Plant and method for the thermal treatment of solids

Abstract

A method and its related plant for the thermal treatment of iron containing oxide, in which fine-grained solids are heated in a preheating calcining stage and exposed to a reduction gas in a subsequent reduction stage. Off-gas from the reduction stage is guided through a separation device wherein water originating from the reduction stage is separated. The water separated in the separation device is recycled into a water treatment section, from which the recycled water is supplied to a water electrolysis plant and/or a steam reforming plant producing hydrogen, and the produced hydrogen is supplied to the reduction stage as reductant and/or to the preheating calcining stage as fuel and/or to the gas heater as fuel and/or from which the recycled water is supplied to the separation device.

Claims

1. A method for the thermal treatment of iron containing oxide, in which fine-grained solids are heated in a preheating calcining stage and are exposed to a reduction gas in a subsequent reduction stage, wherein off-gas from the reduction stage is guided through a separation device, separating water originating from the reduction stage, wherein off-gas from the preheating calcining stage is guided through a venturi scrubber and a packed bed section downstream of the venturi scrubber to condense water vapor, wherein water separated in the separation device is recycled into a water treatment section, from which the recycled water is supplied to at least one of a water electrolysis plant or a steam reforming plant producing hydrogen, and that the produced hydrogen is supplied to at least one of the reduction stage as reductant, to the preheating calcining stage as fuel, to a gas heater as fuel, or from which the recycled water is supplied to the separation device as process water, wherein the process water exchanges heat with the off-gas, is cooled in the water treatment section and is recirculated to the separation device, wherein oxygen generated in the at least one of the water electrolysis plant or the steam reforming plant is supplied to at least one of the preheating calcining stage or the gas heater for the combustion of fuel.

2. The method according to claim 1, wherein at least one of the preheating calcining stage or the reduction stage comprises a fluidized bed reactor.

3. The method according to claim 1, wherein at least one of the preheating calcining stage or the reduction stage comprises a circulating fluidized bed reactor.

4. The method according to claim 1, wherein at least one of the preheating stage or the reduction stage is heated with at least one gas heater, and that the off-gas from the at least one gas heater is guided through a separation device to condense water vapor.

5. The method according to claim 1, wherein the downstream of the reduction stage the reduced solids are cooled in a quench cool, and that the off-gas from the quench cool is guided through a separation device to condense water vapor.

6. The method according to claim, 5 wherein the quench cool is a vibratory cooling conveyor.

7. The method according to claim 1, wherein at least one of the hydrogen content in the reduction gas is at least 85 v/v.-%, and/or the hydrogen content in a fuel gas used in the preheating stage is at least 20 v/v.-%, or the hydrogen content in a fuel gas used in at least one gas heater is at least 20 v/v.-%.

8. The method according to claim 1, wherein water treatment section includes removal of solids, removal of incondensable gases and indirect cooling.

9. A plant for the thermal treatment of iron containing oxide comprising a preheating calcining stage, for heating up fine-grained solids and a reduction stage, for reducing the heated solids by exposure to a reduction gas comprising hydrogen, wherein an off-gas outlet of the reduction stage is connected to a separation device to condense water vapor from the off-gas of the reduction stage, wherein an off-gas outlet of the preheating calcining stage is connected to a venturi scrubber and a packed bed section arranged downstream of the venturi scrubber to condense water vapor wherein a water outlet of at least one separation device is connected to a water treatment section for recycling water, that the water treatment section is coupled to at least one of a water electrolysis plant a steam reforming plant producing hydrogen as a reductant for the reduction stage, as a fuel for the preheating calcining stage, to a gas heater as fuel or that the water treatment section is coupled to at least one separation device to provide process water, which exchanges heat with the off-gas, is cooled in the water treatment section and is recirculated to the separation device, wherein oxygen generated in the at least one of the water electrolysis plant or the steam reforming plant is supplied to at least one of the preheating calcining stage or the gas heater for the combustion of fuel.

10. The plant according to claim 9, wherein the water treatment section comprises at least one of a thickener with a filter press for the filtration of underflow slurry or a covered water surface of the thickener.

11. The plant according to claim 9, wherein the water treatment section comprises a warm water tank for chemical treatment.

12. The plant according to claim 9, wherein water treatment section comprises at least one of an air fan cooler or a chiller for cooling the water.

13. The plant according to claim 9, wherein the water treatment section comprises at least one of a water filter or an ion exchange device.

Description

(1) In the Drawings:

(2) FIG. 1 shows a first embodiment for the thermal treatment of iron containing oxide, and

(3) FIG. 2 shows a second embodiment for the thermal treatment of iron containing oxide.

(4) FIG. 1 shows an inventive plant for the thermal treatment of fine-grained solids, particularly iron containing oxides. An exemplary plant has an output of 500.000 tons per year of hot briquetted iron (HBI). This plant consists of the plant 1 for thermal treatment of solids, the water treatment plant 20 and the water electrolysis plant 40.

(5) The plant 1 comprises a preheating calcining stage 2, which is typically realized as a circulating fluidized bed reactor. However, also other reactor types are possible. Fuel gas and/or oil (natural gas and/or similar gaseous or liquid hydrocarbons and/or hydrogen) and bleed gas from the reduction stage 3 are supplied to the preheating calcining stage 2 and burned with excess of air to heat the moist solids. A typical temperature range is 900 to 1100 C. The off-gases are guided via conduit 125 to a separation device 4. In a typical but not restricting embodiment, 36370 Nm.sup.3/h of off-gas with a temperature of 215 C., is produced in the preheating calcining stage 2.

(6) Separation device 4 comprises a venturi scrubber 11 to remove solids and for cooling to vapor/liquid equilibrium. Further, the separation device 4 comprises a packed bed section 12 to condense the water vapor with an upward flow of the off-gas and a countercurrent downward flow of cold process water. Cold process water is supplied via conduit 119 to the water spray nozzle 13, through which is dispersed above the packed bed in such a way that the water drips downwards through the packed bed 12. The packed bed 12 contains packings that create a large surface area to improve the contact between the falling process water droplets and the rising off-gas, e.g. irregular shaped polyethylene tellerettes or other. For the above given off-gas flow, 190 m.sup.3/h of cold process water with a temperature between 20 and 30 C. are needed.

(7) The water discharged at the bottom of separation device 4 via conduit 121 has a mixing temperature (for the given example values: 55 to 70 C.) and is at least partly recirculated to the venturi scrubber 11 by means of the pump 122 via conduit 124 and at least partly supplied to a water treatment plant 20 via conduit 123. The off-gas, exiting the separation device 4 via conduit 120 is cooled and saturated with water vapor. It is essential that it contains only small amounts of residual water vapor, like for the given example 1.1 t/h. In comparison, the amount of water collected in the preheating calcining stage by condensing water vapor in the packed bed section 12 is approximately 10 t/h for an iron ore moisture of 5%.

(8) Solids from the calcination stage 2 are passed in a not-shown way to the reduction stage 3, in which hydrogen gas is used to remove oxygen from the heated solids, particularly the iron ore, producing metalized iron and water as a byproduct. For the given example, the off-gas generated in the reduction stage 3 and discharged via conduit 118, has a temperature of 275 C. and a volume flow of approximately 310 000 Nm.sup.3/h of off-gas are generated.

(9) The off-gas is guided to the separation device 5 comprising a second venturi scrubber 14 to remove solids and cooling to vapor/liquid equilibrium. The separation device 5 of the reduction stage 3 also comprises a packed bed section 15 to condense the water vapor with an upward flow of the off-gas and a countercurrent flow of cold process water, which is supplied by a water spray nozzle 13 above the packed bed such that the water drips downwards through the packed bed. In principle, the design is similar to the separation device 5. For the given example, approximately 870 t/h of process water at a temperature of 20 to 30 C. are sprayed onto the packed bed 15 via conduit 112 and spray nozzle 13. The amount of water recuperated in the packed bed 15 is 28 t/h. The residual water vapor mass flow in the off-gas discharged via conduit 113 amounts to approximately 3 t/h. The water discharged at the bottom of separation device 5 via conduit 111 is at least partly recirculated to the venturi scrubber 14 by means of the pump 115 via conduits 116 and 117 and at least partly supplied to a water treatment plant 20 via conduit 114.

(10) The reduction section 3 further comprises three fired gas heaters 6, where gaseous and/or liquid fuel (natural gas and/or fuel oil and/or bleed gas and/or hydrogen) is burned with air, oxygen or enriched air, producing CO.sub.2 and/or H.sub.2O vapor in the off-gas, e.g. with a temperature of 230 C. The off-gas of this combustion is guided via conduit 128 through a packed bed column 7 with upward gas flow and with countercurrent flow, e.g. of 160 t/h of cold process water fed via conduit 126 and spray nozzle 13, to condense most of the water vapor contained in the off-gas. Using a mixture of natural gas and bleed gas (from the reduction process) as fuel in the fired gas heaters, the amount of water vapor condensed in the packed bed column 7 is 7 t/h in a Circored plant with a capacity of 500 000 tons HBI per year. The residual water vapor in the off-gas vented to the atmosphere via conduit 127 results in a water loss of 1.4 t/h. The water is collected at the bottom of the packed bed column 7 and discharged to the water treatment plant 20 via conduit 129.

(11) The metallized iron is then transported via a not-shown line to the briquetting machine area 9 to produce HBI, which is then transported into a vibratory cooling conveyor 8, where the briquettes are quench cooled to avoid re-oxidation. For the example, 260 t/h of cold process water are supplied to the vibratory cooling conveyor 8 via conduit 131 and 250 t/h of heated process water are discharged from the vibratory cooling conveyor 8 via conduit 134 and fed to the water treatment plant 20, while 10 t/h of water vapor are leaving the vibratory cooling conveyor via conduit 109. More than 90% of this water vapor are condensed and recovered in the waste gas scrubber 10 comprising a venturi scrubber 16 and a packed bed section 17 arranged downstream of the venturi scrubber 16. Other waste gas streams from the briquetting machine area 9 can also be fed to the waste gas scrubber 10 via conduit 104.

(12) For the values given as an example, approximately 180 t/h of cold process water are supplied to the waste gas scrubber 10 via conduit 105. The water discharged at the bottom of separation device 10 via conduit 106 is at least partly recirculated to the venturi scrubber 16 by means of the pump 107 via conduit 108 and at least partly supplied to a water treatment plant 20 via conduit 110. The cleaned, cooled and water saturated waste gas leaves the waste gas scrubber 10 via conduit 103.

(13) If the dust load of the waste gas streams is low, it is also possible to use a pure packed bed section 17 without a venturi scrubber 16.

(14) The total discharge 19 and 19 of heated process water from the iron ore reduction plant 1 to the water treatment plant 20 amounts to approximately 1700 m.sup.3/h. The smaller flow 19 (approximately 170 t/h) is practically free of solids, while the larger flow 19 is loaded with solid particles.

(15) The first step in the water treatment plant 20 is to separate the solids from the water flow 19 by feeding it into a thickener 21 via conduit 210 and 217, where clear water overflows the rim of the thickener and is fed to the warm water tank 24 via conduit 202, while the sludge is collected at the bottom of the thickener 21. Flocculant and/or coagulant is used in the thickener 21 to accelerate agglomeration of particles to form larger particles or clusters that settle down by gravity. To avoid water losses in the thickener 21 through evaporation, it can be enclosed by a roof installed on top of the water surface with the possibility for condensate to drip back into the thickener 21. The underflow sludge is discharged from the thickener 21 by means of conduit 211 and fed to a filter press 22 by pump 212 and conduit 213, leading to a filter cake 23 with very low water content. The water recovered from the filter press 22 is recycled back to the thickener 21 via conduit 214, pump 215 and conduit 216.

(16) The warm process water flow 19 from the cooling columns 7 of the gas heaters 6 is sent directly to the warm water tank 24 via conduits 201 and 202, bypassing the thickener 21, since this water has low or no solids content and, therefore, no treatment in the thickener 21 is necessary.

(17) A chemical treatment is done in the warm water tank 24 by feeding the appropriate chemicals into the warm water tank 24 via conduit 218 to prevent corrosion in the piping and other equipment, and to control the pH-level. A portion of the water (e.g. 10 m.sup.3/h) is bled from the bottom of the warm water tank 24 via conduit 204 in order to avoid accumulation of salts and impurities in the water loop. The bleed can be utilized to control the temperature of the final product (HBI) by water evaporation at the tail end of the vibratory cooling conveyor. Aeration is also done in the warm water tank 24 to remove small amounts of dissolved gases as CO.sub.2, H.sub.2S and NH.sub.3, by pumping air into the water via conduit 203 and distributing it in the warm water tank by means of a gas distributor plate, perforated pipes, or tubes. The aeration causes the dissolved gases to be transferred to the gas phase. The aeration air together with the removed gases is released to the atmosphere via conduit 205.

(18) After chemical treatment and degasification, the warm water is fed via conduit 206, pump 207 and conduit 208 to an air fan cooler 25 with indirect heat transfer to pre-cool the water, e.g. to 43 C. Subsequently, the water transported via conduit 209, is split into two major conduits 219 and 222. Water transported via conduit 219 (in the example featuring 1660 m.sup.3/h) is sent via conduit 220 to a further cooling system with a chiller 26, a compressor 231, a condenser 27, an expansion valve 234 and the connecting conduits 230, 232, 233 and 235. The main target of the water chiller system is to cool down the process water dedicated for the separation devices 4, 5, 10 and cooling columns 7. The chiller 26 can be a standard vapor-compression chiller or an absorption or adsorption chiller. In the given example, the clear process water is cooled from a range between 40 and 50 C. to a range between 20 and 30 C., preferably between 20 and 28 C., for highly efficient removal of water vapor in the iron ore reduction plant 1.

(19) Chilled process water is then recirculated to the separation devices 4, 5, 10 and the cooling column 7 via conduits 236, 130, 132 and 133 as process water supply 18 to the iron ore reduction plant 1. The specific arrangement and design of the indirect cooling system with an air fan cooler 25 and a chiller 26 together with the used type of refrigerant depends on the ambient conditions at the location of the iron ore reduction plant 1. The chiller 26 can be combined with a closed loop water heat exchanger and air fan cooler to remove heat to the atmosphere in an efficient and safe way.

(20) Water transported via conduit 222, in the example featuring 34 t/h, is treated in additional water treatment systems before it is sent to the water electrolysis plant 40 via conduit 227. The additional treatment of the process water for removal of remaining suspended solids, organic matter, dissolved solids and gases is necessary to accomplish the requirements for water electrolysis. To achieve the required specifications, suspended solids and dissolved solids are removed from the water. In the case of alkaline water electrolysis, the feed water conductivity is typically specified as 5 S/cm. Typically, the standard filtrationdeionization method is used to achieve the required water quality.

(21) Water is fed via conduit 224 to the water filters 28 and 28 to remove suspended solids using silica sand and/or anthracite as filter media. For organic matter activated carbon filter media is also used. Subsequently, water discharged via conduit 225 is fed to a ion exchange device 29/29 like acation/anion resin reactor or a cation/anion resin polymer for removing dissolved solids. Non desirable cations and anions are removed by exchange with hydrogen and hydroxyl ions respectively, forming pure water.

(22) Typically, the ion exchange devices are small plastic beads that are composed of organic polymer chains that have charged functional groups either positive or negative. Ions in the water will be attracted by respective charged functional group in the resin, the polymer or comparable materials and non-harmful weaker ions will be release to the water. Ions can be removed in separated devices connected in series via conduit 226, or in a more efficient mixed bed device containing at least two types of resin, polymer or comparable materials. A regeneration system (not shown in the figures), must be used when removal capacity of the resins becomes exhausted. Typically, strong acid and caustic are used to permeate the resin pores, displace the contaminants and leave the active H+ and OH ions in the respective resin, polymer or comparable material. For more stringent specifications of water purification (e.g. for PEM electrolyzers), other methods as membrane processes can be considered (microfiltration, reverse osmosis, ultrafiltration, etc.).

(23) Additionally, make-up water may be added to the water loop upstream of the water filters 28 via conduit 223 and/or downstream of the air fan cooler system 25 via 221. Typically, the flow of make-up water is adjusted in such a way, that the water level in the warm water tank 24 is kept at the desired height.

(24) Finally, the de-ionized water 32 is sent to a feed water tank 41 in the water electrolysis plant 40 via conduit 401. Preferably, the water electrolysis plant 40 uses the pressurized alkaline water electrolysis technology. Therefore, the purified water (in the example 34 t/h) is fed from the feed water tank 41 to the pump 403 via conduit 402 and then to the electrolysis units 42 via conduit 404. The electrolysis units comprise an anode, a cathode and a diaphragm arranged between the two. An electrical voltage is applied between the anode and the cathode, wherein hydrogen is released at the cathode and oxygen is released at the anode. The oxygen is fed to an oxygen storage tank 43 via conduit 405, while the hydrogen is fed to a hydrogen storage tank 44 via conduit 407. For the given example, 20000 Nm.sup.3/h of oxygen 51 are supplied to the oxygen consumers in the iron ore reduction plant 1 via conduit 406 and 40000 Nm.sup.3/h of hydrogen 50 are supplied to the respective hydrogen consumers in plant 1, especially the reduction stage 3, via conduit 408.

(25) FIG. 2 shows a second embodiment of the invention. It refers to the case of high concentration of dissolved gases as CO, CO.sub.2, NH.sub.3, H.sub.2S in the process water, where an additional water treatment process must be established. For example, a conventional stripping system 30 for degasification of the water by distillation, followed by an acid gas treatment unit 31 are included in the process flowsheet. Because of the high concentration of dissolved gases in the process water, the off gases from pre-degasification done in the warm water tank 24, must be sent via conduit 205, fan 250 and conduit 251 to the acid gas treatment unit 31. Since the stripping system 30 requires an elevated water temperature, e.g. 60 C., the separation of the two partial water streams in conduits 219 and 222 is done upstream of the air fan cooler 25.

(26) In another embodiment of the invention, not shown in any of the figures, hydrogen is not only used as a reductant in the reduction section 3, but also as the single fuel in the preheating section 2 and in the gas heaters 6. In this case, only nitrogen, oxygen and a very small amount of water vapor will be released to the atmosphere without any CO.sub.2 emission. In this case, a higher consumption of hydrogen in plant 1 is expected (e.g. 55000 Nm.sup.3/h). It is also possible to use a mix of fuels (e.g. hydrogen and natural gas) in the preheating section 2 and/or in the gas heaters 6.

(27) A variant to the embodiment shown in FIGS. 1 and 2, is the use of oxygen 51 from the electrolysis plant to enrich the combustion air utilized in the gas heaters 6. For example, the enrichment of the combustion air to e.g. 35 v/v-% O.sub.2 decreases the gas volume flow through the gas heaters 6, e.g. by 35 v/v-%. As a consequence, less process water is required in the cooling columns 7 to cool the off-gas. To avoid an increase of the NO.sub.x emissions, off-gas recycling can be considered.

REFERENCE NUMBERS

(28) 1 Plant for thermal treatment of solids 2 preheating calcining stage 3 reduction stage 4 preheating separation device 5 reduction section separation device 6 gas heater 7 gas heater separation device 8 vibratory cooling conveyor 9 briquetting machine area 10 waste gas separation device 11 preheating venturi scrubber 12 preheating packed bed section 13 water spray nozzle 14 reduction stage venturi scrubber 15 reduction stage packed bed section 16 waste gas venturi scrubber 17 waste gas packed bed section 18 process water supply to plant for thermal treatment of solids 19,19 process water discharge from the plant for thermal treatment of solids 20 water treatment plant 21 thickener 22 filter 23 filter cake 24 warm water tank 25 air fan cooler 26 chiller 27 condenser 28, 28 water filter 29, 29 ion exchange device 30 stripping system 31 acid gas treatment unit 32 de-ionized water supplied to water electrolysis plant 40 water electrolysis plant 41 pure water tank 42 electrolysis unit (s) 43 oxygen storage tank 44 hydrogen storage tank 50 hydrogen supply to the reduction section 51 oxygen supply to preheating section 103-106 conduit 107 pump 108-114 conduit 115 pump 116-121 conduit 122 pump 123-134 conduit 201-206 conduit 207 pump 208-211 conduit 212 pump 213,214 conduit 215 pump 216,217 conduit 219-227 conduit 230 conduit 231 compressor 232,233 conduit 234 valve 235,236 conduit 240 conduit 241 pump 242, 243 conduit 250 fan 251,252 conduit 252 pump 401,402 conduit 403 pump 404-408 conduit