EP0446779B1 - Process and installation for treating oil-containing mill scale and similar materials - Google Patents

Description

The invention relates to a method and a plant for processing oil-containing, wet mill scale and similar substances according to the preamble of claim 1 and claim 7.

Mill scale and mill scale sludge occur in rolling mills when the scale layer has to be removed before the rolling process. Tinder is iron oxide, which can be added to the raw material again in a sintering plant, for example. In the rolling mill, scale mixes with water, which is used as cooling water or as pressurized water for spraying off the scale layer, and with oil or grease, which was used, for example, to lubricate bearings. Larger scale particles can be separated in a wastewater treatment plant for the circulation water of the rolling mill, however, complete deoiling is not possible. Finer scale particles can only be separated from the circulating water to a limited extent by settling, but otherwise by filters. They form the roller scale sludge, which still has a high water content of up to 30% and an oil content of up to more than 6%. If the roller scale sludge is used during sintering, not only does the water evaporate, but at least some of the oil escapes unburned. Some of the oily constituents condense in the electrostatic precipitators that are used today, which are responsible for the flue gases of the sintering plant to clean dust from legally stipulated minimum levels. With high loads of organic material in the flue gas, however, fires or, in the limit, explosions in the filter can occur, which is easily caused by electrical flashovers between the spray and precipitation electrodes of the electrostatic precipitator. Condensed oil components make it more difficult to clean the precipitation electrodes, and oil that is not separated increases the cloudiness of the emitted flue gas indirectly (by impairing the function of the electrostatic filter) and immediately (as a condensed oil mist).

Killmann and Schellberg reported at the 1988 Duisburg Recycling Days about options for treating the mill scale. One solution is two-layer sintering, in which a further layer of oil-containing roller scale sludge is placed on a first, lower sinter mixture without mill scale sludge, which is ignited first, which is to be ignited separately at a suitable time. The oil escapes from the upper layer and is burned as it passes through the lower layer. However, this process can only be carried out if the exact process conditions are observed and would require considerable modifications to existing sintering plants. Attempts have also been made to attempt to free oil from mill scale sludge in an oven heated with coke gas-air mixture at temperatures above 100 ° C, and it has been determined that the original oil content of 7% above 280 ° C oven temperature is less than 0 , 1% could be reduced.

In the area of industrial plants, such as a blast furnace and a steelworks, oil-containing substances such as sands are often produced, from which the oil is difficult to remove and which must therefore be brought to a special landfill. An extraction of these irregularly occurring oil-containing substances can be very complex under certain circumstances. Removal of oil from these substances by simple means would therefore be desirable, even if the substances themselves cannot be fed into the blast furnace.

From US-4 091 545 a method for removing the moisture and fat content of a rolling mill slurry is known for using the same as raw material for sinter. The process provides for the moisture content of the rolling mill sludge to be reduced to about 1 to 10% first by drying with hot exhaust air from the sinter cooler and then for the sludge to be applied in a constant amount to hot, fired sinter. This should vaporize the fat content of the dried sludge. The resulting exhaust gas is collected, fed to a wet dust collector and then discharged. What happens to the oily constituents contained in the exhaust gas is not explained. The rolling mill sludge cleaned of the oily components is added to the sinter mixer with the return fine material. Here, the rolling mill sludge is freed of oil-containing components, but there is no further use of these oil-containing components.

A method for the combustion of hydrocarbons and fission products in sintered exhaust gases is known from US Pat. No. 3,857,694. A sinter mixture containing hydrocarbons is fed into the sintering process. The hydrocarbons expelled due to the ignition of the sinter mixture are collected in the wind boxes arranged after the ignition hood and by means of a blower partly passed through the hot sintered mixture in the cooling section of the sintered belt and partly passed to the ignition furnace of the sintered belt, whereby good combustion of the hydrocarbons and fission products and at the same time good cooling is to be achieved. Apart from this combustion and cooling, the hydrocarbons are not intended to be used.

It is therefore an object of the invention to largely remove oil from oil-containing solids, such as mill scale and in particular mill scale sludge, and to develop a method and a system with which mill scale can be processed in an energy-saving manner and without risk of fire in modern dust filter systems. Another object of the invention is to use the energy content of the oil contained in the mill scale or similar solid.

This object is achieved with the characterizing features of claim 1 and claim 7. Further developments of the invention result from claims 2 to 6 and 8 to 14. Thereafter, a largely dewatered solid, as usual freed from foreign bodies, such as Rolled scale, which may also be in the form of rolled scale sludge, with any pieces sintered together being broken up beforehand, treated in a continuous furnace at temperatures at which the moisture and volatile oils escape and the remaining oils burn, possibly by cracking.

The treatment takes place directly in connection with the operation of an iron ore sintering plant.

It is state of the art to recover hot exhaust air, especially in the first part, from the cooler of the sintering plant and to use it. It is often used as preheated combustion air in the ignition furnace of the same sintering plant. There are also other operationally realized uses, such as extracting steam for the steam network of a hut or preheating combustion media for consumers outside the sintering plant.

According to the invention, the hot exhaust air from the cooler of a sintering system is fed to a continuous furnace in which the hot exhaust air and the oil-containing solid are guided in countercurrent, such as mill scale. The evaporated, partly also burned oil absorbing exhaust air leaves the continuous furnace, has cooled due to the evaporation of moisture from the mill scale and because of the heating of the mill scale by an amount corresponding to the heat capacity flow conditions and is due to its still large heat content as preheated combustion air Ignition furnace fed to the sintering plant. At temperatures above 1,000 ° C, the part of the oil that has not yet burned, which comes from the continuous furnace with the combustion air and where it was thermally released from the mill scale or other solid, also burns there. The high combustion temperatures and the - albeit small - excess of oxygen in the ignition furnace of a sintering plant ensure the complete combustion of the oil. A load on the smoke filter of the sintering plant oil is therefore avoided. The calorific value of the oil is fully used in the continuous furnace, there by partial combustion, and in the ignition furnace, there by residual combustion.

In the preferred treatment of mill scale, the dry, hot and thermally oil-free mill scale coming from the continuous furnace is fed directly to the return material of the sintering system, for example, via discharge channels. B. the hot return material, and in this way reaches the iron ore mixture to be sintered. The mill scale can also be fed directly into the iron ore mixture, bypassing the return material flow, which makes it easier to control the sintering system with regard to balancing the amount of return material and the use of return material. The heat content of the hot mill scale is - apart from the inevitable losses during transport - introduced into the iron ore mixture. It is therefore preheated in accordance with the heat and quantity ratios of mill scale / iron ore mixture and thus passes under the ignition furnace of the sintering plant. A large part of the heat coming from the continuous furnace with the hot mill scale is therefore used.

Are other oily substances, such as. B. sands to treat, the oil can also be removed from them with the help of the hot exhaust air of a sinter cooler and its introduction into the ignition furnace of the sintering plant. However, the oil-free, dried sands are generally not fed to the sintering plant, but are discharged oil-free from the material flow of the sintering plant. They generally correspond to In terms of material and time, not the requirements of the ore mixtures to be processed in the sinter plant.

In an embodiment of the invention it is proposed to use a rotary tube furnace as a continuous furnace. An alternative to this is the use of a deck oven, as was previously often used for roasting sulfidic ores. Both furnaces allow the countercurrent principle to be used for the hot sinter cooler exhaust air and the wet, oily mill scale.

Because of the heterogeneous properties of mill scale - strongly changing wetness, different feed grain, fluctuating oil contents - the addition quantity of the mill scale to the continuous furnace is preferably controlled according to the temperature of the exiting exhaust air fed to the ignition furnace. In this way, constant operation of the ignition furnace is made possible. The dwell time of the feed material in the continuous furnace should preferably be a maximum of about 30 minutes, since this treatment time is sufficient after tests have been carried out.

It is preferably provided that at a cooler exhaust air temperature that is less than a predetermined limit, z. B. above 350 ° C, the supply of wet, oil-rich mill scale is interrupted. It is also preferred for controlling the cleaning function of the continuous furnace, the temperature of the hot mill scale emerging from the continuous furnace z. B. to measure infrared optically and interrupt the flow of wet mill scale to the continuous furnace if this temperature falls below a limit. To enable maintenance work on the continuous furnace also during the operation of the sintering plant, it is expedient in this case to be able to feed the hot cooler exhaust air directly to the ignition furnace by means of a slide and a feed line past the continuous furnace. A cyclone for dust separation is preferably connected upstream of the ignition furnace. The hood for extracting the hot radiator exhaust air is equipped in a known manner with safety devices for blowing off hot radiator exhaust air.

The figure shows the system parts of a preferred example in a highly simplified and only schematic manner.

The sintering machine 1, a traveling grate, is supplied with pretreated mix via a feed device 2, and in the ignition furnace 3 the coke bruise contained in the mix is ignited by ignition gas. The sinter cake is crushed through a spike crusher 4 and into the sinter cooler 5, to which cooling air is fed via a fan 6. After calibration and screening (not shown), the finished sinter is forwarded from the cooler 5 via the conveying path 7 to the blast furnace. The flue gas from the sintering machine 1 is drawn off via an electrostatic filter 8 by a fan 9 and blown off via a chimney 10.
The hot cooler exhaust air is drawn off from the hot part of the sinter cooler 5 via a hood 11 and passed via a line 23 into a continuous furnace 12, through which the mill scale to be treated is countercurrent flows. The cooler exhaust air, which is now cooler, reaches a dust separator 13 from the continuous furnace 12 and is introduced into the ignition furnace 3 as warm combustion air by a fan 14. This combustion air contains the flue gas from the oil burned in the continuous furnace and volatile and low-boiling components of this oil, which burn in the ignition furnace 3 at a temperature above 1000 ° C.

The mill scale is fed to the continuous furnace 12 by a suitable conveyor 15 with an adjustable delivery rate. The hot scale, freed from oil, is z. B. via discharge channels 16 or another suitable transport device to the mixing device 17, in which ore, aggregates and coke breeze are mixed and into which sintered waste can be introduced via a conveyor 18 after separation from the finished sinter behind the cooler 5. The oil-free mill scale can also be added directly to the sintered waste. The mix passes through mixing and rolling drums, not shown, and then arrives at the feed device 2.

The hot, dry mill scale brings heat into the mix. The hot cooler exhaust air cools down when the wet mill scale dries, but is also reheated by partial combustion of oil in the continuous furnace and feeds residual volatile oil to the ignition furnace 3 as a further fuel.

From the dependencies of heat requirements for drying, for thermal cleaning of oil and heating of the mill scale, also taking into account the Oil calorific value, on the one hand, and the air requirement of the ignition furnace of the sintering plant, on the other hand, results in the maximum quantities of mill scale that can be processed in a sintering plant. As described, the maximum amounts are not constant because of the heterogeneous properties. The following example gives an example.

By lowering the ignition air temperature from 300 ° C to lower temperatures, the maximum possible throughput increases: At 200 ° C ignition air temperature there are processable roll quantities of 54 t / day under otherwise identical conditions.

Blast gas from a line 25 is usually used as the ignition gas in the ignition furnace 3, and the combustion air is preheated depending on the blast furnace gas available. In the system according to the invention, separate preheating of the combustion air is not necessary, since there is still sufficient warm cooler exhaust air available. The above examples show the relationship between the pilot air temperature and the possible throughput of mill scale through the continuous furnace.

Because of the different composition of the mill scale, it is advantageous to monitor the temperatures and intervene to control them.
A temperature meter 19 determines the temperature of the cooler exhaust air supplied as combustion air and throttles or stops the supply of mill scale into the continuous furnace 12 via control devices (not shown) or allows the cooler exhaust air to flow through a bypass 20. A temperature meter 21 determines the temperature of the hot radiator exhaust air and intervenes in a similar manner if this z. B. temporarily drops below a predetermined value. Furthermore, the temperature of the mill scale behind the continuous furnace 12 can also be monitored via a temperature meter 22 in order to ensure that sufficient thermal cleaning has taken place. The specified temperature meters as well as other control devices of the sintering plant can work together via a computer to produce optimal process conditions.

Claims (14)

1. Method of processing oil-containing, wet mill scale and similar substances, which are treated in association with an iron ore sintering system and dried by means of hot spent air removed from the sinter cooler, characterised in that the spent air of the cooler is conducted in a continuous-heating furnace in counterflow over supplied mill scale and similar substances, the substances being dried and at least a portion of the oil burning, and the spent air of the cooler, which is charged with moisture and volatile oil, is introduced into the ignition furnace of the sintering system as hot combustion air, the remaining oil burning in the ignition furnace.
2. Method according to claim 1, characterised in that the mill scale, from which oil has been thermally removed therefrom, is added to the material to be mixed for the sintering system.
3. Method according to claim 1, characterised in that the mill scale, from which oil has been thermally removed therefrom, is added to the sintered residue and introduced, together with said residue, into the material to be mixed for the sintering system.
4. Method according to one of claims 1 to 3, characterised in that the temperature of the spent air of the cooler, which flows into the ignition furnace as combustion air, is monitored, and the charge of mill scale through the continuous-heating furnace is controlled in order to maintain predetermined temperature values.
5. Method according to one or more of claims 1 to 4, characterised in that the maximum treatment period for the mill scale in the continuous-heating furnace is substantially 30 minutes.
6. Method according to one or more of claims 1 to 5, characterised in that spent air of the cooler, having a temperature of at least 350° C, is introduced into the continuous-heating furnace.
7. System for accomplishing a method according to one or more of claims 1 to 6 for processing oil-containing, wet mill scale and similar substances in a furnace, which is acted upon by hot spent air from a sinter cooler (5), characterised in that the furnace is in the form of a continuous-heating furnace (12), which is traversed by the spent air in counterflow to the mill scale, and said furnace is connected to the ignition furnace (3) of the sintering machine (1) via a conduit (24) in order to introduce the spent air of the cooler, which has left the continuous-heating furnace (12), as hot combustion air into the ignition furnace (3).
8. System according to claim 7, characterised in that the continuous-heating furnace (12), in which hot spent air of the cooler and mill scale or similar substances are treated in counterflow, is adapted for a treatment period of substantially 30 minutes.
9. System according to claim 8, characterised in that the continuous-heating furnace (12) is a rotary cylindrical furnace.
10. System according to claim 8, characterised in that the continuous-heating furnace (12) is a multilayer furnace.
11. System according to one or more of claims 7 to 10, characterised in that a conveying means (16) is provided for the delivery of the hot mill scale, which has oil removed thereform, from the continuous-heating furnace (12) into the material to be mixed.
12. System according to one or more of claims 7 to 11, characterised in that a temperature gauge (19) is disposed on the conduit (24) for the combustion air, the supply of mill scale or similar substances into the continuous-heating furnace (12) being controllable by said gauge.
13. System according to one or more of claims 7 to 12, characterised in that a temperature gauge (21) is disposed on the conduit (23) for hot spent air of the cooler, said gauge interrupting the treatment in the continuous-heating furnace (12) when the temperature is inadequate.
14. System according to one or more of claims 7 to 13, characterised in that a temperature gauge (22) is disposed at the discharge end of the continuous-heating furnace (12), said gauge reducing or interrupting the feed to the continuous-heating furnace (12) if the treated substance has too low a temperature.

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