WO2006025077A1

WO2006025077A1 - Treatment method of an exhausted pickling bath

Info

Publication number: WO2006025077A1
Application number: PCT/IT2005/000504
Authority: WO
Inventors: Sauro Chiappa
Original assignee: CENTRO STUDI E RICERCHE Srl
Current assignee: CENTRO STUDI E RICERCHE Srl
Priority date: 2004-09-03
Filing date: 2005-09-01
Publication date: 2006-03-09
Anticipated expiration: 2007-03-03
Also published as: ITBS20040093A1

Abstract

The invention refers to a treatment method of exhausted pickling solutions by distillation and irradiation of the intermediate products with microwaves.

Description

DESCRIPTION "Treatment method of an exhausted pickling bath"

[0001] The present invention refers to a process for the treatment of pickling for metal cleaning. [0002] Pickling is an operation for cleaning a metal surface from oxide layers, inorganic deposits, such as inorganic salts, organic substances, minerals, oils, greases, soaps, paints, products from earlier processes, etc. , which form during the standard thermal or mechanical treatments the metals are subject to.

[0003] All these substances cause deposits to form, which make the surface matt rather than bright, and make it impossible to carry out treatments requiring perfect cleaning of the surface, such as: welding, tinning, galvanising, metallization, etc.

[0004] Pickling therefore is an essential metal cleaning operation which is normally carried out by acid or basic chemical or electrolytic treatment. [0005] Chemical acid pickling is normally carried out in cold or hot conditions. In cold pickling, the metal is immersed in a diluted acid bath for a more or less long time and then it is washed in cold water. The concentration of the solution and the acid used respectively depend on the substances to be removed and on the thickness of the relevant layer, as well as on the type of metal. Normally hydrochloric acid is used for iron, steel, galvanised steel and the like, nitric acid or sulphuric acid are used for aluminium and copper alloys and sulphuric acid is used for iron and steel alloys.

[0006] Hot acid pickling is carried out by quick immersion of the metal in a hot bath of concentrated acid and it is basically used for removing the oxides from copper alloys and from precious metals. [0007] The basic pickling consists in immersing the metal in a cold bath of a base and then washing it in cold water. It is mainly used for zinc and aluminium alloys. [0008] In the electrolytic pickling, the metal piece is immersed in an alkaline or acid bath as anode or cathode in an electrolytic cell; also melted-state acid or alkaline compounds may be used as electrolytes. [0009] Pickling baths for metals essentially consist of an aqueous solution of acid or base and metal ions resulting from the metal oxidation during the piece immersion in the pickling bath. In use, pickling baths lose acid or base and gain metal ions, thus losing their effectiveness and becoming exhausted.

[0010] Since cleaning metal surfaces is essential and must be carried out carefully so that the following metal treatments, such as: galvanising, enamelling, nickel- plating, may give the best results, the companies often have a large amount of exhausted pickling baths, poor in acid or base, and rich in heavy metals. For this reason, these are harmful for the environment and for health and must be disposed of at high cost. Moreover, each time pickling must be carried out, it is necessary to provide for new acid or basic baths; of course, this implies considerable additional costs for the companies. [0011] The problem dealt with by the present invention is to avoid the expensive and non-ecological disposal of exhausted pickling baths. This is the object of the present invention, as described hereinafter and claimed in the annexed claims. In fact, it has been seen that exhausted pickling baths can advantageously be treated, thus reducing the need of disposing of them.

[0012] The features and advantages of the process according to the present invention will appear more clearly from the following description of a preferred non-limiting example of embodiment, made with reference to the annexed figures, wherein:

[0013] - figure 1 shows a block diagram of a process for the treatment of exhausted pickling; and

[0014] - figure 2 shows a block diagram of a process and a system for the treatment of exhausted pickling. [0015] The present invention refers to a process for the treatment of exhausted pickling, comprising the following steps:

[0016] a) providing an exhausted acid or basic pickling; [0017] b) separating the base or the free acid from a solid or semi-solid residue;

[0018] c) treating the residue by microwaves. [0019] In step a) , the exhausted pickling has an acid or base concentration comprised between 10 and 200 g/1, preferably between 10 and 100 g/1, more preferably between 40 and 80 g/1.

[0020] Preferably, the exhausted pickling comes from a pickling of hydrochloric acid in a concentration from 30 g/1 to 200 g/1, preferably from 100 g/1 to 140 g/1, more preferably from 120 g/1 to 130 g/1. More preferably, the pickling is an aqueous solution of hydrochloric acid, sulphuric acid or nitric acid, even more preferably it is an aqueous solution of hydrochloric acid in a concentration from 120 to 130 g/1. [0021] Preferably, the exhausted pickling comes from the pickling of ferrous materials or copper; more preferably, from the pickling of iron or steel, even more preferably from the pickling of steel and/or cast iron. [0022] In step b) , the separation of the base or of the acid from the residue occurs by distillation, preferably vacuum distillation, for example by a vacuum evaporator in a suitable material, such as a plastic material. The applied vacuum is normally comprised between 0.4 and 0.9 bar, preferably between 0.7 and 0.8 bar as in the prior art. The temperature at which the vacuum distillation is carried out is normally comprised between 40 and 100 ⁰C, preferably between 50 and 70 ⁰C, even more preferably between 55 and 65 ⁰C. The temperature and the vacuum used vary according to the acid or to the base used and to the concentration of the bath aqueous solution. The man skilled in the art will be able to adjust these parameters to the different operating conditions. Advantageously, the acid solutions separated by distillation, after suitable concentration, are reused for new pickling. [0023] The solid or semi-solid residue obtained after the distillation comprises a portion of residual acid or base, metal salts, hydrated metal salts and crystallisation water. [0024] Preferably, said solid or semi-solid residue is shovellable.

[0025] Favourably, said metal compounds comprise chlorides, oxi-chlorides, hydrated chlorides, oxides, sulphates, nitrates and mixtures thereof, of the metal subject to pickling and which has partly oxidised and dissolved while immersed in the bath. [0026] Preferably, said salts are chlorides, hydrated chlorides and mixtures thereof. Iron chlorides and hydrated iron chlorides are intended as ferrous and/or ferric chlorides and ferrous and/or ferric hydrated chlorides.

[0027] In step c) , the distillation residue is treated with microwave radiations by the use of a special microwave oven comprising a cavity (reactor) for arranging the material to be treated, an energy source (generator) , a transformer-rectifier, a magnetron, an antenna and a waveguide for transporting the microwaves from the generator to the reactor. The reactor comprises a channelling device for recovering the substances produced by the decomposition of the distillation residue. [0028] The distillation residue is placed into the reactor, into special containers, alone or mixed with substances that aid the absorption of microwaves and contribute to concentrating energy and thus, to accelerating the residue decomposition reaction. [0029] The choice of subjecting the distillation residue to microwaves alone or in a mixture depends on its composition. For example, if the residue comprises metal chlorides, metal hydrated chlorides, it must be mixed with suitable substances since these salts are not heated or a heated in a substantially negligible way by the microwaves and therefore, they do not undergo any modifications.

[0030] Most iron oxides (such as magnetite, haematite, cassiterite or mixtures thereof) react positively to microwave heating, thus they can be used as microwave receptors.

[0031] Preferably, in the presence of coal, moreover, there occurs the reduction of oxides due to the reducing action of coal at high temperatures. Coal is a microwave receptive substance and, if mixed with little receptive substances, it still allows quick heating of the mixture. Several authors have proved that iron oxides (haematite and magnetite) mixed with coke coal can be reduced to metal iron (Standish, N., Worner, H; Iron and Steel Maker, 1991, 18, (5) , 59-61 e Gomez, I., Aguilar, J.A. ; Mat. Res. Soc. Proc, 1995, 366,347-352) . [0032] Coal and/or receptive metal oxides (magnetite, haematite, cassiterite or mixtures thereof) are added to the distillation residue, prior to treatment with microwaves, in an amount comprised between 1% and 30% in weight of the total residue and more preferably, between 2% and 10% in weight of the total residue. [0033] At first, heating the semi-solid mass obtained as distillation residue produces an evaporation of the residual humidity followed by decomposition of the metal salts and of the hydrated metal salts with development of gases and fumes that are then recovered. The composition of these gases and fumes depends on the type of pickling used. For example, if the pickling is an aqueous solution of hydrochloric acid, gaseous hydrochloric acid and gaseous chlorine and iron oxides will form, which evolve from the semi-solid mass and are conveniently recovered and recycled for other uses (such as for re-concentrating the diluted solution and/or reused in the solutions of new pickling and/or reused in the storage tanks) or sold. [0034] The solid residue remaining from the decomposition of metal salts and of hydrated metal salts comprises metal oxides and elementary metal. For example, if the distillation residue comprises iron chlorides and/or hydrated iron chlorides, a residue comprising iron oxides and metal iron is obtained after the decomposition. [0035] Metal oxides and elementary metal can be separated by the methods known in the art and easily recycled as they are applied in a number of industrial sectors. If the decomposition residue comprises metal iron and iron oxides, such substances can be separated from the non¬ magnetic substances by magnetic sorting and conveniently recycled. [0036] The distillation residue is irradiated with microwaves at a frequency comprised between 300 MHz and 300 GHz, preferably between 0.5 GHz and 50 GHz, more preferably between 1 GHz and 5 GHz. The time required for obtaining an almost total decomposition of the distillation residue depends on the amount of material treated and in any case, it is comprised between 1 and 120 minutes, preferably between 5 and 60 minutes, more preferably between 10 and 30 minutes. [0037] EXAMPLE [0038] One hundred litres of a solution of exhausted pickling of steel, containing about 40 g/1 of free hydrochloric acid and concentrated in ferrous and ferric ions in an amount of about 120 g/1, are delivered through a pump to a system for treating exhausted pickling and in particular, to a storage tank (ST) and after that, to the distillation apparatus, for example comprising a vacuum evaporator (E) and a crystallizer (C) (see figures 1 and 2) .

[0039] At the distillation system, the solution is subject to distillation in the vacuum distiller or evaporator at a pressure of about 0.8 bar at a temperature of about 55 - 75 ⁰C for the time required for obtaining as leading products gaseous hydrochloric acid and water in the form of vapour. Said leading products, once condensed, give a solution of hydrochloric acid to be reused as reintegration of the pickling itself. [0040] After distillation, a shovellable semi-solid residue is obtained, having a muddy consistency of 25-30 Kg of weight containing free hydrochloric acid, iron chloride and crystallisation water. [0041] Said shovellable distillation residue is transferred to a special container for microwaves and is then mixed with a variable amount comprised in a range between about 2% and 5% in weight of grit and/or coal powder and/or magnetite and/or haematite and/or cassiterite. Afterwards, the container is transferred in a reactor into a microwave oven (MWO) .

[0042] The microwave oven (MWO) favourably comprises at least one energy source, such as a generator, at least one -transformer-rectifier, at least one magnetron, at least one antenna and/or at least one waveguide.

[0043] Transfer into the microwave oven (MWO) may be carried out discontinuously after the insertion of the product to be treated in an alumina melting pot (MP) or continuously by a belt or screw system. The treatment time (heating) depends on the amount of material to be treated and on the oven power.

[0044] The shovellable residue is subject to radiation at a frequency of about 2.45 GHz for a time comprised in a range between about 10 and 30 minutes (the time also depends on the power of the microwave oven and on the amount of product) . Heating occurs suddenly up to about 1000 ⁰C, causing the release and output of volatile products.

[0045] Water in the form of vapour, free residual gaseous hydrochloric acid and then hydrochloric acid and gaseous chlorine coming from the decomposition of hydrated iron chloride develop in a sequence and are collected in a divided manner. Moreover, in the volatile phase, there is comprised haematite and a minimal portion of anhydrous iron chloride, which sublimates.

[0046] The reactor in the microwave oven is subject to vacuum so as to extract - by suitable channelling - the volatile products that evolve while heating the shovellable muddy residue. [0047] At the outlet of the channelling of the volatile phase there is arranged a cyclone for the collection of the haematite and of the sublimated iron chloride and in series there is arranged a heat exchanger for the condensation of steam and of the hydrochloric acid; in this way, a solution of hydrochloric acid is obtained, which is mixed with the solution condensed upstream of the distillation system.

[0048] Moreover, the gaseous chlorine is made to bubble in a solution of ferrous chloride to obtain ferric chloride. [0049] The decomposed residue after thermal microwave treatment, based on the reaction progress and duration, on the amount of coal, on the maximum temperature reached and the like, contains for example about 2% of metal iron and about 90% of iron oxides (Fe2O3, Fe3O4, and FeO) , whereas the remaining amount consists of non-decomposed products. The separation between the various products is carried out by magnetic sorting and/or by other separations known in the art, such as washing. The recovered solid products can be reused in specific sectors.

[0050] Thus, with reference to figures 1 and 2, the recovery process for hydrochloric acid and iron, that is, a mixture comprising iron oxide, magnetite and metal iron, is substantially carried out by two unit systems: a vacuum evaporator of plastic material and a microwave device.

[0051] The exhausted pickling, comprising the iron chloride and the acid, is stored in a storage tank (ST) upstream of the recovery system (step a) of figure 1) . [0052] Afterwards, the stored acid is delivered to the evaporator (E) during the concentration process. [0053] In the vacuum evaporator, the vacuum is obtained for example by a suitable Venturi type pump. ^" [0054] Moreover, in the vacuum evaporator, most of the acid and water are separated from the iron salt and from the residual liquid (step b) of figure 1) and the acid (RA) is recovered and delivered to the storage of recovered acid (RAS) (step bl) of figure 1) .

[0055] To carry out the separation, the required heat or thermal energy are supplied for example by a steam or diathermic oil system (SDO) , or alternatively it is heat recovered from existing processes.

[0056] Similarly, the refrigeration water (RW) required for the process is supplied for example by an evaporative tower or alternatively, by other existing processes at the system.

[0057] At this point, the solution (CA) containing the concentrated iron and the chloride is delivered to a crystallizer (C) (step b2) of figure 1) where there occurs the last separation between the iron crystals and the starting solution; preferably, the starting solution is delivered again to the storage tank (ST) , to recover for example a percentage of iron chloride which does not precipitates (step f) of figure 1) . [0058] Iron crystals are delivered, through screw (AC) , to a melting pot (MP) of suitable material, where they are mixed with a microwave receptive agent; afterwards, the melting pot (MP) is automatically inserted and positioned into the microwave oven (MWO) for decomposition. [0059] The equipment (MWO) is preferably provided with a control panel (MWF) for the control of the parameters and of the progress of the reduction process itself. [0060] At this point, by the microwave equipment (MWO) the final separation (step c) of figure 1) is carried out and during the reaction, a gaseous mixture (G) , or emission fumes, is formed, comprising HCl, C12, chloride and an aerosol comprising iron oxides (step c2) of figure 1) ; these evolving compounds are sucked, separated through cyclone, and the gaseous phase is adsorbed by the recovery acid solution (step e) of figure 1) , whereas the solid (iron oxide, magnetite, metal iron) remaining on the melting pot (steps cl) and d) of figure 1) is extracted through screw. [0061] ADVANTAGES [0062] The process of the present invention allows the total recycling of exhausted pickling, that is, both of the residual acid or base and of the shovellable solid or semi-solid residue obtained after distillation. This is doubtless an advantage over the processes of the prior art which only recover the residual free acid or base by distillation, whereas the distillation residue is regarded as waste and disposed of with expensive methods. [0063] With the process of the invention, besides the recovery of the residual free hydrochloric acid through distillation, gaseous substances are also recovered from the distillation solid residue, such as hydrochloric acid and chlorine from ferrous chloride, which can be reused for other applications or sold, and the elementary metal and the metal oxides that after separation are applied in various industrial sectors; that is, the muddy residue is transformed into raw material to be reused or sold and not to be disposed of. Moreover, the acid solutions recovered by distillation, after suitable concentration, can be reused as pickling. Thus, the process of the invention eliminates the problem of disposing of hazardous industrial waste, with consequent saving for the companies and protection of the environment, and also allows the reuse and/or the sale of important chemicals, which can be applied in a number of industrial sectors, with consequent reduction of the total manufacturing cost.

[0064] The use of microwaves allows optimising the treatment process in terms of time and energy saving. [0065] In fact, as known, microwaves produce a heating of the irradiated material by the transfer of energy rather than heat, thus avoiding dissipation of thermal energy. Moreover, chemical reactions, in this case the decomposition reaction, occur very quickly, thus implying a considerable time saving. [0066] At present, the technology of vacuum distillation of pickling recovers water and free acids thus obtaining a solution with a low acid concentration.

[0067] In order to be reused as pickling, such solution must be integrated with concentrated hydrochloric acid to obtain the suitable concentration.

[0068] This implies an increase of volumes which is difficult to handle at galvanising works. [0069] Advantageously, with the new treatment, the volumes of reintegration of the concentrated acid will be drastically reduced.

[0070] Another advantage of the new method is that the treatment of the exhausted pickling will preferably be in a concentration of free acidity of 40-80 g/1, that is, when the pickling is still active, whereas in the prior art acid is used as much as possible, working in conditions of poor pickling effect, to dispose of it at 10-30 g/1.

[0071] Advantageously, the treatment process of pickling according to the present invention can be carried out both at the company that carries out the pickling and at other companies, for example at companies working in the field of treatment of industrial waste.

[0072] According to a further advantageous aspect, the recovery process of hydrochloric acid and iron oxides and metal iron is successfully applied both to exhausted solutions of hydrochloric acid and iron chloride coming from pickling solutions, and to exhausted solutions of hydrochloric acid and iron chloride from other sources.

Claims

1. A process for the treatment of pickling, comprising the following steps: a) providing an exhausted acid or basic pickling; b) separating the base or the free acid from a solid or semi-solid residue; c) treating the residue by microwaves.

2. A process according to claim 1, wherein said exhausted pickling has an acid or base concentration comprised between 10 and 200 g/1, preferably between 10 and 100 g/1, more preferably between 40 and 80 g/1.

3. A process according to claim 1 or 2, wherein said exhausted pickling is an acid pickling in a concentration between 10 and 200 g/1, preferably between 10 and 100 g/1, more preferably between 40 and 80 g/1.

4. A process according to any one of claims 1 to 3, wherein said pickling is an aqueous solution of hydrochloric acid, sulphuric acid or nitric acid, preferably it is an aqueous solution of hydrochloric acid.

5. A process according to any one of claims 1 to 4, wherein said exhausted pickling comes from the pickling of ferrous materials or copper, preferably from the pickling of iron and/or steel and/or cast iron.

6. A process according to any one of claims 1 to 5, wherein said exhausted pickling comes from the pickling of steel.

7. A process according to any one of claims 1 to 6, wherein in step b) , said separation occurs by distillation of the acid or of the base to produce said solid or semi-solid residue.

8. A process according to claim 7, wherein said distillation is carried out at a temperature in a range from 40 C° to 100 ⁰C.

9. A process according to claim 7 or 8, wherein said distillation is carried out at a temperature in a range from 50 C° to 70 ⁰C.

10. A process according to any one of claims 7 to 9, wherein said distillation is carried out at a temperature in a range from 55 C° to 65 ⁰C.

11. A process according to any one of claims 7 to 10, wherein said distillation is carried out in vacuum.

12. A process according to claim 11, wherein said vacuum is comprised between 0.8 and 0.9 bar.

13. A process according to any one of claims 1 to 12, wherein said solid or semi-solid residue comprises residual acid or base, metal salts, hydrated metal salts and crystallisation water.

14. A process according to any one of claims 1 to 13, wherein said solid or semi-solid residue is shovellable.

15. A process according to claim 13, wherein said metal salts are chlorides, sulphates, nitrates, carbonates, hydroxides of the metal subject to pickling and mixtures thereof.

16. A process according to claim 13, wherein said metal salts are chlorides, hydrated chlorides, sulphates, hydrated sulphates, copper or iron hydroxides.

17. A process according to claim 15 or 16, wherein said metal salts are iron chlorides and/or hydrated iron chlorides.

18. A process according to any one of claims 1 to 17, wherein in step c) , said solid or semi-solid residue is subject to microwave radiation alone or in a mixture with one or more of the substances selected from the following group: magnetite, haematite, cassiterite, coal.

19. A process according to any one of claims 1 to 18, wherein in step c) , said solid or semi-solid residue is irradiated at a frequency comprised between 300 MHz and 300 GHz, preferably between 0.5 GHz and 50 GHz, more preferably between 1 GHz and 5 GHz.

20. A process according to any one of claims 1 to 19, wherein in step c) , said solid or semi-solid residue is irradiated for a time comprised between 1 and 120 minutes, preferably between 5 and 60 minutes, more preferably between 10 and 30 minutes.

21. Use of radiations with typical microwave frequency for the treatment of exhausted pickling according to any one of claims 1 to 20.

22. A system for the treatment of an exhausted pickling comprising:

- at least one pump suitable for handling said pickling;

- at least one storage tank for containing said pickling;

- at least one vacuum distillation apparatus;

- at least one acid container; - at least one microwave oven comprising:

- at least one energy source, such as a generator;

- at least one transformer-rectifier;

- at least one magnetron;

- at least one antenna and/or at least one waveguide; - at least one reactor, said reactor comprising at least one channelling device working in vacuum for the extraction of gaseous and/or volatile products.

23. A system according to claim 22, wherein said distillation apparatus comprises: - at least one vacuum evaporator for separating acid and water from the iron salt and from the residual liquid;

- at least one crystallizer for separating the iron salts from the residual liquid.