Energy savings in the system pickling/acid recovery: the new ECOmode

Transcription

1 Energy savings in the system pickling/acid recovery: the new ECOmode Frank Baerhold, Stefan Mitterecker, Gerhard Jauk, Arthur Stingl Andritz AG Eibesbrunnergasse 20, A-1120 Vienna, Austria Keywords: steel pickling, acid recovery, spray roasting, energy saving ABSTRACT The recovery of hydrochloric acid from a pickling line via pyrohydrolysis in a spray roast reactor is state of the art. A major part of the energy consumption of this process results from the evaporation of water. Although the process uses an internal pre-concentration step to recover energy, the amount of water that has to be evaporated can still be reduced further. A new operating mode for the system pickling/acid recovery was implemented, that saves up to 25% fuel without any additional process steps. This new ECOmode enables the total system to more efficient operation level. The energy consumption of the ARP is mainly a result of water evaporation. The hot off-gas from the roaster is cooled down by direct contact with the waste acid, whereby the waste acid is pre-concentrated and then sprayed into the roaster. This pre-concentration is a very elegant way of process integrated heat recovery. Anyway, the concentration of the liquid sprayed into the roaster is far lower than it could be. This means, that more water is evaporated than necessary. INTRODUCTION Pyrohydrolysis is the conversion of metal halides into metal oxides at elevated temperatures in the presence of water vapor. Reaction products are a hydrogen halide gas and metal oxide. The process was originally developed and introduced in the market by ANDRITZ Ruthner in the 1960 s. It has found wide technical application in the field of acid recovery and oxide production. The introduction of this process has revolutionized pickling technology: the use of hydrochloric acid has strongly increased over the last decades, since pyrohydrolysis is available for the total recovery of waste pickle liquors from mild steel pickling. Chloride media are also used more and more in the metallurgical industry. Oxide production processes for Mg, Co, Ni, and Al have been installed. Also, mixtures of these and other metals may be processed. Processes using hydrochloric acid are known for the leaching of ores as well as for the re-extraction of metals from organic solvents. In both cases the recycling of HCl is of substantial importance to achieve highly economic processes and to reduce waste. PICKLING/LEACHING-PROCESSES Pickling is defined as the removal of inorganic contaminants from a metal surface with a liquid that results in a chemical solving or crack-off of the oxide layers. Pickling is carried out mostly through submersion, flooding or spraying. Additionally, there may be a support of the pickling effect by mechanical or electrolytical processes. The composition and physical structure of these layers depend on the preceding treatment of the steel surface. The oxides, oxohydrates, and cooxides which build up on the carbon steel surface are converted by pickling to an iron chloride containing solution. The occurring reactions are given below 1. Fe 2 O HCl 2 FeCl H 2 O (1) Fe 3 O HCl FeCl FeCl H 2 O (2)

2 FeO + 2 HCl FeCl 2 + H 2 O (3) FeO(OH) + 3 HCl FeCl H 2 O (4) Fe(OH) HCl FeCl H 2 O (5) Fe(OH) HCl FeCl H 2 O (6) The primarily built FeCl 3 is reduced to FeCl 2 by Fe and H 2 which is formed during the pickling on the metallic iron. 2 FeCl 3 + H 2 2 FeCl HCl (7) 2 FeCl 3 + Fe 3 FeCl 2 (8) The free hydrochloric acid at the same time attacks the metallic iron and hydrogen is formed, which involves the risk of undesired hydrogen-embrittlement. Fe + 2 HCl FeCl 2 + H 2 (9) As the chemical solving kinetics slows down during the pickling process due to decreasing free HCl concentration, the solution has to be removed by fresh liquor. A typical spent pickle liquor consists of approximately 190 g/l Cl -, 120 g/l Fe 2+ and 1-3 g/l Fe 3+. Treatment of Spent Acid Due to environmental and economic reasons, disposal through neutralization is applied in very small industrial plants only. Even for small capacity new installations, neutralization is no longer a real option. Some treatment processes recover the free (unbound) acid only, while the metal bound halides are discarded. The feasibility Figure 1: Closed acid loop in a pickling and acid recovery line

3 of these processes depends on plant capacity and the concentration of free acid, and its profitability is limited to smaller capacities. The process is carried out by technologies like ion exchange, retardation on a resin, partial evaporation and condensation, membrane dialysis and others. In some cases, the iron chloride solution can be left free of charge to waste water treatment plants. In some cases also deep welling of the waste acid into the ground used to be common practice (US). Thermochemical background REGENERATION THROUGH PYROHYDROLYSIS Most metal chlorides and fluorides can be hydrolyzed at elevated temperatures. Some metal chlorides, e.g. lead and zinc, cannot be hydrolyzed in the pure form, like most alkaline earth and alkaline metals. For potassium, sodium, calcium, lead and zinc the standard free enthalpy of the hydrolysis reaction is positive over the whole applicable temperature range, thus the reactions will not take place. The aim of acid regeneration is to recover both, the free and the bound acid, thus the acid is processed in a closed loop demonstrated in figure 1. The chemical conversion process in the regeneration plant is called pyrohydrolysis, which is the conversion of metal halides into metal oxides with steam and oxygen and a volatile hydrogen halide gas according to the following reaction equations (here for the iron chloride system). Hydrochloric acid regeneration 2 FeCl H 2 O Fe 2 O HCl (10) 2 FeCl H 2 O + ½ O 2 Fe 2 O HCl (11) A lot of metal chlorides and fluorides can be hydrolyzed at temperatures between 250 and 1000 C. There is a number of applications for pyrohydrolysis outside the steel industry for recovering HCl as solvent for ore treatment (e.g., TiO 2, Ni, Co) or production of special oxide ceramic material (e.g., MgO, Al 2 O 3, Co 3 O 4 ). The used furnaces for this type of high temperature operation are spray roasters and fluidized beds. Both are relying on direct heat transfer from combustion of hydrocarbon fuels to maintain the required elevated temperature for evaporation and reaction, while injecting directly the spent solution via nozzles or pipes. Process Description The spent solution is fed into the pre-concentrator (working as a recuperator), where the hot pyrohydrolysis off gases are cooled by direct heat exchange and the incoming liquid is partially evaporated. The hot and concentrated acid from the preconcentrator is fed into the reactor, which can be a spray roaster or a fluidised bed. A combustion gas with a certain amount of excess air is supplied to the reactor and delivers the heat needed for evaporation and conversion. The reactor off-gas contains the combustion gas, water vapour and gaseous HCl and is separated from entrained oxide particles by a cyclone and a venturi scrubber. The concentrated solution itself serves as washing liquid, thus recovering the sensitive heat and concentrating the spent solution. The hydrogen chloride gas is absorbed in an adiabatic column into wash water coming from the rinsing section of the pickling line, thus also recovering the chlorides of the rinse water. The hydrochloric acid thus formed can be reused for pickling or leaching. A second venturi scrubber removes last traces of dust and HCl from the gas stream using fresh water. As this washing liquid is pumped to the absorber top as well, the process is operated waste water free. Only those quantities of hydrochloric acid which are lost due to the formation of thermally not decomposable chlorides from the impurities of the raw materials, such as alkali and calcium chloride, and those leaving the process via the off gas have to be replaced by fresh hydrochloric acid. A recovery rate of > 99% of all chlorides entering the regeneration plant is achieved while typically producing approximately one litre of 18% HCl and 0.17 kg oxide per litre spent acid. Fuel consumption is directly related to plant capacity as the majority of energy is needed for evaporation of the water content. A specific consumption of 2700 kj per litre spent acid can be achieved.

4 Plant operation is highly automated with little maintenance time and high availability. The flexibility of 75 to 110% of the nominal feed flow can be supplemented by right dimensioning of the tank farm. So far, over 400 pyrohydrolysis process plants (roughly 300 spray roasters and 100 fluidised beds) have been built all around the world with a capacity ranging from 0.3 up to 30 m 3 /h of spent solution. Figure 2: The ANDRITZ METALS Spray Roasting Process The ANDRITZ METALS Spray Roasting Process The spray roasting reactor mainly consists of an empty cylindrical tower lined with refractory material. The bottom is conical or flat and can be equipped with a rabble rake to ensure the flow of processed oxide to the outlet. The chloride solution is sprayed through a ceramic/niobium nozzle assembly into the upper part of the reactor. The droplets descend down, dry up and are pyrohydrolyzed (see figures 4 and 5). The dimension of the reactor diameter is related to various factors, such as terminal velocity of the particles, feed concentration, kind of metal, fuel used for combustion, desired oxide quality parameters, and of course plant capacity. On the outside of the reactor one to four burner chambers are mounted tangentially. The burners, which are operated either with gas, liquefied gas or oil, and a certain amount of excess air for oxidation, charge the interior with hot combustion gases, thus producing a rotating flow pattern. In the reactor very intensive heat yields occur and the temperature profile has a high gradient. The gas outlet temperature is about 400 C for optimum reaction conditions (also depending on the type of feed material) and to avoid condensation of corrosive HCl in the gas duct. A negative pressure must be maintained in the whole plant, so that neither corrosive gases nor oxide powders can escape into the plant environment. The oxide is collected at the bottom, where agglomerates are crushed, and continuously removed by a rotary valve which separates the furnace atmosphere from the environment. The oxide is transported by a pneumatic conveying system to a bag filter mounted on the top of a bin. From here, the oxide powder can be filled into big-bags or directly into trucks for further transportation. Intensive research work has been done to develop realistic CFD-models to optimize the roaster geometry. Particle formation was investigated in a special laboratory scale reactor 2.

5 Figure 3: Typical temperature profile in a spray roast reactor 3 Figure 4: Particle tracks of 10 representative particles (left to right: 33, 317, and 600 μm). Track length has been limited to 10 s residence time 3

6 ENERGY SAVING OPTIONS Several attempts have been made to reduce the energy consumption of the spray roast process. A simple way of process integrated heat recovery is to use the latent heat of the hot reactor off-gas (e.g. after the cyclone) to preheat the combustion air. Unfortunately this is not a technically viable solution, since the dust in the hot off-gas lead either to erosion or to plugging of the heat exchanger. Pre-evaporation in a multi effect evaporator saves energy, but is limited to the crystallization limit of the metal chloride solution in the pre-concentrator. Due to the extremely corrosive nature of the HCl/metal chloride feed solution investment costs are high. Other options are to recover heat from off-gas condensation, using e.g. compression and steam generation, heat pumps, or hot water generation 4. The temperature level of the recovered energy is low or has to be elevated by expensive additional process steps. A disadvantage is also, that a high amount of waste water is created. Energy recovery from the hot oxide is relatively easy to realize, but accounts to only less than 5% of the total fuel requirement. THE ECOmode As described above, the conventional spray roasting process receives a feed solution from the pickling line with approx. 120 g/l Fe. In the pre-concentrator the feed solution has a concentration of g/l Fe. Safe operation of the spray roast process is possible with much higher concentration. This means, that in the normal mode more water is evaporated than necessary. By reducing the amount of water to be evaporated a lot of energy can be saved. The acid recovery process produces a regenerated acid with approx. 200 g/l of free acid and almost no iron. During the pickling process the iron content of the pickling acid increases and the concentration of free acid decreases. In the simplified example described in figure 5 the waste acid leaving the pickling line contains 120 g/l of Fe and still a free acid of 43.3 g/l. Fe tot HCl HCl RA 10 m3/h 0 g/l 200 g/l 200,0 g/l 10 to/h steam 1200 kg/h Fe (as Fe2O3) Pickling/Leaching Fe: 1200 kg/h ARP Fe tot HCl HCl WA 10 m3/h 120 g/l 200 g/l 43,3 g/l Fe tot HCl HCl RW to absorption 10 m3/h 0 g/l 0 g/l 0,0 g/l Figure 5: Standard operating mode of pickling/acid recovery

7 ECOmode The idea of the ECOmode is to shift concentrations and allow acid recovery at higher level. In the pickling line a fixed amount of scale has to be removed, so a fixed input of Fe into the pickling solution is given. This determines the plant capacity of the acid recovery plant. If the concentration in the waste acid is increased to a higher level, the amount of liquid to be fed to the acid regeneration process can be lower, and still the same amount of Fe can be removed from the system. The pickling efficiency is not affected. But due to the effect, that the waste acid has an optimized concentration level compared to the conventional mode, the acid recovery process is much more energy efficient. Figure 6: ANDRITZ METALS spray roast plant In the ECOmode 25% less feed solution has to be fed to the acid regeneration unit to produce the same amount of iron oxide and regenerated acid. The iron separation, which is given by the pickling line, is the same. This results in a reduction of the fuel demand of 25% and a reduction of the electric power consumption (esp. for the exhaust gas fan). The system was already installed in a commercial size production plant. Energy efficiency was proven on a continuous basis since more than a year. The surface quality of the steel strip leaving the pickling line was always perfect. CONCLUSIONS The ECOmode is ANDRITZ METALS new milestone in treating spent pickle liquor and acid management of pickling processes. 25% energy can be saved by ECOmode. The above said focus mainly on pickling operation in the steel industry, but can be applied in the same way to leaching processes in the metallurgical industry. A major issue of HCl-leaching processes is the iron removal. Iron removal and acid

8 recovery via pyrohydrolysis in a spray roaster is a proven and reliable technology, but often consumes a major part of the energy demand of the whole flow-sheet. The ECOmode is the potential to reduce operational costs significantly. Since the first studies and concepts have emerged, the ECOmode process is now entering the market, with the first 4 installations running or being installed. The sophisticated control proved to be both reliable and safe. We have tested on the full-size installation the efficiency and could demonstrate the enormous potential of the process, as the anticipated 25% were clearly exceeded, both in energy reduction and specific emissions reduction. REFERENCES 1. W. Steinbach, W. Karner, Acid regeneration plants, Millenium Steel 2003, p M. Schiemann, S. Wirtz, V. Scherer, F. Baerhold, Laboratory scale experiments and a model for numerical simulation, Powder Technology 228 (2012), p M. Schiemann, S. Wirtz, V. Scherer, F. Baerhold, Numerical modeling of industrial scale reactors, Powder Technology 245 (2013), p K. Adham and C. Lee, Energy Recovery in the Metal Chloride Pyrohydrolysers, Chloride Metallurgy 2002, METSOC, Montreal, Canada, Oct , 2002.