METHOD AND STRAND SINTERING EQUIPMENT FOR CONTINUOUS SINTERING AND PRE-REDUCTION OF PELLETIZED MINERAL MATERIAL

FIELD OF THE INVENTION

The invention relates to a method defined in the preamble of Claim 1. The invention further relates to equipment defined in the preamble of Claim 8.

BACKGROUND OF THE INVENTION

Continuous strand sintering is used for agglomerating pellets after pelletizing powdery mineral material, improving the strength and reactivity of the pellets. The mineral material herein refers to a min- eral, which has similar crystal chemistry properties to those of the oxide group and contains the metal to be recovered, the metal mainly comprising compounds of metal and oxygen.

As an example of a strand sintering tech- nique, the strand sintering furnace 1 shown in Fig. 1 should be mentioned, being divided into several sequential zones I to VII, different temperature conditions prevailing in each one of them. The strand sintering equipment includes a perforated conveyor belt 2, which is conveyed as an endless loop around two deflector rolls 3, 4. At the left forward end of the furnace, wet fresh pellets are fed onto the conveyor belt 2 to form a bed with a thickness of a few decimetres. The conveyor belt 2 conveys the bed of pellets through the drying I, heating II, sintering III, equalizing IV zones of the sintering furnace and further through sequential cooling zones V, VI, VII. After travelling through the cooling zones, the pellets exit at the far end of the strand sintering equipment in a sintered form. To optimize the energy economy, the energy contained in the cooling gases at the tail

end of the furnace is used for drying, heating, and sintering at the forward end of the furnace, which is why the strand sintering equipment includes overhead circulation gas ducts 5, 6, 30 for implementing the gas circulation mentioned above. Burners 7, 8 have been placed in the circulation gas ducts 5 and 6 and are used to increase the temperature of the conducted gas to the sintering temperature needed in the sintering. Below the conveyor belt 2, there are lower ex- haust gas ducts for dissipating, through washers, the gas that exits each zone I, II, II and has been conducted through the pellet bed and the conveyor belt 2. Below the conveyor belt, there are lower inlet gas ducts 13, 14, 31 for conducting the gas to the cooling zones V, VI, VII. The movement of the gas in the ducts is provided by means of blowers 11, 12, 32 to 36, which are arranged in the lower exhaust and inlet gas ducts. In the known technology, gas is conducted to the atmosphere from the exhaust gas ducts after the blowers. Correspondingly, the gas that is to be conducted to the cooling zones has been taken to the inlet gas ducts from the atmosphere.

PURPOSE OF THE INVENTION The purpose of the invention is to disclose a method and equipment, which can be used to improve the process according to the known technology so that, in connection with sintering, pre-reduction of the mineral material is provided, whereby at the subsequent stages of the process, no separate pre-reduction equipment is needed before the actual reduction, which takes place in an electric fusion furnace or the like.

Another purpose of the invention is to disclose a method and equipment, which can be used to re- duce the energy consumption of the process.

SUMMARY OF THE INVENTION

The method according to the invention is characterized in that which is presented in Claim 1.

The strand sintering equipment according to the inven- tion is characterized in that which is presented in

Claim 8.

In the method according to the invention, an essentially smooth pellet bed is formed from the pellets onto a sintering base. On the sintering base, the pellet bed is conveyed through the process zones of different temperatures, including at least the heating/sintering zone and at least one subsequent cooling zone. During the conveyance, gas is conducted through the pellet bed. The gas that is conducted from the cooling zone through the pellet bed is circulated to the heating/sintering zone.

According to the invention, in the method, at least part of the gas that has travelled in the heating/sintering zone through the pellet bed is conducted to the cooling zone. The gas composition is rendered reductive for pre-reducing the pellets in the heating/sintering zone. The gas composition is measured and the gas composition is altered on the basis of the measurement to keep the composition reductive. One advantage of the invention is the fact that, simultaneously with the sintering, pre-reduction of the material can be effected, when a reductive atmosphere is arranged in the sintering furnace, whereby at the subsequent stages of the process, no separate pre-reduction equipment is needed. The actual reduction takes place in the electric fusion furnace or the like. As part of the material has already been pre- reduced, when delivered into the smelting furnace, a lesser amount of energy is needed in the smelting fur- nace and, thus, the total energy consumption of the process can be reduced.

In one application of the invention, the gas composition is measured by defining the carbon monoxide and/or the oxygen content of the gas.

In one application of the method, the gas is heated in the direction of rotation after the cooling zone and before the heating and sintering zones by burning a fuel in the gas, and the gas composition is kept reductive by adjusting the air coefficient of the combustion on the basis of the gas composition meas- ured.

In one application of the method, the gas composition is adjusted by conducting air to the gas in the direction of rotation after the heating/sintering zone and before the cooling zone on the basis of the gas composition measured.

In one application of the method, the gas pressure is adjusted to a predefined level by removing from the gas circulation the excess part of gas that is formed in the process during the combustion of the fuel.

In one application of the method, the gas pressure is defined in the heating/sintering zone, and on the basis of the defined pressure, the excess part of gas is conducted to the atmosphere in the direction of rotation of the gas after the heating and sintering zones and before the cooling zone.

In one application of the method, the gas is rendered reductive by means of a carbonaceous material that is arranged on the surface of the pellet bed and/or among the pellets, and/or inside the pellets.

The strand sintering equipment according to the invention includes a strand sintering furnace, which is divided into a number of sequential process zones of different temperature conditions, the zones including at least one heating/sintering zone, wherein the pellets are sintered, and at least one subsequent cooling zone, wherein the sintered pellets are cooled.

The conveyor belt is directed as an endless loop around a deflector roll and a drive roll for conveying the bed of pellets through the process zones of the strand sintering furnace. The conveyor belt is gas permeable. An overhead circulation gas duct is located above the conveyor belt for conducting the gas from at least one cooling zone to the heating/sintering zone and onto the bed of pellets. A burner is placed in the overhead circulation gas duct to heat the gas that travels in the duct. A lower exhaust gas duct is located below the conveyor belt for conducting the gas that exits the heating/sintering zone and has been conducted through the pellet bed and the conveyor belt. A blower is arranged in the lower exhaust gas duct to provide a movement of the gas. A lower inlet gas duct is located below the conveyor belt for conducting the gas to the cooling zone.

According to the invention, the strand sintering equipment includes a connecting channel, which provides flow communication from the lower exhaust gas duct to the inlet gas duct to conduct at least part of the gas exiting the heating/sintering zone to the cooling zone. The equipment further includes a gas sensor, which is arranged in the connecting channel to detect the gas composition. The equipment further includes a leakage air channel, which provides flow communication between the lower exhaust gas duct and the atmosphere. The equipment further includes a leakage air valve for opening and closing flow communication in the leakage air channel. In addition, the equipment includes a control device, which, in order to render the gas composition reductive, is arranged to monitor the gas compositions detected by the gas sensor and, on the basis of that, to control the leakage air valve for controlling the access of air from the atmosphere to the exhaust gas duct for adjusting the oxygen content of the gas, and/or to adjust the combustion air

coefficient of the burner for adjusting the carbon monoxide content of the gas.

In one application of the equipment, the equipment includes a pressure sensor, which is ar- ranged to measure the gas pressure in the heating/sintering zone. A pressure reduction channel is arranged after the blower in the flow direction to provide flow communication between the connecting channel and the atmosphere. A pressure-controlled valve is arranged in the pressure reduction channel and, controlled by the pressure detected by the pressure sensor, it is arranged to allow gas to exit the connecting channel to reduce the gas pressure to a predefined level. In one application of the equipment, the strand sintering equipment includes a heating zone, a sintering zone, a first cooling zone and a second cooling zone, which are separated from each other by walls . In one application of the equipment, the control device comprises a gas analyzer that measures the carbon monoxide and/or oxygen content of the gas.

The other advantageous features of the method and the equipment are disclosed in the appended claims.

LIST OF FIGURES

In the following, the invention is described in detail by means of exemplary embodiments and with reference to the appended drawing, wherein

Fig. 1 shows schematically a strand sintering equipment according to the known technology, and

Fig. 2 shows schematically an embodiment of the strand sintering equipment according to the inven- tion.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 2 shows a strand sintering furnace 1, which is divided into several sequential zones I to VII, different temperature conditions prevailing in each one of them. The zones in this example comprise a drying zone I, a heating zone II, a sintering zone III, an equalizing zone IV, and three cooling zones V, VI, VII. The conveyor belt 2 is directed as an endless loop around a deflector roll 3 and a drive roll 4 to convey a bed of pellets through the zones I to VII of the strand sintering device 1. The conveyor belt 2 is a perforated steel band, the perforation allowing the gas to pass through. At the forward end of the furnace (to the left in the figure) , wet fresh pellets are fed onto the conveyor belt 2 to form a bed with a thickness of several decimetres. The conveyor belt 2 conveys the pellet bed through the drying zone I, heating zone II, and sintering zone III of the furnace to the stabilizing or equalizing zone IV, after which the pellet bed further travels through the sequential first cooling zone V, second cooling zone VI, and third cooling zone VII. After travelling through the cooling zones, the pellets exit the tail end of the strand sintering equipment in a sintered form. A first circulation gas duct 5 is arranged above the conveyor belt 2 to conduct gas from the second cooling zone VI to the heating zone II and onto the bed of pellets. A first burner 7 is arranged in the first circulation gas duct. Below the conveyor belt, there is a first exhaust gas duct 9, which receives the gas that comes from the heating zone II and has been directed through the pellet bed and the conveyor belt. A first blower 11 is arranged in the first exhaust gas duct 9 to provide a movement of the gas. A first inlet gas duct 13 is located below the conveyor belt 2 for conducting the gas to the second cooling zone VI. A first connecting channel 15 provides flow

communication from the first exhaust gas duct 9 to the first inlet gas duct 13 so that at least part of the gas exiting the heating zone II enters the second cooling zone VI. In the first connecting channel 15, there is a first gas sensor 17 for detecting the gas composition. A first leakage air channel 19, which provides flow communication between the first exhaust gas duct 9 and the atmosphere, is provided with a first leakage air valve 21 for opening and closing flow communication in the first leakage air channel 19. To render the gas reductive in a controlled manner, a control device 23 is arranged to monitor the gas compositions detected by the first gas sensor 17 and, on the basis of that, to control 23 the combus- tion air coefficient of the first burner 7 for adjusting the carbon monoxide content of the gas and to control the first leakage air valve 21 to allow air from the atmosphere to enter the first exhaust gas duct 9 for adjusting the oxygen control of the gas. Correspondingly, a second circulation gas duct 6 above the conveyor belt 2 conducts the gas from the first cooling zone V to the sintering zone III and onto the pellet bed. A second burner 8 is arranged in the second circulation gas duct. A second exhaust gas duct 10 is located below the conveyor belt for conducting the gas that comes from the sintering zone III and has been conducted through the pellet bed and the conveyor belt. A second blower 12 is arranged in the second exhaust gas duct to provide a movement of the gas. A second inlet gas duct 14 is located below the conveyor belt for conducting the gas to the first cooling zone V. A second connecting channel 16 provides flow communication from the second exhaust gas duct 10 to the second inlet gas duct 14 so that at least part of the gas exiting the sintering zone III enters the first cooling zone V. A second gas sensor 18 is arranged in the second connecting channel 16 for

detecting the gas composition. A second leakage air channel 20 provides flow communication between the second exhaust gas duct 10 and the atmosphere. In the second leakage air channel 20, there is a second leak- age air valve 22 for opening and closing the flow communication. To render the gas composition reductive, a control device 23 is arranged to monitor the gas compositions detected by the second gas sensor 18 and, on the basis of that, to adjust the combustion air coef- ficient of the second burner 8 to adjust the carbon monoxide content of the gas and to control the second leakage air valve 22 to control the entry of air from the atmosphere to the second exhaust gas duct 10 to adjust the oxygen content of the gas. In this way, at least part of the gas that has travelled through the pellet bed in the heating/sintering zone is conducted to the cooling zone, the gas composition is rendered reductive for pre- reducing the pellets in the heating/sintering zone, and the gas composition is measured. The gas composition is measured by defining the carbon monoxide and/or the oxygen content of the gas. On the basis of the measurement, the gas composition is altered to keep the composition reductive. The gas is heated in the direction of rotation after the cooling zone and before the heating/sintering zone by burning a fuel in the gas, and the gas composition is kept reductive by adjusting the combustion air coefficient on the basis of the gas composition measured. Furthermore, the gas composition is adjusted by conducting air to the gas in the direction of rotation after the heating/sintering zone and before the cooling zone on the basis of the gas composition measured.

When burning the fuel by the burners in the circulation gas channels to heat the gas, the pressure in the closed gas circulations formed according to the invention increases cumulatively, unless the pressure

is decreased every now and then. Hence, the gas pressure is adjusted to a predefined level by removing from the gas circulation a volume of gas that corresponds to the volume of gas generated in the process during the combustion of the fuel. Therefore, the gas pressure is measured in the heating/sintering zone, and on the basis of the pressure defined, the excess portion of gas is conducted to the atmosphere in the direction of rotation of the gas after the heating and sintering zones and before the cooling zone.

Therefore, the equipment includes a first pressure sensor 24, which is arranged to measure the gas pressure in the heating zone II . A first pressure reduction channel 26, which in the flow direction is located after the first blower 11, provides flow communication between the first connecting channel 15 and the atmosphere. A first pressure-controlled valve 28 is arranged in the first pressure reduction channel 26. The first pressure-controlled valve 28 releases gas from the first connecting channel 15, being controlled by the pressure detected by the first pressure sensor 24, to reduce the pressure to a predefined level .

Correspondingly, a second pressure sensor 25 is arranged to measure the gas pressure in the sintering zone III. A second pressure reduction channel 27, which in the flow direction is located after the second blower 12, provides flow communication between the second connecting channel 16 and the atmosphere. A second pressure-controlled valve 29 is arranged in the second pressure reduction channel 27. The second pressure-controlled valve 29 releases gas from the second connecting channel 16, being controlled by the pressure detected by the second pressure sensor 25, to re- duce the pressure to a predefined level.

To render the gas reductive, carbonaceous material, such as coke, can also be arranged on the sur-

face of the pellet bed and/or among the pellets and/or inside the pellets.

The invention is not limited to the exemplary embodiment disclosed above only, but various modifications are feasible within the inventive idea defined by the claims.