PICKLE LIQUORS

BACKGROUND OF THE INVENTION

THIS invention relates to a pickling process, and in particular to regenerating spent pickling liquors.

The process of pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, rust or scale from ferrous metals, copper, and aluminum alloys. A solution called pickle liquor, which contains strong acids, is used to remove the surface impurities. It is commonly used to de-scale or clean steel in various steelmaking and steel processing processes.

Acid reacts with the oxides and base metal to produce dissolved metal salts, thereby neutralizing the acid.

Many hot working processes and other processes that occur at high temperatures leave a discolouring oxide layer or scale on the surface of the steel. In order to remove the scale, the work piece is dipped into a vat of pickle liquor. The primary acid used is hydrochloric acid, although sulfuric acid was previously more common. Hydrochloric acid is more expensive than sulfuric acid, but it pickles much faster while minimizing base metal loss.

Speed is an important requirement for integration in automatic steel mills that run production at high speed; speeds as high as 800 ft/min ( ^« 243 metres/min) have been reported.

Carbon steels, with an alloy content less than or equal to 6%, are often pickled in hydrochloric or sulfuric acid. Steels with an alloy content greater than 6% must be pickled in two steps and other acids are used, such as phosphoric, nitric and hydrofluoric acid. Rust- and acid-resistant chromium- nickel steels are pickled in a bath of hydrochloric and nitric acid.

The pickling process generates a considerable quantity of spent pickle liquor (SPL) containing the dissolved metal salts as well as residual free acid (a weak ferrous chloride solution). This SPL must be disposed of by chemical neutralization with alkali or regenerated.

Although carbon steel can be pickled in either sulfuric or hydrochloric acid, for quite some time hydrochloric acid has been the acid of choice, particularly for large, continuous, strip lines. Hydrochloric acid offers a number of potential advantages over sulfuric acid, including the following:

1 . It consistently produces a uniform light gray surface on high-carbon steel.

2. Less chance of over-pickling.

3. Iron concentrations can be as high as 13%.

4. Rinsing is facilitated because of the high solubility of iron chloride

5. Safer to handle than sulfuric acid.

6. Lower operating temperatures can be employed.

Notwithstanding these benefits, the preponderance of hydrochloric acid is in no small measure the result of the availability of effective means to regenerate the spent pickle liquor. The recognised standard for regeneration of spent hydrochloric pickle liquor has been pyro-hydrolysis or roasting. This process not only provides a way to totally recover the residual free acid values but, it also liberates the combined acid from the salt and in so doing converts the dissolved iron to an oxide powder which holds some monetary value. There is currently in South Africa a premier exponent of this technology which provides a well organised service to its portfolio of steel producers and processors. However, as with all technologies, there are some drawbacks and the attractiveness of pyro-hydrolysis for this application has been diminished somewhat recently for a number of reasons, such as:

• The largest traditional market for the powdered ferric oxide co product has been magnetic media such as recording tapes. The replacement of magnetic recording tapes by other storage media such as Compact Disc (CD’s) has dramatically reduced this market and severely depressed the value of the iron oxide by-product.

• The pyro-hydrolysis process is very energy intensive. It requires natural gas and/or electricity and the price of this fuel has increased appreciably in recent years. Eskom (the South African electricity public utility) is looking to increase electricity prices from 50,3c/kWh to 97,51 c/kWh over the next five years.

• The equipment required for this process is physically large and expensive. The trend towards avoidance of major capital investment in the steel industry has limited installation of these systems to only the largest mills.

• Recent increases in fuel costs have made transport to central regeneration plants an expensive proposition.

• Being highly acidic and corrosive, transport of spent pickle liquor is always under scrutiny. • The regeneration plant is very maintenance intensive due to the highly corrosive nature of the liquor at elevated operating temperatures.

• Metals other than iron such as zinc, lead or chromium present in the SPL interfere with the regeneration process and may cause the SPL to be rejected by processors.

Over the years many other regeneration processes have been developed but have been limited in their uptake. These include:

• Distillation/adiabatic absorption has been used where there is a significant level of free acid remaining in the spent pickle liquor. Purified HCI, at the azeotropic concentration of about 15% w/w is recovered from the ‘overs’ while concentrated ferrous chloride liquor is recovered from the ‘bottoms’. Of course, this process only recovers the ‘free acid’ values and only 2-4% HCI may be available from some mills. Although operating temperatures are less than pyrolysis, corrosion is still very much an issue.

• The acid retardation ion exchange process can be used to reduce the acidity of the liquid ferrous chloride byproduct.

It is accordingly an object of the invention to provide an alternative pickling process, and in particular to regenerating spent pickling liquors.

It is accordingly an object of the invention to provide a pickling process, and in particular to regenerating spent pickling liquors site that will, at least partially, alleviate the above disadvantages. SUMMARY OF THE INVENTION

According to the present invention, there is provided a pickling process including the steps of:

1 . treating ferrous metal in a hydrochloric acid pickling liquor solution to provide a solution comprising hydrochloric acid and ferric chloride (FeCh) in a pickling tank;

2. removing solution comprising hydrochloric acid containing ferric chloride from the pickling tank and adding hydrochloric acid to the pickling tank to maintain the concentration of hydrochloric acid in the range of 14 to 17wt%, preferably 15 to 16wt% and maintaining the concentrion of iron in the range 5 to 15wt%, typically 8 to 11 wt%, for example about 10wt%;

3. treating the solution removed at step 2) comprising hydrochloric acid containing ferric chloride with sulphuric acid (H ₂ SO ₄ ) thereby to effect the Reaction (1 ):

FeCI + H2SO4 - ► 2HCI + FeS0 ₄ (1 ) and obtain a solution comprising hydrochloric acid containing ferrous sulphate (FeSC );

4. and cooling the solution comprising hydrochloric acid containing ferrous sulphate to below 0°C and obtain a ferrous sulphate precipitate;

5. separating the ferrous sulphate precipitate from the solution to provide a ferrous sulphate product and a regenerated hydrochloric acid solution; and

6. recycling the regenerated hydrochloric acid solution, typically with a concentration of above 14%wt, for example from 15 to 20wt%, typically from 15 to 18wt%, to the pickling tank at step 2).

At step 4), the solution is typically cooled to -1 °C to -15°C, -5°C to -15°C typically -10°C to -15°C.

At step 3), the dosing of sulphuric acid is preferably in molar proportion to the ferric chloride in the solution, at a molar ratio of 1 :1. The hydrochloric acid pickling liquor solution at Step 1 ) typically has an ambient temperature.

Preferably, the the solution comprising hydrochloric acid containing ferric chloride (FeCh) removed from the pickling tank at step 2), and the regenerated hydrochloric acid solution recycled to the pickling tank at step 6) are passed through a cross-flow heat exchanger, for the solution comprising hydrochloric acid containing ferric chloride (FeCh) to heat the regenerated hydrochloric acid solution.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flow diagram of a process of the present invention; and

Figure 2 is a graph showing a comparison of concentration of hydrochloric acid between the current method of regenerating SPL, and the process of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a process for the regeneration of spent pickle liquor that can be carried out on-site where the pickling process takes place. For a generator of waste and hence carrying environmental liabilities, costs would be reduced as equipment on site would reduce down time, service costs would be less and product quality would be more consistent. The equipment may be containerized utilizing technology that operates by exploiting a double decomposition technique with additional technology to create an iron sulphate hepta-hydrate by-product and regenerated hydrochloric acid.

When sulphuric acid is added to spent hydrochloric acid (spent pickle liquor) it causes ferrous chloride in the liquor to effect a double decomposition according to the follow reaction:

Liberated hydrochloric acid is recovered along with the original free hydrochloric acid, while the iron is crystallized out as ferrous sulphate mono hydrate.

The sulfuric acid reacts with the ferrous chloride according to equation (1 ) thereby liberating free hydrochloric acid in solution. The mixed acid is then cooled to below the freezing point of water, causing crystallization of ferrous sulphate hepta-hydrate. The drop in temperature converts crystallized ferrous sulphate mono-hydrate to crystallized ferrous sulphate hepta- hydrate. The mother liquor, now fortified with hydrochloric acid, but containing a residual of sulfuric acid is then separated from the crystals and recycled to the pickling process. The level of residual sulphuric acid in the regenerated hydrochloric acid is dependent on the concentration of iron and the temperature. This is controlled by computer to ensure that the dosing of sulphuric acid is in proportion to the iron content and the temperatures are controlled.

The process according to the invention may be known as the NUgen process (Trade Mark).

There are a number of advantages using this methodology and these are as follows:

• The process operates at low temperatures, thereby minimizing corrosion and allowing the use of inexpensive plastics for piping and other equipment.

• Energy expenditures for cooling and crystallization are inherently less than pyro-hydrolysis. • The ferrous sulphate hepta-hydrate is a readily marketable material. There are a number of markets for this material including water treatment coagulants as well as sewage de-odorization, animal feed soil micro-nutrients, catalyst manufacture, iron oxide pigments, magnetic oxides, heavy metal stabilization, cyanide destruction, arsenic stabilization and chrome control in cement.

• At least 80% less energy intensive.

The capital investment for this technology is considerably less than for spray roaster pyro-hydrolysis, and lends itself very well to being built into a forty foot (12.192 m) container.

The process is more forgiving towards contamination with other metals such as zinc.

With reference to Figure 1 , steel (not shown) is continuously fed through a pickling line 10 including at least one pickling tank 11 initially containing 18wt% hydrochloric acid pickling solution. The picking tank may hold from 7,000 to 25,000 litres of pickling solution. Ferrous metal is submerged in the pickling tank 11 for a period of 10 to 40 mins. The hydrochloric acid reacts with the oxides and base metal to produce dissolved metal salts, thereby neutralizing the hydochloric acid. Solution 12 comprising hydrochloric acid containing ferric chloride (FeCh) is removed from the pickling tank 11 and regenerated hydrochloric acid/regenerated pickle liquor (RPL) 16 is recycled to the pickling tank to maintain the concentration of hydrochloric acid in the pickling tank 11 in the range of 15 to 16wt% and maintaining the concentrion of iron at about 10wt%. Solution 12 typically containing about about 10wt% iron (in the form of dissolved ferrous chloride) and 15 to 16wt% hydrochloric acid is withdrawn from the pickling tank 11 . The temperature of the exiting solution 12 is at ambient for a continuous pickle line but may be less for a batch pickling operation. The solution 12 is introduced to a cross-flow heat exchanger 14. The solution 12 exchanges heat with a regenerated pickle liquor (RPL) solution 16 returning to the pickling tank 11 in the pickling line.

The acid solution to be regenerated 18 then flows into a reactor 20 where it is contacted with 98wt% sulphuric acid (H ₂ SO ₄ ) 22, thereby initiating Reaction (1 ) and obtaining a reaction mix 24 containg HCI and ferrous sulphate (FeSC ).

FeCI + H2SO4 - ► 2HCI + FeS0 ₄ (1 )

The Stoichiometry for Reaction 1 is provided in Table 1 below: Table 1

The final temperature reduction is accomplished by chilling the reaction mix 24 to temperatures of -15 to -1 °C in a crystallizer tank 26 which is cooled by a chiller 28. This temperature reduction reduces the solubility of ferrous sulphate, causing it to precipitate out of solution, leaving behind the hydrogen ions from the sulphuric acid and the chloride ions from the ferrous chloride.

Under these conditions the ferrous sulphate drops out as the hepta-hydrate salt (FeS0 ₄ .7H ₂ 0) which is a coarse crystalline material 30, easily separated from the mother liquor by vacuum filtration 32. A small amount of water is used to wash the residual mother liquor from the crystals to provide a ferrous sulphate product 34. This water compensates for the water of hydration that is combined with the ferrous sulfate. Regenerated pickle liquor (RPL) 36 comprising hydrochloric acid with a concentration in the range of 15 to 18wt% flows to the heat exchanger 14 where it is heated, and heated regenerated pickle liquor (RPL) 16 is returned to pickling tank 11 .

Figure 2 is a graph showing a comparison of concentration of hydrochloric acid between the current method of regenerating SPL, and the process of the present invention.

The process of the present invention shown by the line 36:

• Consistent concentration with hydrochloric acid in the range of 15 to 18wt%.

• Consistent production rate.

• Consistent pickle rate.

• Consistent finished product.

• Consistent energy costs.

• Security of supply of HCI (regenerated).

• Reduction in liability associated with disposal.

• Reduction in costs associated with transport.

• Eliminating environmental liability associated with road transportation of hazardous waste.

The lines 38 showing current pickling process:

• Reduction of concentration of hydrochloric acid over time to 3wt%.

• Reduction of production over time.

• Reduction of quality of Pickle over time.

• Reduction of quality of Finish over time.

• Increase in energy costs over time.

• Increase in carbon footprint over time.

Line 40 showing current regeneration process:

• Production loss due to emptying of tank. • Production loss due to filling of tank.

• Hazardous waste disposal liability.

• Increase waste cost due to change in legislation.

• Transportation of hazardous waste liability.

• HCI supply issues.