MXPA99008276A - Method for recycling iron bearing sludges in a steelmaking operation - Google Patents

Abstract

The invention is directed to a process for dehydrating wet BOF scrubber sludge to produce a steelmaking revert having an improved flow rate when handled in a recycle stream. Wet sludge is combined with hot BOF slag to provide a slag/sludge mixture. The wet sludge causes the mixture to have a moisture content greater than 10%water by weight, and the hot slag, having a temperature below the molten liquid state, vaporizes the water in the mixture and reduces the moisture content to about 4%water by weight or less. The dehydrated mixture has improved flow rate properties when it is recycled as a steelmaking revert.

Description

METHOD FOR RECYCLING IRON CARBON SLUDES IN AN STEEL MANUFACTURING OPERATION Description of the invention The present invention is concerned with a process for dehydrating wet iron carrier mud according to the preamble of claim 1. It is well known in the art of steelmaking or steelmaking that the iron-bearing powders and sludges generated by steelmaking ovens are valuable recovery materials suitable for recycling to steelmaking operations. Such waste materials contain iron oxides in an amount of up to about 50% by weight and it is very desirable to recover the iron for use as a steel filler material. However, in the case of wet sludge and especially in the case of the sludge from the BOF scrubber (basic oxygen furnace), the high moisture content makes the wet sludge very difficult to handle or handle in a recycle stream. The filter retort produced from the wet scrubber sludge normally has a moisture content of about 30% by weight. The high viscosity of such wet sludges causes them to have poor handling characteristics. They adhere to the conveyors and the machinery when attempts are made by REF .: 30171 to transport them as recovery materials in a recycling stream. They move poorly and often form sticky agglomerations that clog and paralyze equipment and machinery. For example, under test conditions, it has been found that wet sludge having a moisture content greater than 10% has a flow velocity of less than 4.46 Kg (10 pounds) of sludge per minute. Such slow flow rates make the wet material very inappropriate for recycling as a steel recovery material. In instances where the waste steel mill powder is recycled back to the operation, the dry powdery condition of the material causes environmental problems of dust formation. To control the formation of dust, either water is added or the dried material is mixed with wet sludge to eliminate the formation of dust. However, when several different wet or dry waste materials are combined to produce a steel recovery material, high levels of undesirable elements and compounds can be introduced into the steelmaking process. For example, if hot dip coating sludge is introduced to the recycle stream, the zinc in the recycle stream can be raised to a level where the waste material is unsuitable for use in a steel mill.

Therefore, such a combination of waste from the steel plant must be carefully checked for chemistry to avoid introducing elements detrimental to the steelmaking process. ~~ Several devices and methods have been developed in the past to reduce the moisture content and / or recover the iron from the wet sludge. For example, Reissue of U.S. Patent No. 30,060 teaches a process that instantly vaporizes water in the sludge by spraying the sludge into a stream of hot gas (649 ° C (1200 ° F)). U.S. Patent Nos. 4,091,545, and 4,133,756 also teach the use of a hot gas to reduce the moisture content of the wet slurry. U.S. Patent Nos. 5,114,474, 4,725,307, 4,711,662 and 2,710,796 teach the mixing of wet sludge and powder with molten slag to produce recovery materials. The mixture is ground for recycling after the slag cools and solidifies. An article in "33 METALPRODÜCING", March 1997, describes a process that forms BOF waste sludge in briquettes. The apparatus used in the process includes a rotary kiln or dryer to separate the water from the mud, a roller press, roller conveyors and kneading mills. Such recycling plants require large capital investments. The use of a rotary kiln consumes extensive energy to generate heat to dry the mud. The article also describes the use of heated molasses as a binder to form briquettes. The heated molasses also adds to the cost of the recycling process. Therefore, as shown in the patents, it is well known in steelmaking or steelmaking that slurries and steelworks powders can be processed and recycled as a valuable recovery material. It is also recognized that wet sludge flows poorly and creates logistical problems in a recycle stream due to its poor flow rate properties. The patents also teach that dry steelmaking waste materials present dusting problems during recycling. Finally, the above patents teach the solution of these well-known problems by dehydrating the wet sludge with hot gases to produce a sintered or appropriate loading material. The current state of the art requires complex recycling facilities and hot gas blowers that consume large amounts of expensive energy to dry the wet sludge. The hot gas vaporizes the water found in the wet sludge and reduces the moisture content of the sludge to a level where the sludge can be used as a steel recovery material.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow chart illustrating the "process steps for treating the wet slurry of the steelworks to produce a steel recovery material." Figure 2 is a graph showing the speed measurements of flow in relation to the moisture content of the mud.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to Figure 1 of the drawings, pollution control devices in modern steelmaking, such as bag filters, precipitators, cyclones and scrubbers, generate large quantities of iron-carrying powders and sludges. Such waste materials have a high value as a recovery material for reloading to steelmaking ovens. However, many of these waste materials have very high water content due to wet environmental processes, such as wet scrubbers, which are used to control emissions from steelmaking.

In the preferred embodiment, Figure 1 shows a basic oxygen furnace (BOF) 10 and hood 11 positioned above the mouth of the BOF to collect the smoke and gas that is emitted during the steel refining process. The smoke and hot gases are collected in a wet scrubber 12 and the sludge from the wet scrubber is sent to a thickener 13 where the water is separated. An additional downstream stage in the recycling process usually involves either batch or continuous wet slurry filtration. This filtering step is carried out in the press 14 where a retort 15 of the filter is produced. The retort of the filter or sludge has a moisture content of approximately 30% by weight of water. As mentioned hitherto, the BOF wet sludge contains iron oxides in amounts of up to about 50% by weight and it is highly desirable to recover the iron for reuse as a filler in the steelmaking operation. However, the high viscosity of the wet mud makes it very difficult to handle the material as a recovery material of the steelworks. It has been found that if the moisture content of the wet sludge can be reduced to less than about 10% by weight in water, the flow velocity properties of the sludge are improved as far as the sludge can conveniently be handled as a recovery material from the sludge. steelworks.

It has also been found that a preferred moisture content of between about 3% -4% water produces a flow velocity of the upper sludge as a recovery material. For example, in the graph shown in Figure 2, the flow velocity is plotted against the moisture content of four different mixing ratios ranging from a slag / mud ratio of 0.05: 1 to a ratio of 10: 1. The flow velocity tests were carried out in a 0.557 m3 (2 cubic feet) tray having a 650 sloped floor to discharge the slag / sludge mixture through an opening of 1612.9 mtr (2 ^ square inches) in the tray. The data plotted in Figure 2 clearly shows that a preferred ratio of 1: 1, the slag / sludge flow rate decreases rapidly when the moisture content of the mixture is greater than 7% by weight of water. At a level greater than about 8% water content, it is considered that the flow velocity of the material is only marginally acceptable and at more than 10%, the flow velocity of the material is unacceptable. At more than 10% flow rates become very deficient and at a moisture content greater than 11% or higher there is no material flow. It can be seen that in order to use a BOF wet sludge as a recovery material in a steelmaking process, it is necessary to first reduce the moisture content of the wet sludge to a level where the water in the slurry is lower of 10% by weight. As clearly shown in figure 2, the water level for all four tested mixing ratios should be reduced to a preferred range of between about 3% -4% to obtain optimum flow rates as shown in the plotted data. It has also been discovered, during actual use in a pilot test, that when the moisture content of the wet mud material falls to a level of less than 3%, the formation of dust can become a problem. If this happens, water should be added to the slag / sludge mixture to adjust the moisture content back to the preferred moisture range of 3% -4% to eliminate dusting. Dehydration of moist mud is extremely intense in energy. The prior art patents describe sludge drying operations that consume large amounts of energy to generate heat to vaporize the water found in the mud. It has been found that the hot slag, at a lower temperature of the molten liquid state, provides a free heat source which can be combined with the wet BOF mud to vaporize the water and decrease the moisture content of the mud.

As described hitherto, prior art patents teach the mixing of the molten slag with powder and steelmaking mud to recover the iron from the steel mill waste. Such a practice is extremely dangerous. Mixing the molten slag with water can cause terrible explosions. In the past, such explosions in steelmaking operations have resulted in injuries and deaths to employees. Prior art patents still prevent this problem. For example, Pinkerton describes, in U.S. Patent 2,710,796, that "excess water, however, must be avoided, the generation of water vapor is too violent ...". Explosive conditions are completely avoided when the hot slag, not molten, is combined with the wet mud to expel the water from the mud. Referring again to Figure 1, the hot slag from supply 16 is combined with the wet slurry from supply 15 to form a hot slag mix, wet slurry at 17. The preferred and most convenient method for combining hot slag and Wet sludge is mixing alternating lots taken from supplies 15 and 16. This produces the preferred 1: 1 slag / sludge mixing ratio. However, it has been found that the careful combination of the hot slag and the retort of the wet filter or sludge is required to avoid the rapid generation of water vapor. The procedure developed to avoid the rapid generation of water vapor involves combining the materials in a layer or layer comprising alternating thick layers of 0.305-0.61 m (1-2 feet) in thickness of the slag and retort of the wet filter or sludge. This procedure allows for safe steam release and uniform drying of the filter or mud retort. The recovery of the stack 17 in layers after curing for about 16 hours results in a uniform mixing of the two materials suitable for downstream processing, ie crushing, filtering and / or magnetic separation. This procedure is easily performed with a front end loader bucket used in most slag processing sites in the steel industry. The slag / sludge mixture ratio can be changed to produce slag / mud ratios of up to about 10: 1 to about 0.5: 1. However, when the proportion of slag sludge is changed to increase the mud content in the mixture, the metallurgical impact on the quality of the finished product must be considered. It must be remembered that the slag additions replace the separated impurities back into the steel vessel. For example, in most instances, phosphorus is considered detrimental to the quality of steel products. Metallurgical technicians try to trap phosphorus and other impurities in the slag cover that floats on the surface of the molten steel bath contained in a steel vessel. These impurities are removed from the molten steel as the slag is systematically derived. Table A shows the chemical composition for a combined 1: 1 slag / sludge mixture of BOF slag and sludge and BOF scrubber. The table shows that the slag contains approximately 0.7% phosphorus by weight and the sludge contains approximately 0.06% phosphorus. The resulting blended mixture has approximately 0.10% phosphorus in a slag / sludge mixture ratio of 1: 1. This is an acceptable phosphorus level for the BOF filler. If the 1: 1 mixing ratio is changed -'to increase the slag content, the phosphorus level will increase. For example, if the slag taken from the feed 16 is combined with the feed slurry 15 at a slag / sludge ratio of 2: 1, the slag / sludge mixture will contain approximately 0.49% phosphorus, at a ratio of 5: 1 the mixture will contain approximately 0.59% phosphorus and at a ratio of 10: 1 will contain approximately 0.64% phosphorus. TABLE A Various other undesirable elements and compounds may be inadvertently introduced to the recycle stream when different waste materials found in a steelmaking operation are added to the slag / sludge mixture. In the previous case it was shown that the slag / sludge mixture contains an acceptable phosphorus level of 0.3%. However, if the hot dip and tin mill slurries were added to the slag / sludge mixture, the zinc and chromium levels of the mixture would increase. Excessive amounts of either zinc or chromium could have detrimental effects on steel. Therefore, it is easy to see that steelmakers must carefully check chemical recovery compositions in consideration of end use to avoid introducing undesirable impurities into finished steel products. The slag / sludge mixture shown at 17 is allowed to stand or cure for a prolonged period of time to allow the radiant energy emitted from the hot slag to vaporize the water in the sludge and reduce the total water content in the mixture to a lower level of approximately 10% by weight. The water content of the slag / sludge is verified with meters 18 to determine when the moisture content of the mixture falls to a level of less than 10%. The actual operations of the pilot plant have shown that the slag / sludge mixture should be allowed to stand or cure for a period of time up to approximately 16 hours to vaporize a sufficient amount of water to reduce the moisture content at the preferred moisture level. of 3% -4%. After the slag / sludge mixture is dehydrated to the moisture range of 3% -4%, its flow properties are improved and it is sent downstream for further processing. These additional processing steps may include magnetic separation 19, sieving 20 and / or sintering 21. In mixtures containing high zinc levels of about 0.9% and greater, the mixture is not considered suitable for use in a bed operation of 21 sintering and such slag / sludge mixtures are charged directly to the BOF _with or without magnetic separation and / or sieving as shown by lines 22 and 22a. In slag / slurry mixtures wherein the zinc concentration is less than about 0.9% by weight, the mixture can be added to the sintering bed 21 with or without magnetic separation and / or sieving as shown by lines 23 and 23a . However, it should be understood that low zinc content sludge / slurry mixtures can be directly charged to a BOF without sintering. It has been discovered that fine particles of 20 mesh (1.52 mm (approx 0.03 inch)) in the recovered slag / sludge mixture have particles that fluctuate up to 12.75 mm (0.5 inches) in size may present a problem if the recovery material Slag / sludge is directly charged to a BOF. It has also been discovered that such fine particles can be fed directly to a sintering plant without presenting any known problem in the sintering operation. When smaller slag / sludge fines (1.52 mm (20 mesh)) are loaded directly to a BOF, they are transported out of the vessel with the release gases. This overload to the gas cleaning scrubber system and prevents the recycling effort. In order to solve this problem lime can be added to the wet filter or mud retort 15 in an amount of about 1% by weight. It is believed that the addition of lime causes the micro-packing of the fine slag / slurry powders during the texturing and selection or filtration operations downstream of the mixing process shown at 17. The many transfer points from conveyor to conveyor and the various sprays of water located in a recycling operation cause the lime acts as a binder and improves the agglomeration of fine slag / sludge powders into micropaints. This reduces the amount of fine powders (20 mesh) (1.54 mm) in the dry slag / sludge mixture and makes the recovery material more suitable for loading directly into a BOF vessel. Under actual plant conditions the mixed lime and agglomerated scoria / mud mix was loaded to a BOF without any noticeable increase of fine dust in the release gases. The technique of combination with lime also reduces the problems of dust formation during the handling and loading of the mixed material. As a result, the moisture content of the slag / sludge mixture can be additionally reduced to a preferred range of between about 2% -4% by weight of water when the lime additions are combined with the filter retort. It should be understood that this process is not limited to steelmaking operations. Any hot impurity can be used as a thermal source to dehydrate the wet sludge produced in any metal refining or smelting operation and in such a way that such impurity / sludge mixtures can be recycled back to their respective refining or smelting operations. While this invention has been described with a preferred design, it will be understood that it has the possibility of further modifications, uses and / or adaptations, generally following the principle of the invention and in which such derivations of the present description are included. as they enter into the knowledge or practice customary in the art with which the invention is concerned and as they may be applied to the essential features described herein and which fall within the scope of the invention limited by the appended claims. It is noted that, in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (17)

1. Claims Having described the invention as above, the content of the following claims is claimed as property: 1. A process for dewatering iron-carrying wet sludge to produce a recovery material having an improved flow rate, when handled in a stream of recycling, the process steps are characterized in that they comprise: a) combining the wet sludge with hot slag to provide a slag / sludge mixture having a moisture content greater than 10% by weight of water, the hot slag has a lower temperature than the molten liquid state; b) discontinue the step of combining the wet sludge and the hot slag and causes the slag / sludge mixture to stand for a period of time to allow the radiant energy emitted by the hot slag to vaporize the water in the slag / sludge mixture and reduce the moisture content to a range of less than 10% but greater than 3% by weight of water; and c) recycling the slag / sludge mixture as a recovery material having a reduced moisture content, the reduced moisture content improves the flow velocity of the slag / sludge mixture recovery material.
2. 2. The process according to claim 1, characterized in that the wet sludge is filtered before the step of combining the wet sludge and the hot slag to provide a retort of the sludge filter having a moisture content greater than 10% in water weight.
3. 3. The process according to claim 1 or 2, characterized in that the wet sludge is basic oxygen furnace scrubber (BOF) sludge.
4. 4. The process according to claim 3, characterized in that the sludge from the basic oxygen furnace scrubber (BOF) is combined with selected steel mill waste sludge, not generated in a BOF operation and / or waste powder from selected steelworks.
5. 5. The process according to any of the preceding claims, characterized in that the slag / sludge mixture recovery material is charged to a basic oxygen furnace (BOF).
6. 6. The process in accordance with the claim 5, characterized in that lime is added to the wet slurry during the combination step before the slag / sludge mixture is put to rest to reduce the moisture content.
7. The process according to any of the preceding claims, characterized in that the moisture content is reduced to a level of approximately less than or equal to 4% by weight of water.
8. The process according to any of the preceding claims, characterized in that the material for recovering the slag / sludge mixture is sintered before being charged to an oven.
9. The process according to any of the preceding claims, characterized in that the flow rate is improved to at least 22,321 Kg (50 pounds) per minute when the slag / sludge mixture is moved along an inclined plane at 65 °.
10. The process according to any of the preceding claims, characterized in that the slag / sludge mixture is combined in a slag / sludge ratio less than or equal to 10: 1.
11. The process according to claim 10, characterized in that the slag / sludge ratio is 1: 1 and the reduced moisture content is approximately less than or equal to 4% by weight in water.
12. The process according to any of the preceding claims, characterized in that the slag / sludge mixture is put to rest for a period of time of at least 16 hours to allow the radiant energy emitted from the hot slag to vaporize the water in the slag / mud mixture.
13. 13. The process according to any of the preceding claims, characterized in that the slag is a hot waste having a lower temperature than the molten liquid state.
14. 14. A recovery material, characterized in that it comprises: (a) a steelmaking sludge or dry steelmaking at a moisture content of less than about 10% by weight, the dried slurry has a particle size greater than 20 mesh. according to claim 14, characterized in that: (a) the dry slurry has a moisture content of about 3% by weight to about 4% by weight. 16. The recovery material according to claim 1, characterized in that: (a) the dry sludge contains iron in an amount greater than about 50% by weight. 17. The recovery material according to claim 16, characterized in that: (a) the dry sludge contains zinc in an amount of less than about 0.9% by weight. The recovery material according to claim 16, characterized in that: (a) the dried sludge contains zinc in an amount greater than about 0.9% by weight. 19. The recovery material according to claim 14 / characterized in that: (a) the dry sludge contains phosphorus in an amount of less than about 0.3% by weight. 20. The recovery material according to claim 14, characterized in that: (a) the dry sludge contains at least one substance selected from a group consisting of iron, manganese, phosphorus, zinc, silicon dioxide, calcium oxide, magnesium oxide, aluminum oxide and titanium oxide. 21. The recovery material according to claim 14, characterized in that: (a) the steelmaking sludge is basic oxygen furnace sludge. 22. The recovery material according to claim 14, characterized in that: (a) the dried mud has a particle size of less than about 1.27 centimeters (0.5 inches). 23. The recovery material according to claim 14, characterized in that: (a) the dried mud contains lime. 24. The recovery material according to claim 15, characterized in that: (a) the dry sludge contains iron in an amount greater than about 50% by weight. 25. The recovery material according to claim 24, characterized in that: (a) the dry sludge contains zinc in an amount of less than about 0.9% by weight. 26. The recovery material according to claim 24, characterized in that: (a) the dry slurry contains zinc in an amount greater than about 0.9% by weight. 27. The recovery material according to claim 24, characterized in that: (a) the dry mud has a particle size of less than about 1.27 cm (0.5 inches). 28. The recovery material according to claim 24, characterized in that: (a) the dry sludge contains at least one substance selected from the group consisting of iron, manganese, phosphorus, zinc, silicon dioxide, calcium oxide, magnesium oxide, aluminum oxide and titanium oxide. 29. An intermediate product characterized in that it comprises: w (a) a steel slurry having a moisture content of less than about 10% by weight, (b) a particle size greater than about 20 mesh and (c) a content of iron of at least 50% by weight. 30. The product according to claim 29, characterized in that: (a) the dried mud has a moisture content of between about 3% by weight to about 4% by weight. • The product according to claim 30, characterized in that: (a) the dry sludge contains zinc in an amount less than about 0.9% by weight. 32. The product _ in accordance with claim 29, characterized in that: (a) the dried mud has a particle size of less than about 1.27 cm (0.5 inches). 33. The product according to claim 30, characterized in that: (a) the dry sludge contains at least one substance selected from the group consisting of iron, manganese, phosphorus, zinc, silicon dioxide, calcium oxide, magnesium oxide, aluminum oxide and titanium oxide. 34. A product made by the process according to claim 1, characterized in that it comprises: (a) a steel mill slurry having a moisture content of less than about 10% by weight, (b) a larger particle size of approximately 20 mesh and (c) an iron content of at least 50% by weight. The product according to claim 34, characterized in that: (a) the dried mud has a moisture content of between about 3% by weight to about 4% by weight. 36. The product according to claim 35, characterized in that: (a) the dry sludge contains zinc in an amount of less than about 0.9% by weight. 37. The product according to claim 35, characterized in that: (a) the dried mud has a particle size of less than about 1.27 centimeters (0.5 inches). 38. The product according to claim 35, characterized in that: (a) the dry sludge contains at least one substance selected from the group consisting of iron, manganese, phosphorus, zinc, silicon dioxide, calcium oxide, magnesium, aluminum oxide and titanium oxide.