Acid Regeneration for Spent Hydrochloric Pickle Liquor

Acid Regeneration for Spent Hydrochloric Pickle Liquor

Steel pickling is one of the important steps in steel manufacturing industry.  It is part of the finishing process in the production of certain steel products in which oxide and scale are removed from the surface of strip steel, steel wire, and some other forms of steel, by dissolution in acid. During the process, the acid reacts to dissolve surface oxides, thus metals ions are accumulated in the pickling solution.

Pickling is a process which consists of chemical removal of scale (surface oxides) and other dirt from steel by immersion in aqueous acid solution. During the pickling process, acid reacts with scale as well as base steel to produce dissolved metal salts. To this end, pickling solutions are employed, mainly consisting of mineral acids. Pickling acid baths are used to remove, modify, passiva­te or clean steel surfaces in a defined manner. A solution of either hydrochloric (HCl) acid or sulphuric (H2SO4) acid is normally used for the pickling of the carbon steel products. The concentration of these acids decreases during the pickling process, whereas the percentage of the pickling products in the pickling bath increases.

Pickling of carbon steel thus is a process consuming fresh sulphuric acid or hydrochloric acid and delivering ferrous sulphate or ferrous chloride. Both, the procurement of fresh acid as well as the disposal of sulphates and chlorides, typically go along with high cost and complex logistics.

For acid pickling of the carbon steel and steel products, sulphuric acid was primarily in use, upto the middle of the nineteenth century. Starting in 1964, several steel pickling facilities have switched over from sulphuric acid pickling to hydrochloric acid pickling. At present, hydrochloric acid is the most commonly used acid for carbon steel pickling.

A freshly prepared pickling bath typically contains 12 % to 16 % hydrochloric acid, although this concentration is progressively reduced along with the use of the acid. The pickling bath is considered spent when the acid concentration decreases between 75 % and 85 % of its initial value, and the metals concentration in solution increases to 150 grams per litre to 250 grams per litre.

Hydrochloric acid is now preferred over the sulphuric acid because of (i) it provides optimal surface quality and fast pickling, (ii) it consistently produces a uniform light gray surface on the carbon steel, (iii)  it has abilities to dissolve all compounds of the scale layer, (iv) probability of over-pickling is much less, (v) iron concentrations can be as high as 13 %, (vi) rinsing is facilitated because of high solubility of iron chloride, (vii) the acid is safer to handle when compared with the sulphuric acid, (viii) pickling is carried out at lower operating temperatures, (ix) has the advantage of lower costs, (x) hydrochloric acid pickling offers faster and cleaner pickling, lower acid consumption, and higher utilization of the acid, (xi) less steam consumption and reduced quantities of spent pickle liquor generation, and (xii) greater versatility and more uniform product quality than sulphuric acid pickling.

The major by-product of the steel pickling process is the generation of the spent pickling liquor or the spent acid. The spent pickling liquor can be managed in several ways namely (i) transporting it away to a processing organization which recovers and converts ferrous chloride to ferric chloride and sells the product as a precipitant to the waste water treatment plants, (ii) treating it on-site with caustics and transporting the resultant sludge away, (iii) regenerating it by an acid regeneration process on site, or at an off-site facility, and reusing the regenerated acid, (iv) recovering the free acid by several commercially available recovery processes, and (v) injecting it by deep well injection.

Hydrochloric acid regeneration refers to the process for the reclamation of bound and unbound hydrochloric acid from metal chloride solutions such as ferrous chloride. Regenerated acid has no adverse effect on metal cleaning efficiency compared to the virgin acid.  It pickles as efficiently as the virgin acid.

Regeneration of the spent pickling liquor of the hydrochloric acid is an ancillary process in which the spent pickle liquor, which contains iron chloride plus hydrochloric acid solution, is converted by a process such as a spray oxidation process into a marketable iron oxide product plus hydrochloric acid solution which can be recycled for the pickling operation. Acid regeneration process eliminates the need for and cost of disposal of spent acid and the cost of replacement of hydrochloric pickle liquors, making the plant virtually self-reliant. At the same time the process reduces emissions and thereby the impact of the plant on the environment.

Depending on the composition of spent pickling liquor, various methods of regeneration are used. Some of the methods enable the recovery of only hydrochloric acid.

Total hydrochloric acid regeneration plants provide a means to virtually eliminating the cost and complex logistics associated with fresh and spent acid supply and disposal. For the regeneration of the hydrochloric acid regeneration, there are a range of processes for the total regeneration of the spent hydrochloric pickle liquors, yielding recovery rates of upto 99.5 %. All these processes present significant improvements in feasibility over traditional evaporative processes for the reclamation of only unbound hydrochloric acid. Besides that, total acid regeneration is part of the environmental responsibility of the plant which is operating the pickling line.

There are several regeneration technologies which are available for the regeneration of hydrochloric acid. These are (i) pyro-hydrolysis, (ii) crystallization, (iii) hydrolytic precipitation, (iv) solvent extraction, and (v) Kleingarn acid management system

Pyro-hydrolysis – Pyro-hydrolysis is a process where the spent pickle liquor is thermally decomposed in order to convert the spent pickle liquor back into hydrochloric acid and iron oxide. This process is carried out at a very high temperature along with water vapour and oxygen. The spent pickle liquor is pumped into the pyro-hydrolysers which convert the ferrous chloride (FeCl2) into components of ferric oxide (Fe2O3) and hydrochloric acid. Pyro-hydrolysis plants are very energy intensive, mainly because a large amount of fuel combustion is needed to evaporate the metal chloride solution and to heat the roaster contents. Fig 1 shows schematic diagram for pyro-hydrolysis process.

Fig 1 Schematic diagram for pyro-hydrolysis process

The recovery of hydrochloric acid from spent pickling liquor by pyro-hydrolysis is environmentally advantageous in that it removes the need to neutralize and dispose the spent acid and in that it reduces water consumption. It is also highly cost-effective, as it eliminates disposal costs, reduces the cost of replacement acid and generates a valuable oxide by-product.

Pyro-hydrolysis is the chemical conversion of metal salts using steam and oxygen at high temperatures. Depending on the temperature of pyro-hydrolysis, the oxide product is either in granules if treated in a fluidised bed plant, or as a powder, if treated at a lower temperature in a spray roast plant. The spray roast process works at a temperature well below the sintering temperature of iron oxide so that the oxide is obtained as a red powder, typically less than 1 micrometer in size.

Hydrochloric acid regeneration using pyro-hydrolysis provides total recovery of the spent acid solution into a metal-free product. The process not only recovers hydrochloric acid in its free and bonded state, but also obtains high-quality iron oxide, either as pellets or fine powder, which is in strong demand by the ferrite, pigment, and other industries.

Regeneration of hydrochloric acid using pyro-hydrolysis method is normally considered by the large pickling plants since this method is costly because of the high energy cost involved in the operation of the pyro-hydrolysers. This method is not environmental friendly due to the corrosive chloride salts which exist in the dust emitted by this process. Hence, this process technology needs installation of dust collection system consisting of cyclone or electrostatic precipitator (ESP).

Crystallization – Regeneration of acid using crystallization method was initially used for regeneration of waste sulphuric acid. However, due to later development, regeneration using crystallization was also able to be performed for hydrochloric acid spent pickling liquor. The applicability of regeneration using crystallization for hydrochloric acid has been confirmed upon conducting some technical feasibility studies which have concluded that multi-stage crystallization is needed to be conducted in a series of continuous stirred tank reactor type crystallizers. The hydrochloric acid waste also needs to go through crystal recycling process in order to yield impurity-free crystals.

The regeneration process through crystallization of ferrous chloride involves a standard technique which does not have a size limitation. The regenerated hydrochloric acid can have some impact on the pickling process due to the dead-load of chloride. This problem can be eliminated by adjusting the conditions of pickling rates to be at least equal to pure hydrochloric acid with an acceptable surface finish.

Hydrolytic precipitation – The regeneration of hydrochloric spent pickling liquors using hydrolytic precipitation technology involves the process of vapour distillation under evaporative hydrolysis conditions at temperatures as high as 250 deg C. When there are no other chloride salts present, hydrolytic distillation process proceeds to completion at around 175 deg C. However, when magnesium chloride is present, a higher temperature is needed for the hydrolytic distillation process be completed.

Solvent extraction technology – The solvent extraction route technology is a popular regeneration technology. This regeneration technology is preferred since it produces less hazardous by-products in the process of treating spent pickling liquor. By using the solvent extraction technology, ferrous chloride can be separated from hydrochloric acid. The by-product produced from the regeneration of hydrochloric spent pickling liquor is required to go through post-treatment.

Kleingarn acid management system – By adopting this regeneration technology, the costs of replacement of the spent pickling liquors with new acid can be reduced. This technology needs less initial investment. Application of Kleingarn acid management system as regeneration method helps in reducing waste volume by saving the amount of hydrochloric acid being used. This regeneration method also can ease the recycling of acid wastes.

Kleingarn acid management system not only needs less initial investment but at the same time it helps in reducing spent pickling liquor volume. Regeneration of spent pickling liquor using Kleingarn acid management system can assist in increasing the acid strength and reducing the iron concentration at the same time. Experiments are needed to be carried out in order to obtain the optimum pickle rate using this regeneration method. This regeneration process can be repeated until the dedicated hydrochloric acid bath tank needs to be emptied for cleaning or repair. Once the dedicated hydrochloric acid bath tank is emptied, fresh solution is to be made up using partly spent acid from other tanks plus fresh acid. The regeneration of hydrochloric acid using Kleingarn acid management system has ecological advantages.

Processes for the regeneration of spent pickling liquor

There are several processes for the regeneration of spent hydrochloric acid pickling liquor which have been developed since the late 1960s, but none matches the broad commercial acceptance of the spray roaster process, which has demonstrated its long term feasibility and viability in a large number of industrial implementations. The spray roaster process and some other processes for the hydrochloric acid regeneration are described below.

Spray roaster process

Among all known processes for hydrochloric acid regeneration such as crystallization and fluid bed pyro-hydrolysis, the spray roaster process is the most feasible in terms of energy consumption, operating cost, maintenance cost, availability, and by-product-marketability.

The spray roaster process is a pyro-hydrolytic process in which the spent acid is spray atomized into a directly fired furnace (by contact with oxygen) and splitted into ferric oxide powder (solid phase) and hydrochloric acid (gas phase).  The gaseous hydrochloric acid is absorbed in water to form regenerated hydrochloric acid of around 18 % strength which can be reused for pickling. It is common and attractive to use the slightly acidic rinse water from the pickling line as absorption liquid.

A spray roaster is simply a large refractory lined steel vessel with direct-fired burners near the bottom to heat the roaster contents. The number of burners and their positions depend on the size of the roaster and the heat needed. The flame temperatures can reach in the range of 1,200 deg C to 1,750 deg C depending on the air to fuel ratio. Because of the cooling effect of the feed spray, the average temperature in the reaction zone is typically between 600 deg C to 700 deg C. In this type of pyro-hydrolysis roaster, the metal chloride solution is sprayed into the free board of the empty cylindrical vessel, while the required energy is supplied by the up flow of hot gases generated in the bottom burners.

The spray roaster is to be designed to allow enough drying time for the largest liquid droplet to hydrolyze before reaching the bottom of the vessel. Commercial spray roasters range from 5 meters to 8 meters in height to provide this drying time. Further, the roaster diameter is to be designed for an upward gas (space) velocity which is high enough to prevent droplets from wetting the bottom of the vessel, but low enough to prevent high dust losses to the off-gas system. The space velocity is typically 0.3 meter per second to 1 meter per second for commercial units and can be even lower for smaller units.

Fig 2 shows a sketch of a spray roaster. Spray roasters typically have very large diameters to keep the gas velocities low. If the gas velocity becomes too high, too many particles are carried away with the roaster off-gas, and the product quality and the efficiency of the roaster drop. In the spray roaster, the off-gas and oxides leave the roaster counter-currently at around 400 deg C to 500 deg C. Because of the counter-current flow, the exit temperature in the spray roaster is less than the reaction zone temperature.

Fig 2 Sketch of spray roaster and typical structure of spray roaster oxide

The residence time of the sprayed particles in the high-temperature reaction zone is very short and hence, very small liquid droplets, which can be quickly heated, are to be created by atomization. The fast heat up of the spayed particles results in the formation of a solid metal oxide crust on the surface of each droplet. As the bulk of the droplet heats, the water content vapourizes and breaks through the oxide shell. Hence, the spray roasted oxides are frequently composed of very fine (several micro-meters), ‘fluffy’, hollow spheres. Typical structure of spray roaster oxide is shown in Fig 2.

The three basic process steps of the spray roster process are (i) pre-concentration, (ii) roasting, and (iii) absorption. In the pre-concentration step, the incoming waste acid undergoes direct heat and mass exchange with the hot exhaust gas from the roaster furnace. The direct heat exchange is accomplished in a venturi evaporator where the waste acid is atomized and turbulently inter-mixed with the roaster gas at high velocity. The waste gas is thus partially evaporated, leaving behind a pre-concentrated waste acid to be used as liquid feed to the spray roasting furnace.

In the spray roasting furnace, the pre-concentrated waste acid which is injected from the top by means of high pressure atomizing nozzles undergoes a drop by drop evaporation of water and hydrochloric acid as well as pyro-hydrolysis reaction of remaining iron chlorides and excess oxygen provided by the burners. These burners are tangentially aligned around the furnace circumference in order to form a specific ‘swirl’ flow pattern which increases the droplet retention time by increasing the length of its path through the furnace. Almost any common kind of industrial grade fossil fuel such as e.g. natural gas, liquefied petroleum gas (LPG), liquefied natural gas (LNG), coke oven gas, or fuel oil can be used.

In the pyro-hydrolysis regeneration system, the iron chloride (FeCl2) is converted into hydrochloric acid and iron oxide by hydrolytic decomposition. The reaction takes place in the reactor at temperatures ranging from 600 deg C to 800 deg C. At reaction temperatures the iron chloride solution is split into hydrogen chloride and iron oxide by means of water vapour and atmospheric oxygen. The chemical reactions are (i) 12 FeCl2 + 3 O2 = 8 FeCl3 + 2 Fe2O3, (ii) 2FeCl3 + 3 H2O = 6 HCl + Fe2O3, and (iii) 4FeCl2 + 4H2O + O2 = Fe2O3 + 4 HCl.

In the absorption column, the cooled roaster gas from the gas exit of the pre-concentrator undergoes adiabatic heat exchange with the rinse water in a packed column and forms regenerated acid of typical concentration of around 18 % hydrochloric acid, which can be reused for pickling. The roaster is normally equipped with an extensive off-gas system, including gas / liquid contactor (venturi) for partial evaporation of the fresh feed, an absorber for recovering the gaseous hydrochloric acid as recovered acid, and dust removal equipment such as cyclone or ESP. Typical flow sheet of a spray roasting process is given in Fig 3.

Fig 3 Typical flow sheet of a spray roasting process

The chemical reaction products of the pyro-hydrolysis reactions consist of hydrochloric acid which is extracted from the top of the furnace together with the steam and combustion products and ferric oxide powder which settles at the conical bottom of the furnace is pneumatically conveyed into a storage bin, from where it can be filled in a variety of commercial transport means such as into big bags or onto trucks.

The spray roasted iron oxide powder has high oxide purity and a good surface structure. Due to these properties it is a valuable input material for the downstream industries such as producers of architectural paints, building products, styrene catalysts, toner for laser printers and ferrites.

Fluidized bed process

The fluidized bed process is also a pyro-hydrolytic process like the spray roaster process and it takes place in a directly heated furnace. In a fluidized bed roaster, the metal chloride solution is introduced onto a large bed of hot metal oxides, while the required thermal energy is provided by the hot fluidizing combustion gases. Fig 4 shows a schematic sketch of a fluidized bed roaster.

Fig 4 Sketch of fluidized bed roaster and typical structure of fluidized bed particles

The fluidized bed roaster is to be designed for a space velocity which is 3 times to 10 times of the minimum fluidizing velocity of the oxide bed. For iron oxide pellets of 200 micrometers to 2,000 micrometers in diameter, space velocities of 2 meters per second to 2.5 meters per second are common. The roaster height is selected to minimize the amount of dust carry over to the off-gas system. Typical total heights of the fluidized bed roaster are 5 meters to 6 meters.

The hot combustion gas is normally produced by submerged tuyeres which directly inject the air and fuel into the bottom of the fluidized bed. The hot gas flows upwards and fluidizes the bed of particles. As the combustion gas flows through the well agitated bed of oxides, it quickly reaches thermal equilibrium with the bed. The liquid feed is not sprayed, but is directly fed (poured) on top of, or inside, the bed of oxides. According to one explanation, the liquid feed wets the outer layer of the hot oxide particles (of the  order of 0.5 mm) and is quickly evaporated to form an onion-like layer of new solid oxide on top of the existing oxide, thereby producing dense homogeneous particles, as shown in Fig 4.

Since the off-gas from the fluidized bed roaster is hotter than the spray roaster, higher quantity of water is pre-evaporated in the venturi. Thus, some dilution water is needed to be added to the venturi, to control the ferrous chloride, concentration below the saturation level. Without dilution water, the recirculating venturi liquor forms crystals which can cause plugging and damage in the venturi contactor. Fuel requirement in the fluidized bed process is higher than the spray roaster process, mainly because of the requirement of the dilution water. In some cases, the amount of dilution water can be reduced by (i) operating the fluidized bed at a lower temperature (e.g. 800 deg C), (ii) increasing the oxide dust capture efficiency in the cyclone (e.g. using multi-cyclones), (iii) lowering the amount of fine dust generation in the fluidized bed. Since the safe combustion temperature for natural gas is around 760 deg C, bed temperatures of less than 800 deg C are not typically used for a natural gas operated system. Fig 5 shows a simple flow diagram for a fluidized bed process.

Fig 5 Simplified flow diagram for the fluidized bed process

In the fluidized bed process the conversion of waste acid into iron oxide and hydrogen chloride takes place in a fluidized bed at a temperature of around 800 deg C to 900 deg C. Because to it, the iron oxide obtained from a fluidized bed hydrochloric acid regeneration plant is of a granular, sintered consistency and is called pellet. Average diameter of pellet is 200 micrometers to 2,000 micrometers depending on the reactor set up. The pellets are iron ore substitute and can be recycled back within the integrated steel plant.

In the fluidized bed process, absorption of hydrochloric acid is done in similar way as in spray roasting, but obtainable acid concentration is slightly lower typically at 17 %. Fluidized bed pyro-hydrolytic process has a few distinct advantages over spray roaster process. These are (i) the oxide product is a granular solid and normally dust free because of the onion layer-like growth of the particles in the fluidized bed roaster, (ii) no separate combustion unit is needed as the pyro-hydrolysis and combustion reactions occur simultaneously in the fluidized bed, (iii) control over particle size is possible through the residence time control, partial recycling of the particulate product, or by varying the feed delivery, (iv) good  mixing and temperature control which are normally associated with the fluidized beds, and (v) fluidized bed roasters are typically smaller than spray roasters.

PHAR process

PHAR (Pickliq hydrochloric acid regeneration) is a process for regeneration of spent hydrochloric acid from steel pickling. The process is applicable to any size pickling operation. PHAR technology eliminates the disposal problem, creating considerable reductions in operating, environmental, and capital costs. The process uses sulphuric acid to restore hydrochloric acid for reuse. PHAR produces ferrous sulphate crystals (sulphate hepta-hydrate), an economically viable by-product, which can be sold for the industrial purposes. By eliminating transportation and / or treatment of spent pickling liquor, along with costs associated with generating hydrochloric acid to replace the spent liquor solution, PHAR produces energy savings of 95 %, cost savings of 52 %, and a 91 % reduction in CO2 emissions, compared to the existing technology.

In PHAR process spent pickle liquor typically exits from the pickling line at around 11 % to 13 % iron and 2 % to 4 % free hydrochloric acid. The temperature of the acid is around 80 deg C to 90 deg C for a continuous pickling line but can be less for a batch pickling operation. Using a cross flow heat exchanger, the exiting spent pickle liquor exchanges  heat  with  the ‘regenerated pickle liquor’ (RPL) returning to the pickling line.

The spent pickle liquor to be regenerated then flows into a reactor where it is contacted with concentrated sulphuric acid (93 %). The sulphuric acid reacts with the ferrous chloride thereby liberating free hydrochloric acid in solution. The final temperature reduction is accompanied by chilling the reaction mix to temperatures of -15 deg C to -1 deg C in a crystallizer tank. The temperature reduction reduces the solubility of ferrous sulphate, causing it to precipitate out of solution. The resulting iron sulphate is separated by crystallization. The mother liquor, now fortified with hydrochloric acid, but containing a residual of sulphuric acid is then separated from the crystals by vacuum filtration or centrifugation and recycled to the pickling process.

A small amount of water is used to wash the residual mother liquor from the crystals.   This water compensates for the water of hydration which is combined with the ferrous sulphate. The level of residual sulphuric acid in the regenerated hydrochloric acid is dependent on the concentration of iron and the temperature. The process operates at low temperatures and produces ferrous sulphate hepta-hydrate (FeSO4.7H2O). Fig 6 shows the typical layout of PHAR process.

Fig 6 Typical layout of PHAR process

The PHAR process has a number of potential advantages compared to the other alternatives. These are (i) the process operates at low temperatures, thereby minimizing corrosion and allowing the use of inexpensive plastics for piping and other equipment, (ii) energy consumptions for cooling and crystallization are inherently less than evaporation and can be minimized by reclaiming heat from the spent pickle liquor with heat exchangers, (iii) the ferrous sulphate hepta-hydrate is a readily marketable material, which is used as water treatment coagulants as well as sewage de-odourization, (iv) the capital investment for the system is considerably less than other alternatives and it is physically much smaller, (v) a supply of fuel gas is not needed for the operation, and (vi) the process is more forgiving towards contamination with other metals such as zinc.

Distillation process

Distillation process has been used where there is a significant level of free acid remaining in the spent pickle liquor. Purified hydrochloric acid, at the azeotropic concentration of around 15 % is recovered from the ‘overs’ while the concentrated ferrous chloride liquor is recovered from the ‘bottoms’. This process only recovers the ‘free acid’ values. In one variation of this process, the ferric chloride accumulating in the bottom is crystallized out. In another variation, the acid retardation ion exchange process is used to reduce the acidity of the liquid ferrous chloride by product.

Sulphuric acid distillation Process

In this process sulphuric acid is added to spent pickle liquor. This causes the reaction FeCl2 + H2SO4 = 2HCl + FeSO4 to take place. The liberated hydrochloric acid is recovered along with the original free hydrochloric acid by distillation, while the iron is crystallized out as ferrous sulphate monohydrate.

Hybrid pyro-hydrolysis processes

When the main purpose in the operation of a pyro-hydrolysis plant is the production of high quality iron oxide powder, then a reactor design which combines the energy efficiency of a spray roasting furnace with the homogeneous and stable process conditions of a fluidized bed process is adopted. This needs higher investments in the de-dusting and gas quenching technologies.

Hydro-thermal regeneration process

Hydro-thermal regeneration process is relatively newer technology. It replaces the directly fired furnace and gas / liquid absorption by an alternative process route consisting of oxidation and hydrolysis. Formation of ferric oxide takes place in liquid phase which reduces the consumption of heat energy. The concentration of regenerated acid is equal to waste acid total HCl concentration .This concentration of regenerated acid can be increased to a level higher than 30 % by using a pre-concentrator. The iron oxide quality produced by this process is comparable to pyro-hydrolytic processes in terms of the chloride ion contamination. However specific surface of particles is adjustable to much higher figures by tuning of the hydrolysis conditions.

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