sulfuric acid at 800 to 1150 ℃ to produce a decomposition gas containing metal oxides and SO2, and cooling the metal oxide and SO2 containing decomposition gas to 270 to 350 ℃ in a waste heat steam boiler, electrostatic gas purification The apparatus removes dust of the SO 2 containing cracked gas having a temperature of 270 to 350 ° C., thermally cools the dedusted SO 2 containing gas in a quenching machine, cools the quenched gas to a temperature of 25 to 50 ° C., and decomposes SO 2 containing The waste sulfuric acid containing the metal sulfate is removed by drying the mist by removing the gas and then treating the cracked gas to produce sulfuric acid. A process for recovering sulfuric acid from a process comprising: quenching dedusted cracked gas using spent sulfuric acid containing metal sulphate but not solid iron sulphate, introducing the spent sulfuric acid into a quench and the corresponding amount of sulfuric acid concentration. Is used to discharge higher concentrations of spent sulfuric acid, up to 28 to 32% by weight, from the quenching machine, while simultaneously removing and removing the ultrafine dust removed from the cracked gas, and removing the concentrated waste sulfuric acid suspended therein. A process for recovering sulfuric acid from spent sulfuric acid containing metal sulphate, characterized by continuing to evaporate further to a sulfuric acid concentration of 50 to 75% by weight. The method of claim 1, The total amount of waste acid to be treated is introduced into the quencher and a corresponding amount of spent acid with a higher sulfuric acid concentration is discharged from the quencher for subsequent evaporation. The method of claim 1, Wherein only a portion of the spent acid to be treated is introduced into the quench and the corresponding amount of spent acid having a higher sulfuric acid concentration is discharged from the quench and combined with the remaining spent acid, followed by further processing for subsequent evaporation.