Method for recovering hydrochloric acid from iron chloride solution

Field of the invention

The present invention relates to a method for recovering hydrochloric acid from iron chloride solution by reacting the iron chloride with sulfuric acid to produce iron sulfate monohydrate. More particularly the present invention relates to such a method wherein the hydrochloric acid is evaporated in reduced pressure and subsequently recovered by condensing.

Background of the invention

In certain processes, such as industrial processes, the recovery of hydrochloric acid from solution containing iron chloride is required. One such industrial process is the pickling of steel with hydrochloric acid in the processing of steel. Aqueous waste solution containing ferrous chloride and hydrochloric acid is produced. Spent pickle liquor is considered hazardous waste. For e.g. environmental reasons the hydrochloric acid needs to be recovered and possibly recycled.

Methods for recovering hydrochloric acid from iron chloride solution are known in the art. US 5 417 955 discloses a two-step method for converting ferrous chloride from a pickling liquor. The ferrous chloride is first mixed with sulfuric acid to produce ferrous sulfate and HCI. The HCI is carried to an absorption tower where most of the HCI is collected and the remainder is returned to the reactor. The fer- rous sulfate is separated from the sulfuric acid. The unreacted sulfuric acid is returned to the reactor and the ferrous sulfate is transported to a second reactor where it is reacted with sulfuric acid and air (O ₂ ) to produce ferric sulfate. The ferric sulfate is removed from the second reactor. Both reactions are carried out at relatively low temperatures under atmospheric pressures. The air treatment used in US 5 417 955 is generally called air stripping.

US 3 635 664 discloses a method for recovering hydrochloric acid from a hydrochloric acid pickling waste by distillation comprising the steps of adding sulfuric acid to hydrochloric acid waste to convert FeCI ₂ in said waste to HCI and FeSO ₄ and to obtain a mixture containing at least 38 percent by weight of free sulfuric acid; distilling said resultant mixture to vaporize substantially all amount of the HCI therefrom together with water and to precipitate ferrous sulfate; condensing said HCI and water thus vaporized to recover hydrochloric acid; separating the precipi-

tated ferrous sulfate from the residual liquid; and circulating the resultant liquid free of ferrous sulfate as a sulfuric acid source. In Example 1 there is described a method wherein part of the HCI is removed in a flash distillation unit and the rest of the HCI is removed in another distillation apparatus. In another example the chemicals are mixed in a reactor and the HCI is distilled in evaporator/distillation tower combination.

DE 4 122 920 A1 discloses a method wherein the iron sulfate is recovered as hep- tahydrate and fed to a separate vacuum crystallizer outside the mixing vessel to crystallize and recover the iron sulfate heptahydrate. It is specifically mentioned that it is not desired to obtain the iron sulfate as monohydrate because it is not water soluble and therefore it is not suitable for the method described because the method aims at obtaining soluble iron sulfate to get it out from the vessel to be precipitated elsewhere. In DE 4 122 920 A1 very low temperatures are used and therefore only little water is evaporated with hydrochloric acid. In consequence no regular condenser can be used.

However, the methods and device arrangements described have some disadvantages. For example in Example 3 of US 3 635 664 there is described the use of an apparatus shown in Fig. 3 wherein the HCI is distilled in evaporator/distillation tower combination. With the reaction conditions described the solids may precipitate in the system. As there is only weak termosyphone mixing in the distillation apparatus, it may be blocked.

Furthermore, the air stripping method described in US 5 417 955 includes a pressurized reactor. In such method there is always a hazard of leakage in which case the pressurized strongly acidic mixture is spread widely from the reactor. Further, in the known methods multiple vessels, reactors, vacuum crystallizers or the like are required thus making the device arrangements complex, space-requiring and expensive. Also very high temperatures used in some methods may cause corrosion and need energy.

Summary of the invention

The present invention provides a method for recovering hydrochloric acid from iron(ll)chloride solution comprising mixing the iron chloride with sulfuric acid to obtain reacting mixture wherein the sulfuric acid concentration is at least 55% (w/w) to produce hydrochloric acid which is subsequently evaporated at reduced pres-

sure of 225 mbar or less and recovered by condensing, and iron sulfate monohy- drate, wherein the mixing, the evaporation and the precipitation of iron sulfate monohydrate are carried out in the same single vessel. In one embodiment the sulfuric acid concentration is about 60% (w/w) or more. At least part of the iron sulfate monohydrate is separated and recovered. In one embodiment the iron chloride solution is spent pickling liquor.

It was discovered that to recover substantially all the hydrochloric acid only evaporation in reduced pressure was required. Furthermore it was surprisingly discov- ered that the mixing of the solutions, the evaporation of the hydrochloric acid and the precipitation of most of the iron sulfate monohydrate can be carried out in the same container as the reaction. This has several advantages when compared to the state of the art. Thus, the present invention also provides a method for precipitation and recovery of iron sulfate.

One advantage is that as the process is simpler and can be carried out in simple and compact apparatus, it requires smaller investments and saves space and time. Furthermore, it is more reliable. For example, the separate vacuum crystal- lizer used in the method of DE 4 122 920 A1 may make the investment costs con- siderably higher.

Another advantage is that the mixing of the reacting compounds occurs in said same vessel, for example by utilizing the flow of reacting mixture or by a mechanical mixer.

Another advantage is that because low temperatures can be used the process of the present invention saves energy. Also with lower temperatures there are fewer problems with corrosion.

Still another advantage is that when evaporation at low pressure is utilized the system is safe to use when compared to pressurized systems, such as systems utilizing air stripping.

Brief description of the drawings

Figure 1 shows a schematic view of one embodiment of a device setup useful in the process of the present invention.

Figure 2 shows a schematic view of another embodiment of a device setup useful in the process of the present invention.

Figure 3 shows a schematic view of still another embodiment of a device setup useful in the process of the present invention.

Detailed description of the invention

The iron chloride solution which may be used in the method of the present inven- tion contains 2-valent (ferrous) iron chloride. With the amounts of sulfuric acid and temperatures used herein the main product is ferrous sulfate monohydrate (FeSO ₄ -H ₂ O). The reaction with sulfuric acid in may be the following:

FeCI ₂ + H ₂ SO ₄ + H ₂ O → FeSO ₄ -H ₂ O +2 HCI

One example of a starting material containing ferrous chloride is spent pickling liquor. The spent pickling liquor is mixed with sulfuric acid to obtain reacting mixture wherein the sulfuric acid concentration is at least 55% (w/w) to produce hydrochloric acid which is subsequently evaporated at reduced pressure of 225 mbar or less and recovered by condensing, and iron sulfate monohydrate, wherein the mixing, the evaporation and the precipitation of most of the iron sulfate monohydrate, e.g. substantially all, are carried out in the same single vessel. At least part of the iron sulfate monohydrate is separated and recovered and the unreacted sulfuric acid may be recycled.

In the method of the present invention the reacting, the evaporating and the precipitating of iron sulfate monohydrate steps are carried out in the same single vessel (or reactor as the terms may herein be used interchangeably). The reacting compounds are also mixed in said vessel. Term "vessel" as used herein refers to any suitable container wherein said reaction will occur, such as a reactor or the like. In one embodiment the reacting mixture is recycled and led back to the reactor by a flow high enough to cause the mixing of the reacting solutions in the reactor, for example 20-60 t/h to a vessel that has a size of about 3 m ³ . In another embodiment a mixer, such as a mechanical mixer, is used.

The pickling liquor and sulfuric acid are fed to the vessel in such proportions that the concentration of the free sulfuric acid should be at least about 55% by weight in the reaction mixture in the vessel. The sulfuric acid concentration in the vessel

can be adjusted to the desired level by feeding fresh concentrated sulfuric acid into the vessel. It is also essential to remove excess water originated from the spent pickling liquor in the evaporation step to maintain the sulfuric acid concentration at desired level. Generally an advantageous concentration of the sulfuric acid is at least about 60% by weight in the reaction mixture in the vessel. If the concentration is significantly lower the reaction will slow down and the concentration of chloride in iron sulfate monohydrate product will increase. In certain uses of iron sulfate monohydrate, such as the use in animal feed, low chloride concentration is required. Further, the corrosion caused by chloride is avoided.

The temperature of the reacting mixture in the vessel may be in the range of 70- 100 ⁰ C. In one embodiment the advantageous range of 80-95 ⁰ C is used. The heating may be carried out by using any suitable heating method known in the art. Examples of heating methods are heating mantel, heating coil and combinations thereof. However, especially in large scale process it is difficult to get large enough heating surface needed for the heat transfer. In one embodiment an external heat exchanger is used for heating and reacting mixture is fed through the heat exchanger and recycled back to the reactor/vessel. Generally, when the heat exchanger is used the recycled reacting mixture is heated to about 20 ⁰ C higher than the temperature of the reacting mixture in the vessel.

In the evaporating of hydrochloric acid and water the pressure should be low, for example 225 mbar or less. If higher pressures are used, the temperature needed for evaporation will be too high, for example at 300 mbar the temperature would be about 120 ⁰ C. In such high temperatures corrosion increases and some materials, such as plastics or the like, may not last. In one embodiment the pressure is about 110 mbar or less. Then smaller heat exchangers can be used. Also the corrosion of the equipment is further reduced.

After the evaporation the hydrochloric acid is recovered for example by condensing using any suitable condenser known in the art, such as a plate or tube condenser.

Further, at least part of the iron sulfate monohydrate which is precipitated in the sulfuric acid treatment is settled, separated and recovered. The settling is mostly achieved in the same vessel where the mixing and evaporation takes place. Generally the coarse iron sulfate crystals are settled and are easily recoverable. Some fine matter flows with the reacting mixture in the circulation through the heat ex-

changer and may later act as nuclei to facilitate further crystallization. The precipitation of iron sulfate monohydrate crystals onto the nucleated particle is generally called crystallization (see Kirk-Ottmer's Encyclopedia of Chemical Technology). To facilitate the settling in the vessel it may be designed specifically to obtain efficient settling of coarse iron sulfate crystals. The outlet and inlet of the reacting mixture circulation are in the upper part of the vessel wherein the mixing occurs. The settling takes place in the more sedate lower part. The flows may be directed to obtain a circulating movement of the solution in which case the centrifugal force will drive the heavier particles first onto the wall of the vessel where they will settle down.

In the present invention the settling takes place in the reactor and the reacting mixture that contains only small amount of iron sulfate monohydrate is fed to the heat exchanger and is recycled back to the reactor. The process may be a continuous process or a batch process. Different embodiments of device setups may be utilized in the process, such as ones described below.

An example of a device arrangement useful in the method of the invention is shown in Figure 1. In this setup settling occurs already in the reactor 15 and no settling tank is required. Designated at 1 is the line for feeding aqueous acidic waste solution containing ferrous chloride to the reactor 15. The reactor may be provided with means for mixing the content (not shown). The reactor is further provided via line 2 for feeding fresh sulfuric acid from the tank 21 , line 3 that is connected to a condenser 20, line 7 for recycling the heated reacting mixture, and line 23 for recycling filtrate from break tank 18. The reactor 15 is directly connected via line 8 to separating means 17, such as a filter or a centrifuge. Line 10 is for discharge of the iron(ll)sulfate monohydrate from the separating means 17 and line 9 leads the filtrate to the break tank 18. Line 25 is connected between reactor and heat exchanger 19. Line 11 is the inlet and line 12 is the outlet for the heating me- dia of the heat exchanger. Line 13 is the inlet and line 14 is the outlet for the cooling media of the condenser. The vacuum is maintained in the reactor and condenser with the vacuum pump 22 via line 24. The condensed water-HCI mixture is discharged via line 4.

Another example of a device arrangement useful in the method of the invention is shown in Figure 2. Designated at 1 is the line for feeding aqueous acidic waste solution containing ferrous chloride to the reactor 15. The reactor may be provided with means for mixing the content (not shown). The reactor is further provided via

line 2 for feeding fresh sulfuric acid from the tank 21 , line 3 that is connected to a condenser 20, line 7 for recycling the heated reacting mixture, line 23 for recycling filtrate from break tank 18 and line 5 that is connected to a settling tank 16. The settling tank is connected via line 8 to separating means 17, such as a filter or a centrifuge. Line 10 is for discharge of the iron(ll)sulfate monohydrate from the separating means 17 and line 9 leads the filtrate to the break tank 18. Line 6 is connected between settling tank and heat exchanger 19. Line 11 is the inlet and line 12 is the outlet for the heating media of the heat exchanger. Line 13 is the inlet and line 14 is the outlet for the cooling media of the condenser. The vacuum is maintained in the reactor and condenser with the vacuum pump 22 via line 24. The condensed water-HCI mixture is discharged via line 4.

Another example of a device arrangement useful in the method of the invention is shown in Figure 3. In this setup there is a feed from the settling tank 16 to the re- actor 15. Designated at 1 is the line for feeding aqueous acidic waste solution containing ferrous chloride to the reactor 15. Line 25 is connected between reactor and heat exchanger 19. The reactor may be provided with means for mixing the content (not shown). The reactor is further provided via line 2 for feeding fresh sulfuric acid from the tank 21 , a line 3 that is connected to a condenser 20, line 7 for recycling the heated reacting mixture, and line 23 for recycling filtrate from break tank 18. The reactor 15 is connected to the settling tank 16 with a line 5 and the settling tank 16 is also connected back to the reactor 15 with a line 6. The settling tank 16 is connected via line 8 to separating means 17, such as a filter or a centrifuge. Line 10 is for discharge of the iron(ll)sulfate monohydrate from the separat- ing means 17 and line 9 leads the filtrate to the break tank 18. Line 25 is connected between the reactor 15 and the heat exchanger 19. Line 11 is the inlet and line 12 is the outlet for the heating media of the heat exchanger. Line 13 is the inlet and line 14 is the outlet for the cooling media of the condenser. The vacuum is maintained in the reactor and condenser with the vacuum pump 22 via line 24. The condensed water-HCI mixture is discharged via line 4.

Pumps may be used in the setup as necessary, for example in most of the lines indicated with an arrow in the figures. In some cases the stock feeds may operate by vacuum without pumps, but to maintain steady feeding it is generally advanta- geous to use pumps.

The principle of a continuous process shown in a non-limiting example of Figure 1 is the following. Spent pickling liquor is fed through the line 1 typically at flow rate

0.5-3 t/h, for example at flow rate 1.7 t/h, into the reactor 15 where it reacts with sulfuric acid. The sulfuric acid concentration is kept in the reactor 15 at desired level, which is at least 55% (w/w), by dosing fresh sulfuric acid through the line 2 at flow rate 0.05-1 t/h, typically at flow rate 0.3 t/h from the storage tank 21 and heated reacting mixture from the heat exchanger circulation through the line 7 at flow rate 10-100 t/h, typically at flow rate 60 t/h into the reactor. Heat, which is needed to evaporate the hydrochloric acid, is introduced into the reaction mixture using an external heat exchanger 19. The reacting mixture is fed to the heat exchanger via line 25 and recycled into the reactor via line 7. The temperature in the reacting mixture in the reactor is kept on the desired level, 80-95 ⁰ C by heating it by heat exchanger to 90-130 ⁰ C, typically to about 110-120°C. In this embodiment the heating medium of heat exchanger is steam, but other heating media can be used as well.

The pressure in the reactor 15 is kept at a desired level, such as 100-200 mbar, with a vacuum pump 22. With said pressure the temperature in the reactor is in the range of 80-95 ⁰ C, depending on the sulfuric acid concentration. The hydrochloric acid/water vapor evaporates through the line 3 into a condenser 20 where it is condensed and led to storage tank through the line 4. For example at flow rate 1.7 t/h of spent pickling liquor about 1.5 t of 19-20% hydrochloric acid will be obtained.

The ferrous sulfate monohydrate precipitates in the reacting mixture in the reactor. The main portion of ferrous sulfate monohydrate product is settled in the reactor 15. The settled fraction is led to a pressure filter through the line 8. The final separation of ferrous sulfate monohydrate from the solution is carried out by filtration, preferably by pressure filtration. Also other means of filtration may be used, such as a belt filter or a drum filter. The ferrous sulfate monohydrate crystals are discharged via line 10. For example at flow rate 1.7 t/h of spent pickling liquor about 0.5 t iron sulfate monohydrate will be obtained. The filtrate, which comprises mainly unreacted sulfuric acid, is recycled back into the reactor preferably via the break tank 18.

The principle of a continuous process shown in a non-limiting example of Figure 2 is the following. Spent pickling liquor is fed through the line 1 typically at flow rate 0.5-3 t/h, for example at flow rate 1.7 t/h, into the reactor 15 where it reacts with sulfuric acid. The sulfuric acid concentration is kept in the reactor 15 at desired level, which is at least 55% (w/w), by dosing fresh sulfuric acid through the line 2

at flow rate 0.05-1 t/h, typically at flow rate 0.3 t/h from the storage tank 21 and heated reacting mixture from the circulation through the line 7 at flow rate 10-100 t/h, typically at flow rate 60 t/h into the reactor. Heat, which is needed to evaporate the hydrochloric acid, is introduced into the reaction mixture using an external heat exchanger 19. The reacting mixture is fed to the heat exchanger via line 6 and recycled into the reactor via line 7. The reacting mixture is heated to 90-130 ⁰ C, typically to about 110-120 ⁰ C in the heat exchanger in order to maintain the optimal temperature 80-95 ⁰ C in the reactor. In this embodiment the heating medium of heat exchanger is steam, but other heating medium can be used as well.

The pressure in the reactor 15 is kept at a desired level, such as 100-200 mbar, with a vacuum pump 22. With said pressure the temperature in the reactor is in the range of 80-95 ⁰ C, depending on the sulfuric acid concentration. The hydrochloric acid/water vapor evaporates through the line 3 into a condenser 20 where it is condensed and led to storage tank through the line 4. For example at flow rate 1.7 t/h of spent pickling liquor about 1.5 t of 19-20% hydrochloric acid will be obtained.

The ferrous sulfate monohydrate precipitates in the reacting mixture. The mixture is then continuously led to a settling tank 16 through the line 5. The main portion of ferrous sulfate monohydrate product is settled in the settling tank 16. It can be separated from the main stream 5 using hydro cyclone, settling tank, centrifuge or any other suitable separation method known in the art. The overflow is led through a heat exchanger back to the reactor and the settled fraction is led to a pressure filter through the line 8 directly or via a break tank (not shown). The final separa- tion of ferrous sulfate monohydrate from the solution is carried out by filtration, preferably by pressure filtration. Also other means of filtration may be used, such as a belt filter or a drum filter. The ferrous sulfate monohydrate crystals are discharged via line 10. For example at flow rate 1.7 t/h of spent pickling liquor about 0.5 t iron sulfate monohydrate will be obtained. The filtrate is recycled back into the reactor preferably via the break tank 18.

Example 1

A test production wherein the process was run at a steady state was carried out in a pilot reactor. Spent pickling liquor that contained 9.1% (w/w) of iron (II) and free HCI 4.8% (w/w) was fed continuously (15 kg/h) into a reactor having a volume of 50 I. The reactor was equipped with a mixer and an internal heating coil. The sulfuric acid concentration in the reactor was kept at 60% (w/w) by feeding fresh 93%

(w/w) sulfuric acid continuously into the reactor at a rate of 8.5 kg/h. The pressure of the reactor was kept under 150 mbar by a vacuum pump. The temperature in the reactor was about 95°C, corresponding to the pressure and sulfuric acid concentration. Said feed rate of the spent pickling acid was kept at a level where the residence time in the reactor was high enough to keep the chloride residue in the solution at 1%. The ferrous sulfate monohydrate product was separated from the overflow of the reactor by settling before the pressure filtration. The yield of the ferrous sulfate monohydrate after filtration was about 4,3 kg/h (as 100%) The chloride residue of the product was 0.1 %. The hydrochloric acid was evaporated by introducing low pressure steam into the heating coil. The yield was about 10 kg/h and the average concentration of the condensed HCI was 20.5% corresponding to the chloride concentration of the spent pickling liquor and the water balance of the process.

Example 2

Spent pickling liquor that contained 9.3% (w/w) of iron (II) and 5.0% (w/w) of free HCI was fed continuously (650 kg/h) into a reactor having a volume of 3500 I. In the beginning the liquor reactor was heated up with external steam (3 bar). The sulfuric acid concentration in the reactor was kept at 60% (w/w) by feeding fresh 93% (w/w) sulfuric acid continuously into the reactor at a rate of 133 kg/h. The pressure of the reactor was kept under 150 mbar. The temperature in the reactor was kept at about 85°C by circulating the reacting mixture through an external heat exchanger. The flow through the exchanger was in the range of 25-40 t/h. Said feed rate of the spent pickling acid was kept at a level where the residence time in the reactor was high enough to keep the chloride residue in the solution at 1 % or below. The ferrous sulfate monohydrate product which was precipitated in the reactor was pumped from the reactor to the break tank and from there forward to the filter press. Filtered ferrous sulfate monohydrate was produced 190 kg/h. (Fe 29%, Cl < 0.5 %). The hydrochloric acid that was evaporated had average concentration 19.5% after condensing and the flow was (590 kg/h) corresponding to the chloride concentration of the spent pickling liquor and the water balance of the process.