FIELD OF THE INVENTION The present invention relates to the recovery of metals and acids coming from exhausted pickling solutions and/or neutralization sludge of stainless steel in their total separation into secondary raw materials that can be reused, such as refractory oxides, iron sulphates and hydroxides, chromium hydroxide or metallic chromium or chromates, metallic nickel and hydrofluoric, nitric and/or sulphuric acids. BACKGROUND OF THE INVENTION

Many industrial processes require use of acidic baths. During use these baths are enriched with metals, their activity is reduced and must be partially supplemented with fresh acids. The discharged portion of exhausted solution is generally neutralized thus generating sludge. Said sludge comprising metal hydroxides and acid salts must be disposed of with modes and costs depending from concentration and danger of die contained chemicals. For instance, when pickling stainless steel mixtures of nitric/hydrofluoric or sulphuric/hydrofluoric acids are mainly used. When these baths contain more than 60-80 g/1 of dissolved metals, they lose their effectiveness and their management becomes problematic, the working tanks must be partially emptied and supplemented with fresh solution restoring the acid concentration and reducing the metal concentration. The discharged bath portion is generally neutralized with kmewash for obtaining a sludge comprising metals, mainly iron, chromium and nickel in the form of hydroxides and anions, fluorides and sulphates in the form of calcium salts. When nitric acid is present, waste treatment is even more problematic in view of the high solubility of the nitric acid salts, making impossible a total removal of nitrates from waste water.

Only recently in large plants where the volumes to be disposed of are high, techniques such as diffusion dialysis or acid retard are used, apt to recover free acids contained in waste, so as to reduce consumption of kmewash and volume of sludge produced in the subsequent neutralization. Other techniques for treating steelworks waste are known, such as electrodialysis, pyrolysis, cryogenic separation systems, solvent extraction. However all said techniques did not find a really wide diffusion in view of the plant and management costs or laboπousness and poor efficiency of said methods. Another restraining factor is also the low value added of the recovered acids and metals. However a dramatic increase of the world demand of raw materials is presently occurring, with considerable increase of their market prices and on the other hand storage of steelworks sludge is increasingly cost burdensome and logistically difficult in view of lack of storage sites accepting this waste classified as carcinogenic because of the nickel inner contents.

Treatment of this waste therefore becomes favourable, to win the expensive raw materials (often being the dangerous ones), so as to reduce or even eliminate thek volume and environmental impact. From this standpoint even the dump sites, where this steelworks sludge were stored for years, become mines of raw materials of commercial value.

Several studies were conducted in this direction and are summarized as follows.

A paper by E. Gomes, E. Valles in J. Appl. Electrochem, 29 (1999) p. 805-812, discloses a process of recovery and separation of nickel and chromium from industrial exhausted pickling baths by electrodeposition. The complexity and laboriousness of the disclosed process however strongly limit its use in an industrial environment.

Documents IT2006A0042 (Condoroil) and DE4435232 (Hahnewold GmbH) disclose a process of recovering acids and metals from stainless steel pickling baths by means of electrodialysis.

Document EP 00463671 (Metallgesellschaft) discloses a cascade treatment with diffusion dialysis, metal electrolysis and evaporation.

The foregoing three patent documents provide for a first stage of recovery of free acids contained in the pickling waste and a subsequent stage of treating the remnant solution of metal salts. This solution undergoes electrodialysis to further recover free acidity and deposit metals in elementary form. However, working direcdy on the solution depleted of free acids has a considerable drawback: the component having the greater concentration in the waste is the iron ion, thus the bigger portion of the energy consumption cost of the process, on the basis of the current required for the metal deposition, is due to the iron deposition. Only 20% of the energy cost is used for depositing the more expensive and demanded chromium and nickel components.

Document WO 2004/007801 (Klein) disclosing treatment not only of exhausted pickling baths but also of rinsing water and abatement agents, discloses a first stage of salification of the present free acids, followed by a concentration stage of metal salts to a limited volume, to be then fed to a pyrolytic oven, where metal oxides to be recovered in the steelworks and acids to be recycled to the pickling stages are produced. Use of pyrolytic systems is however limited by the extremely burdensome costs of management and maintenance of the plant.

Documents GB 2036573 (Oy W. Rosenlew AB) and US 5500098 (Eco-Tec Limited) disclose improvements to the known process of distillation of volatile acids in presence of sulphuric acid. Such improvements mainly concern the efficiency of the acid recovery process and the obtainment of a solution or crystallization of metal sulphates. The possible use and recovery of such metal sulphates is however only mentioned and falls outside the scope of said patent documents. Document WO 95/04844 (Outokumpu Oy) discloses a process of treating exhausted pickling solutions based on nitric acid and hydrofluoric acid where the free and bound acids are recovered by the distillation process in presence of sulphuric acid, while the metal salts crystallized in the form of sulphates, undergo a set of redissolutions, admixture with other steelworks wastes and precipitations so as to obtain a first iron precipitate in the form of jarosite and chromium in the form of hydroxide and a second precipitate of nickel hydroxide.

Document EP 0339401 (Outokumpu Oy) discloses a similar distillation process in presence of sulphuric acid, not applied to stainless steel pickling baths but to hydrochloric acid baths for pickling carbon steel before the galvanizing stage. In this connection the precipitate of iron sulphate and zinc, obtained as distillation by-product, can be used in the process of producing zinc.

DESCRIPTION OF THE INVENTION

The object of the present invention is a process of treating and recovering almost all the components of pickling waste, allowing to split it into fully recyclable secondary raw materials, avoiding to forward it to the dump site.

The present invention relates also to the treatment of sludge produced by neutralization of waste, even when already stored in a dump site, where it is necessary to provide for a previous redissolution in an acidic medium as disclosed hereinafter.

The first stage of the process firstly provides for the separation of the insoluble oxides included in waste, through a decantation, washing and compaction unit. These oxides may be reused in the smelting furnace. By the same unit it is also possible to recover the oxides lost in the quenching section and in the rinsing steps after pickling.

The pickling solution, after oxide separation, is forwarded to a distillation unit where it is brought to boiling after addition of a suitable quantity of 98% sulphuric acid. Indeed it is well known that volatile acids, such as nitric or hydrofluoric acid, can be obtained through attack of sulphuric acid on their salts and subsequent distillation. More particularly, hydrofluoric acid is obtained industrially by attack of sulphuric acid on fluorite.

Nitric acid too, at a higher concentration than the azeotropic one, is produced by distillation in presence of sulphuric acid.

Such a technique used also for regeneration of exhausted acidic baths, exploits the higher volatility of nitric and hydrofluoric acids in comparison with sulphuric acid (nitric acid, 68.8% azeotropic, boiling T=121.8°C; hydrofluoric acid, 38.2% azeotropic, boiling T=112°C; sulphuric acid, 96-98% azeotropic, boiling T=317°C). Moreover during distillation, sulphuric acid moves nitric and hydrofluoric acids from their salts and complexes, reforming the free acids allowing their almost complete distillation. The distilled nitric and hydrofluoric acids are condensed and forwarded to the work tanks or storage.

During distillation, according to attained concentration of sulphuric acid, massive precipitation of iron sulphate and partial precipitation of chromium and nickel sulphates, takes place.

The clear solution, essentially comprising high concentration sulphuric acid, part of chromium and nickel sulphates and traces of iron sulphates is added with 98% sulphuric acid until the acid concentration required by the process is restored, and then is forwarded to the evaporator in place of fresh sulphuric acid to treat another pickling solution. The precipitated salts are rinsed with an aqueous solution of sulphuric acid of suitable concentration. This treatment causes the redissolution of the precipitated nickel and chromium salts while iron sulphate remains undissolved and may be recovered. At this point the acidic solution containing chromium and nickel and iron traces is forwarded to a first neutralization stage, after oxidation of divalent to trivalent iron, if required.

In this first neutralization stage the solution is brought to a pH of 3.5 to 4.5 by use of sodium hydroxide. At this pH range nickel remains almost completely in the solution, while chromium and trivalent iron are almost totally precipitated. Use of sodium hydroxide in place of the most common limewash avoids precipitation of sulphates which remain in the clear solution together with nickel. The obtained sludge, after rinsing and filter pressing, consisting of chromium and iron hydroxides, has the specifications requited for its industrial use as secondary raw material.

The clear solution and the sludge rinsing waters are additionally neutralized up to pH 8-9, i.e. to complete nickel precipitation. The precipitated hydroxide is then dewatered by microfiltration, filter pressing or other suitable means.

According to the technique used to dewater nickel hydroxide, neutralization is conducted eidier with sodium hydroxide or hmewash.

When hydroxide is dewatered by microfiltration, use of sodium hydroxide is required to avoid a massive formation of calcium sulphate that could obstruct the membranes. The sludge obtained in this stage consists of nickel hydroxide. After precipitation the solution comprises only sodium sulphate. With this technique sludge with water contents of 85-90% is obtained.

On the contrary, when a filter press is used, this solution allowing to obtain sludge with greater dry contents, bodi sodium hydroxide and kmewash may be used as neutralizers. Sodium hydroxide has the advantage to produce a sludge free of calcium sulphate but lighter and more difficult to filter press. Limewash besides being cheaper produces a heavier and more compact sludge that is easy to filter press but heavily polluted by calcium sulphate. The sludge coming out from filter press has a water contents much lower than sludge by microfiltration. Water contents of a filter press sludge is between 50-

60%.

Nickel hydroxide may be further treated until metal in its elementary form is obtained. In such a case the hydroxide is taken to a dissolution equipment kept at pH controlled by sulphuric acid. Dissolution pH is kept at about 3.5-4.5, so that nickel hydroxide goes in solution as sulphate, while the possible traces of iron and chromium hydroxides due to incomplete precipitation during the first neutralization, as well as calcium sulphate present when limewash is used in the second neutralization, remain undissolved.

The sludge coming from microfiltration or filter pressing, after neutralization with sodium hydroxide, essentially consists of nickel hydroxide that is totally dissolved in sulphuric acid leaving only a litde residue. On the contrary sludge from filter press, after neutralization with limewash, leaves a heavy residue of calcium sulphate. However this precipitate is heavy and compact, so that it may be separated from the clear solution of nickel sulphate in a quick and complete manner by simple settling. As the sludge coming from filter press contain less water than sludge from microfiltration, after dissolution they produce solutions which have more nickel, thus easier to be treated by electrodeposition.

The nickel sulphate solution is then fed to the cathode area of an electrodialysis cell, where nickel in metal form is deposited on the cathode. The working conditions are such that the deposit is dendritic and can be easily removed by stripping plates passing near the cathode at fixed times. The metal stripped from the electrode in the form of pellets is collected in a suitable container at the cell bottom. After filtering and rinsing steps, these nickel pellets may be recovered directly in the steelworks.

At the same time of the nickel deposition in the cathodic compartment, concentration of sulphuric acid takes place in the anodic compartment.

When this acid reaches a fixed concentration, it is discharged to be recovered in the above described stage of hydroxide dissolution. Use of a membrane cell was preferred in place of an undivided cell, as it allows to avoid the anodic oxidation of the chromium ion, traces of which are possibly present in the solution of nickel sulphate, into chromate and to keep low the acid concentration at the cathode, so as to have always a very high faradic yield. In experimental tests it was found that the addition of some components like chelating and buffer agents in the sulphate solution, allows to optimize the faradic yield.

A further integration of the disclosed process concerns the separation of chromium from iron in the first produced hydroxide sludge, this separation being possible because of the amphoteric behaviour of chromium, having a trend to return in solution at strongly alkaline pHs, as shown in the Pourbaix diagram illustrated in the figure of the annexed drawing. In this case, the sludge precipitated at a pH between 3.5 and 4.5, after settling, rinsing and removal of the clear solution, is further alkalized with sodium hydroxide up to pH 10, thus redissolving chromium hydroxide. After removal of the clear solution, the undissolved iron hydroxide is rinsed and again filter pressed. The alkaline solution of trivalent chromium, combined with the sludge rinsing water, may be again neutralized up to pH 7.5-8 where trivalent chromium is again fully precipitated in the form of hydroxide or the solution may again undergo electrochemical reduction to obtain chromium in metal state or even undergo oxidation to obtain a chromate solution. If one decides to use this technique for sludge produced by neutralization of pickling baths already stored in authorized dump sites, it is necessary to add to the process a preliminary passage of dissolving sludge in an acid.

In order to keep low the process costs and for compatibility with the subsequent treatments, sulphuric acid must be used being careful that sulphuric acid does not solubilize the calcium salts certainly contained in the sludge coming from neutralization normally made with limewash, of pickling baths with sulphuric acid. Therefore in the fraction of insoluble oxides there is also a considerable amount of calcium sulphate that must be removed if one decides to recover the insoluble oxides to be reused in the steelworks.

Then the sulphuric solution containing the metal salts initially present in the sludge, follows step by step the process above described for the exhausted pickling baths. On the contrary the insoluble fraction before going to the settling, rinsing and compacting stage as hereinbefore described, must be treated with hydrochloric acid so as to dissolve calcium sulphate contained in the fraction. The obtained acidic solution of calcium salts after neutralization and drying steps, may be used as road antifreeze.

The process of the invention will be better understood by reading the following explanatory but not limiting examples. EXAMPLE 1

A pickling bath solution of the following composition is used: Iron (III) 22 g/1

Chromium (III) 3.2 g/1 Nickel (II) 3.9 g/1

Fluorides 40 g/1

Nitrates 55 g/1 To 1 1 of this solution an equal volume of 98% sulphuric acid is added. Distillation is conducted at atmospheric pressure for 5 hr, up to constant volume. Boiling temperature attains 12O ⁰ C. At the end of distillation the concentrated solution is placed in a IMOF cone to determine the existing liquid fraction. Details are as follows:

Final volume 1 1 Clear solution 570 ml

Precipitate 430 ml

The clear fraction has the following composition indicated as percentage on the total of each component initially present:

Iron (III) 0.48 % Chromium (III) 55 %

Nickel (II) 43 %

Fluorides 0 %

Nitrates 0 %

Sulphuric acid 74% The solid fraction was totally dissolved in water and from analysis the following composition was found:

Iron (III) 98 %

Chromium (III) 45 % Nickel (II) 55 % Fluorides 1 %

Nitrates 0 %

The clear fraction was forwarded again to the distiller together with another liter of pickling bath and concentrated sulphuric acid was added until the acid concentration required for distillation is restored.

The results are the following: Final volume 950 ml

Clear solution 460ml Precipitate 490 ml The clear fraction has the following composition indicated as percentage on the total of each component initially present: Iron (III) 1.5 %

Chromium (III) 49 % Nickel (II) 43 % Fluorides 0 %

Nitrates 0 %

Sulphuric acid 75 %

The solid fraction was totally dissolved in water and from analysis the following composition was found: Iron (III) 98 %

Chromium (III) 46 % Nickel (II) 53 %

Fluorides 0.32 %

Nitrates 0 % Therefore the fact of using the concentrate clear fraction in place of the clean sulphuric acid, for distillation of the volatile acids, does not affect the process quality. EXAMPLE 2

The experiment illustrated in Example 1 was repeated as described. However the solid fraction was not redissolved in water but underwent consecutive extractions with a solution of 1 % sulphuric acid. This treatment causes the total redissolution of nickel and chromium salts, leaving undissolved the iron salts. The obtained sulphuric solution contains as percentage:

Iron (III) 2 % of iron present in the precipitate

Chromium (III) 99 % of chromium present in the precipitate Nickel (II) 99 % of nickel present in the precipitate

In this way a solid fraction is obtained, exclusively comprising iron sulphates and a clear fraction full of chromium and nickel, forwarded to the subsequent treatment. EXAMPLE 3

The salty fraction of the concentrate of a distillation test, after dissolution in a solution of 1% sulphuric acid, has the following composition: Iron (III) 0.05 g/1

Chromium (III) 0.372 g/1 Nickel (II) 0.433 g/1

The first neutralization phase at pH between 3.5 and 4.5 was conducted either with IN sodium hydroxide or 10% limewash. The obtained results are shown in the following tables.

Table IA: Precipitation with IN sodium hydroxide

Iron (III), g/1 Chromium (III), g/i Nickel (II), g/i Sulphates, g/1

Starting sol 0.05 0.372 0.433 6.11 pH 3,5 0.0024 0.046 0.413 pH 4,0 0 0.019 0.405

_P H 4,5 0 0 0.388 6.15

Table IB: Precipitation with 10 % limewash

Iron (III), g/1 Chromium (III), g/i Nickel (II), g/i Sulphates, g/1

Starting sol 0.05 0.372 0.433 6.U pH 3,5 0.0014 0.032 0.433 pH 4,0 0 0 0.430

_P H 4,5 0 0 0.424 2.5

From the tests it resulted that for the tested solution, separation of chromium and iron from nickel is already optimal at pH 4 with both alkalis. Moreover use of sodium hydroxide prevents precipitation of sulphates.

EXAMPLE 4

By comparison the details are shown, obtained by analyzing two sludges, both obtained by neutralizing up to pH 10 with 30% sodium hydroxide, a waste coming from the first neutralization at pH 4-4.5. The first sludge was then concentrated by microfiltration, the second by filter press.

Sludge by microfiltration

Dry = 8.1 %

The sludge was dissolved in nitromuriatic acid and the sample was analyzed obtaining the following results.

Sludge by filter press Dry = 49.8 %

The sludge was dissolved in nitromuriatic acid and the sample was analyzed obtaining the following results

EXAMPLE 5

The clear solution coming from neutralization at pH 4 with sodium hydroxide, as described in Example 4, containing about 0.4 g/1 nickel and chromium and iron traces, was further neutralized up to pH 8-9 to obtain the complete nickel precipitation. The obtained sludge, very light and hardly apt to be settled, was dehydrated through a microfiltration plant and redissolved in the least possible volume of sulphuric acid. The obtained solution has the following composition Nickel (II) 12 g/1

Iron (III) 50 ppm

Chromium (III) 0.6 g/1 PH 4 and is electrolyzed in a two compartment cell after having added 0.25% citric acid. Electrolysis is conducted at 200 A/m ² using as a cathode a stainless steel mesh. Nickel is deposited on the mesh in dendritic form and is stripped with a mild mechanical action. The recovered nickel undergoes analysis to assess its market value. The following table shows the main details of the analysis.

EXAMPLE 6 A steelworks sludge coming from a storage dump site and having a dry contents of

56% is treated in 50% sulphuric acid using variable ratios of sludge weight to acid weight. All the samples are kept under stirring for 30 minutes at a temperature of 60°C.

In all cases an intense green solution and a black sludge are obtained. The ratio of sludge quantity to acid quantity used determinates the metal extraction percentage. For the tested sludge the best ratio sludge to acid is 1:1.

By treating 20Og of sludge (56% dry) with 20Og of 50% sulphuric acid, a final volume of 330 ml is obtained, from which 170 ml of green solution is separated having the following composition:

Iron 4.8% (based on the weight of starting sludge) Chromium 0.77%

Nickel 0.5%

Calcium 0% as well as I49g of sludge with a 62 % dry contents.

A small sample of this sludge, after rinsing and drying steps, was further attacked with both sulphuric acid and nitromuriatic acid and did not show any further release of metals. The remnant sludge was on the contrary treated with 37% by weight hydrochloric acid. The filtered solution contains 4.45% calcium in relation with the starting sludge. After rinsing the sludge was again treated with hydrochloric acid but the calcium release in this second passage resulted totally negligible.