DE69906377T2 - METHOD AND DEVICE FOR OBTAINING IRON BY DIRECT REDUCTION - Google Patents

Description

Scope of the invention

The present invention relates to a process for the production of a ferrous metal based on a ferrous mineral in which the iron is enriched in the form of an oxide, and the corresponding device, which includes a reduction furnace having one or more inlets for the reducing gas and within which the process of direct reduction of iron (DRI) is carried out. The reducing gas is produced by mixing a part of the process gas produced in the reduction furnace with an additional gas supplied from an externally arranged processing circuit.

Background of the invention

The prior art includes direct reduction processes using the introduction of hydrocarbons into the reducing gas stream to enable a reaction for upgrading the methane in the furnace with the H₂O and CO₂ in the gas; direct reduction processes are also known which involve the introduction of hydrocarbons with C > 5 directly into the furnace in the zone between the reducing gas introduction and the outlet from it above the burned gas.

Further patent documents for other processes for the direct reduction of iron-containing minerals are given below:

US-A-2,189,260, US-A-3,601,381, US-A-3,748,120, US-A-3,749,386, US-A-3,764,123, US-A-3,770,421, US-A-4,054,444, US-A-4,173,465, US-A-4,188,022, US-A-4,234,169, US-A-4,201,571, US-A-4,270,739, US-A-4,374,585, US-A-4,528,030 US-A -4,556,417, US-A-4,720,299, US-A-4,900,356, US-A-5,064,467, US-A-5,078,788, US-A-5,387,274 and US-A-5,407,460.

The prior art also includes processes wherein hot ferrous metal is produced in a shaft furnace type reduction furnace having a vertical and gravitational flow for the material which is subsequently fed into the melting furnace by means of a closed pneumatic transport system in an internal atmosphere.

Document EP-A-0262353 discloses a method and apparatus for the production of hot sponge iron in a vertical shaft furnace, wherein the reducing gas for the reduction is produced in a preparation unit by catalytic conversion of a mixture of steam and natural gas to a reducing gas consisting essentially of CO and H₂. The feed gas or the process gas leaving the reduction zone of the reactor flows to a quenching-cooling device, where it is cooled and dehydrated, and to a carbon dioxide unit, where the CO2 is removed. A back pressure regulator makes it possible to retain the overflowing gas and to keep the reactor and the associated equipment under a certain, desired pressure. Therefore, the process gas is essentially completely "cleaned" of H2O and CO2 in the sense of being recycled as a portion of the active gas (H2 and CO) contained in the feed gas. What happens is simply that something that is no longer required (H2O and CO2) is taken out of the process gas, while the remaining parts are fed into the main gas supply line into which the reducing gas produced by the steam conditioning unit flows.

Summary of the invention

The process for producing ferrous metal by the direct reduction of iron oxides and the corresponding apparatus relating to the invention are described and characterized in the respective main claims, with the dependent claims describing further innovative features of the present invention.

The method related to the present invention consists in bringing iron-containing mineral with different granule geometry into contact with the supplied gas in the shaft furnace type reduction furnace, whereby both the gas and the material are continuously supplied so that a vertical and gravitational flow of the material is created and the direct reduction of the mineral is achieved. The material can be removed from the reactor either cold or preferably hot to be subsequently fed to a melting furnace or the like so that it can be formed into hot iron briquettes (HBI) or cooled and subsequently formed into direct reduced iron (DRI).

The reduction furnace is equipped with means for feeding the ferrous mineral and with means for discharging and reducing the ferrous metal; it is equipped with at least one inlet manifold for feeding the reducing gas in correspondence with a reduction zone or reactor within the furnace.

The reaction gas fed into the reactor contains hydrocarbons, which are introduced into the stream after partial combustion of hydrogen and carbon monoxide with oxygen and is achieved by mixing part of the process gas present in the reduction furnace with additional gas supplied from an external treatment circuit.

In one variant, the hydrocarbons are added before partial combustion is achieved, with the purpose of increasing the temperature of the gas fed into the reactor.

According to a further variant, the hydrocarbons are at least partially fed into a zone between the reduction zone and the zone in which the material to be reduced is discharged.

In all cases, the supplied hydrocarbons react to reduce the iron oxide (FeO) to iron metal, producing more H2 and CO.

The direct reduction of the iron oxide is achieved in two different, continuous stages within the reduction reactor. In a specific embodiment, the furnace is equipped with a first stage, defined as the preheating and prereduction stage, in which the fresh iron oxides, that is, those which have just been fed into the furnace, come into contact with a mixture of reducing gas consisting of partially burned gas coming from the lower parts of the furnace and of fresh hot gas, that is, that gas which is fed from the outside, arriving from a collector which supplies fresh reducing gas and possibly CH₄ or other neutral gases. This first stage takes place in a corresponding first zone arranged in the upper part of the furnace.

In the second stage the real reduction now takes place, the total reduction of the iron oxide is achieved due to the action on the oxides which have already been partially reduced in the first stage, by a mixture of a reducing gas based on H₂ and CO and at least one hydrocarbon, preferably natural gas, supplied in the middle zone of the reduction reactor. This second stage takes place in a corresponding second zone, located below the first zone.

The two inlets for the furnace through which the gas is supplied can be regulated independently of each other, with both influencing the flow of the fresh reducing gas and the additional natural gas in the supplied stream.

Furthermore, the inlet temperature of the two reducing gas streams can be regulated independently by the addition of O₂ before they enter the reduction reactor.

The oxidation reaction required to raise the temperature of the gas results in a change in the concentration of oxidation of the gas from a normal value of 0.04 to 0.08 to 0.06 to 0.15.

The following ratio is intended for the oxidation concentration of the reducing gas:

Nox = (H2 O + CO2 /(H2 O + CO2 + H2 + CO).

In the second reaction zone of the furnace, where the reduction of the iron oxide is completed, a gas is produced with a high proportion of H₂ and CO and with an oxidation concentration between 0.15 and 0.25 depending on the reduction reaction of the iron oxides with H₂, CO and CH₄.

After this gas has left the second reaction zone, it enters the first reaction zone, which is located higher up, and mixes with the hot gas which is added to the preheated and prereduced iron oxides in the first zone.

The gas produced in the reduction reactor is partly recycled and partly used as fuel.

The gas returned to the circuit has a volume composition in the following ranges:

H&sub2; = 20-41%, CO = 15-28%, CO&sub2; = 15-25%, CH4 = 3-10%, N2 = 0-8%, H2 O = 2-7%.

According to a characteristic of the present invention, the gas fed to the reduction reactor consists of a mixture of a natural gas, with the recirculated gas, also known as process gas or blast furnace gas, present in the reactor itself, and reclaimed gas; the recirculated gas is preheated to a temperature of between 650 and 950°C; the gas resulting in the preheater is mixed in turn with fresh reclaimed gas and subsequently with air or with oxygen-enriched air or with pure oxygen in order to achieve partial combustion of the H₂ and CO in the reduction gas, for the purpose of increasing the temperature to values of between 800 and 1150°C, preferably between 1000 and 1150°C; and the oxidation concentration of the resulting gas fed into the furnace ranges between 0.06 and 0.15.

The methane represents between 6 and 20% of the volume of the reducing gas mixture.

When the supplied gas in the reduction zone comes into contact with the hot, partially reduced material, which consequently consists partly of iron metal and partly of iron oxides, a highly endothermic reaction is induced.

An endothermic reaction also occurs in the preheating and prereduction zone when the gas comes into contact with the iron oxide.

An advantage of this invention is that the first preheating and prereduction zone is extended, which makes it possible for the transformation of ematite (Fe2O3) to wustite (FeO) to be initiated much more quickly.

The entire reactor operates at a higher average temperature, which is constant across both zones, thus promoting a higher reaction rate in both the pre-reduction and reduction zones, consequently achieving the effect of reducing consumption and increasing productivity.

In the case where the furnace has two inlets for the supply of the reducing gas, the first inlet is arranged at a determined distance (x) with respect to the second inlet, which is arranged in the central area of the furnace, in correspondence with the second reduction zone. This distance (x) can be between 1 and 6 meters, preferably between 2 and 4 meters, in order to promote the reactions in the most suitable zone between the reducing gas and the iron oxides.

The first gas inlet also has the function of pushing the gas flowing from the second reduction zone towards the center of the furnace in order to achieve a uniform distribution of the gas in this section of the reactor.

According to one variant, there are several or more than two inlets for the reducing gas into the furnace. The first stream of reducing gas is fed into the middle of the reactor, preferably into the reduction zone, while the other streams are arranged in the zone between the inflow of the first stream of gas and the outlet of the fuel gas, in the upper part of the furnace. This middle zone is called the preheating and prereduction zone for the iron oxide-based materials.

The flow of gas into the reactor, which is thus composed, allows the entire reduction and pre-reduction zone to be kept at as constant a temperature as possible and to maintain a gas inside the furnace that has a high reducing power at all times, thus ensuring greater productivity and lower gas consumption; which also allows the final metallization of the product to be improved.

In this way, it is also achieved that the iron oxide in the reduction zone is already partially reduced, which promotes the completion of the final reduction reaction from FeO to Fe.

Short description of the drawings

These and other characteristics of the present invention will become even more understood from the following description of some preferred forms of embodiments, given as non-restrictive examples, with the aid of the accompanying figures, in which:

Fig. 1a shows in diagrammatic form an apparatus for the direct reduction of iron oxides relating to the present invention in a first form of embodiment;

Fig. 1b shows in diagrammatic form an apparatus for the direct reduction of iron oxides relating to the present invention in a second form of embodiment;

Fig. 2 shows a first variant of an oven which is part of the device in Fig. 1a;

Fig. 3 is a diagram showing the temperature inside the furnace shown in Figs. 1a and 2;

Fig. 4 shows a second variant of a furnace arranged in the device in Fig. 1a;

Fig. 5 is a graph showing the temperature inside the furnace as shown in Fig. 4;

Detailed description of preferred embodiments

With reference to Fig. 1a, an apparatus for the direct reduction of iron oxides, according to the present invention, comprises a reduction furnace, which is of the shaft furnace type, or of the reduction reactor type 10, comprising in turn an upper opening 11 for the top feed through which it is possible to feed the minerals (iron oxides), a first preheating and prereduction zone 12, a second zone or middle zone 14 in which the final reduction reaction of the iron oxide takes place, and a lower zone or discharge zone 15, shaped like a truncated cone, detachably arranged at the bottom side in a lower opening 16 through which the iron is discharged.

The iron-based metal oxides are fed into the reactor 10 in the form of grains or crude mineral of appropriate sizes; the iron therein usually being between 63 and 68% by weight.

At the end of the process according to the present invention, the iron content in the reduced material coming out of the reactor 10 is normally between 80 and 90% by weight.

Corresponding to the two zones 12 and 14 of the reactor 10 are two independent inlets 17 and 18, respectively, through which a mixture of gas can be supplied in a suitable manner, as will be described in more detail below.

In this upper part, above zone 12, the reactor 10 is equipped with an opening 19 through which the combusted gas or the process gas flows out. This gas normally has the following characteristics:

Composition: H₂ = 20-41%, CO = 15-28%, CO₂ = 12-25%, CH₄ = 2-10%, N₂ = 0-8%, H₂O = 2-15%; the temperature is between 500 and 700ºC; the oxidation concentration is between 0.3 and 0.50, preferably between 0.40 and 0.45; and the reduction ratio R is between 1 and 1.8, the reduction ratio being calculated as follows:

R = (H2 + CO)/(H2 O + CO2).

In the embodiment shown in Fig. 1b, the furnace 10 comprises only one reaction zone 14 and only one inlet 18 through which the reducing gas is introduced into the furnace. In both versions, that is, the one shown in Fig. 1a and the other shown in Fig. 1b, the burnt gas escaping from the reactor 10 is led through a pipe 20 to a cooling unit 21, suitable for recovering the heat given off by it; from the cooling unit 21 it passes, through another pipe 22, to a cooling and condensing unit 24. In this unit 24, the burnt gas is washed at a temperature between 40°C and 65°C and the quantity of water present in the gas itself is partially replaced. The percentage of water remaining in the gas at the outlet of unit 24 is between 2 and 7%.

The gas at the outlet of the unit 24 is sent through a line 30, partly through a preheater 36, partly through a catalytic reprocessor 44 to be used as fuel, and partly through a compressor 26.

The gas exiting the compressor 26 is used in turn as recycle gas and is sent through a line 28, inside the unit 21, and partially through a line 46, mixed with a natural gas containing methane (CH4) or pure methane, supplied through a line 34 in a ratio of about 4:1 (it being stated here that for each part of natural gas, about 4 parts of the gas entering through line 46 are supplied) and fed into the regenerator 44 so that the recycle reaction of the methane (CH4) with H2O and CO2 can begin.

The portion of the gas supplied to the unit 21 through line 28 is preheated and is then passed through line 32 to the preheater 36 where it is further preheated to a temperature between 650 and 950ºC. CH₄ may also be supplied into line 32.

The gas emerging from the preheater 36, which has a feed rate of between 600 Nm³/ton DRI and 1500 Nm³/ton DRI, is mixed in a line 38 with the gas which is fed from the reprocessor 44 through the line 50.

The gas resulting from this mixture is divided into two parts and distributed between two pipes 40 and 41 connected to the inlets 17 and 18 of the furnace 10. The supply of reducing gas is controlled in each of the zones 12, 14 by means in the form of regulating valves 55 and 56.

In each line 40 and 41 is introduced air, or air enriched with oxygen, or pure oxygen and natural gas in variable percentages, for the purpose of achieving partial compression of the CO and H₂ and raising the temperature of the gas.

A stream of CH4 or natural gas is introduced into the gas before it is fed into the reactor.

In a variant, shown by a dashed line in Figures 1a and 1b, the CH4 is introduced before being fed to the partial combustion, for the purpose of increasing the temperature of the gas fed into the reactor.

The CH4 may also be introduced into a zone between the reduction zone 14 and the material outlet cone, through line 81. In this case, the introduced CH4 partially cools the reduced iron before it enters the zone 14 in which the reduction reactions are carried out before the iron is tapped.

The resulting mixtures are then fed into the reduction zone 14 and optionally into the preheating and prereduction zone 12.

Referring to the furnace 10 with two inlets (Fig. 1a) for each zone 12 and 14, the corresponding mixture of gas is regulated in an automatic and independent manner.

To be more precise, the gas flow in the first zone 12 is between 500 Nm³/ton DRI and 800 Nm³/ton DRI and enters the reactor 10 at a temperature between 800ºC and 1150ºC, preferably between 1000ºC and 1150ºC, while the gas flow in the second zone 14 is between 1000 Nm³/ton DIR and 1500 Nm³/ton DRI and also enters the reduction reactor 10 at a temperature between 800ºC and 1150ºC, preferably between 1000ºC and 1150ºC.

The consumption of oxygen required to raise the temperature of the reducing gas from 650ºC to 950ºC to 800ºC to 1150ºC is preferably pure oxygen plus that contained in the air if air is also blown in and is between 8 Nm³/ton DRI and 60 Nm³/ton DRI, preferably between 20 and 60 Nm³/ton DRI.

The consumption of CH₄ is between 50 and 120 Nm³/ton DRI, preferably between 90 and 110 Nm³/ton DRI. In terms of volume, CH₄ represents between 6 and 20% of the mixture of reducing gases fed into the reactor.

The reactions involved in the reduction zone 14 are as follows:

FeO + CH4 = Fe + 2H2 + CO (1).

At the same time, in the same zone 14, the following reduction reaction takes place, with hydrogen and carbon monoxide:

FeO + H2 = Fe + H2 O (2)

FeO + CO = Fe + CO₂ (3).

The consequence of these three endothermic reactions is that the temperature of the gas in the reduction zone drops from 800ºC to 1150ºC to 700ºC to 900ºC, but the reaction temperature remains higher than in state of the art furnaces and the gas leaving the reduction zone 14 has a reducing concentration between 0.15 and 0.35 and a reducing power between 1.1 and 2.8.

The reactions involved in the pre-reduction zone 12 are as follows:

Fe&sub2;O&sub3; + H2 = 2FeO + H2 O (4)

Fe&sub2;O&sub3; + CO = 2FeO + CO2 (5).

In the lower zone 15, which is shaped like a truncated cone, it is also possible to supply gas, which includes natural gas, in order to regulate the residual carbon in the hot reduced iron to values between 1.5% and 3.0%.

In a variant, as shown in Fig. 2, the furnace 10, instead of a single lower part shaped like a truncated cone, has at least two, and preferably three or four, lower ends shaped like a cone or truncated cone 15a, 15b and 15c through which reduced ferrous metal can be withdrawn in a controlled and independent manner. In this case, the CH4 can also be supplied by means or devices arranged at the central zone of the ends of the truncated cone 15a, 15b and 15c, thus exploiting the geometric shape of the system.

The evolution of the temperature inside the furnace 10, both in the version as shown in Fig. 1a and in the version as shown in Fig. 2, is shown in Fig. 3, from which it can be seen how the temperature is higher and more constant in the segment affected by zones 12 and 14.

Regarding a further variant, as shown in Fig. 4, the furnace 10 is equipped with a plurality of inlets, namely more than two, instead of two inlets for supplying the reducing gas. In this case, a first gas flow is supplied to the lower inlet 18 through the line 41, a second gas flow is supplied to the inlet 17 through the line 40 and other gas flows, each of which can be autonomously controlled, are supplied through lines 42 and corresponding inlets 43 arranged between the inlets 17 and the upper opening 19.

The evolution of the temperature inside the furnace 10, in the embodiment as shown in Fig. 4, is shown in the diagram shown in Fig. 5, from which it can be seen how the temperature is higher and more constant in the whole segment influenced by the lines 40, 41 and 42.

It is obvious that it is possible to make modifications and extensions to this process for the direct reduction of iron-containing minerals and the corresponding apparatus as described above, but this does not deviate from the technical teaching of the present invention.

Claims (26)

1. Process for the direct reduction of iron-containing mineral within a vertical reduction furnace (10) of the type with a gravitational loading, comprising the following steps: Feeding the iron-containing mineral into the furnace (10) from above, blowing in a mixture of high-temperature reducing gas based on H₂ and CO into said furnace (10), said reducing gas being in counterflow with respect to said iron-containing mineral fed into the furnace, the process gas or top gas being withdrawn from the top of said furnace (10) and the reduced mineral being withdrawn from the bottom of said furnace (10), said reducing gas being obtained by mixing variable parts, depending on the requirements of the iron reducing process, at least a part of said process gas and additional gas fed from an external regeneration circuit (44), the method being characterized in that said process gas is divided into two parts, a first part of which is fed to a preheater (36) to be subsequently fed back into the furnace (10) and a second part is fed to said regeneration circuit (44) to be reconditioned and subsequently fed to said furnace (10), wherein the gas escaping from said preheater (36) and coming from said recycling circuit (44) are mixed together to form a mixed gas before being fed into said furnace (10). 2. Process according to claim 1, characterized in that before being divided into two parts, said process gas is washed, cooled and subjected to a substantial reduction in the water content therein. 3. Method according to claim 1, characterized in that said preheater (36) preheats the temperature of the first part of the process gas to a value which is between 650 and 950ºC. 4. Process according to claim 1, characterized in that said gas mixture, before being fed into the furnace, is further mixed with O₂ or air enriched with O₂, for the purpose of obtaining a partial combustion of the H₂ and CO present in the reducing gas, and thus increasing the temperature in the reduction zone of the furnace to values between 800°C and 1150°C, advantageously between 1000 and 1150°C. 5. Process according to claim 1, characterized in that at least one hydrocarbon is added to said gas mixture before it is introduced into the reduction zone of the furnace. 6. A process according to claim 5, characterized in that said hydrocarbon consists of CH₄ for the purpose of cooperating in the reduction of the iron oxide to iron metal, at the same time producing further H₂ and CO. 7. Process as in claim 1, characterized in that said reducing gas is supplied to at least two zones (12, 14) of said furnace (10), arranged one above the other, in a controlled manner, a first stage serving for preheating and prereduction in the upper part (12) of the furnace (10), and a second stage for final reduction in the lower part (14) of the furnace (10). 8. Process according to claim 6, characterized in that said methane represents between 6 and 20% in the volume of said mixture of reducing gas. 9. A method according to claim 7, characterized in that the supply of said gas mixture, in different percentages, is controlled in different reduction zones (12, 14) along the length of said furnace (10). 10. A process according to claims 5 and 7, characterized in that said hydrocarbon preferably consists of natural gas and in that said hydrocarbon in said gas mixture is proportioned and controlled independently fed into the different reduction zones (12, 14) along the length of said furnace (10). 11. A method according to claim 7, characterized in that said gas mixture is heated independently in different zones (12, 14) along the length of said furnace (10). 12. Process according to claim 1, characterized in that said gas mixture which is introduced into said reactor has an oxidation concentration between 0.06 and 0.25. 13. A method according to claim 7, characterized in that CH₄ is further partially fed into said furnace (10) in a zone between said lower part (14) and an underlying discharge zone (15). 14. Apparatus for the direct reduction of ferrous mineral, comprising a vertical reduction furnace (10) of the type with a gravitational loading for achieving therein reactions for the reduction of the ferrous mineral, feeding means (11) for feeding the ferrous mineral into said furnace (10) from above, first feeding means (17, 18) for feeding a mixture of a high temperature gas consisting of pure reducing gas based on H₂ and CO, first means (19) for sucking the process gas or the top gas from the top of said furnace (10) and second means (16) for removing the reduced mineral from the lower part of said furnace (10), wherein first mixing means are available for mixing, in variable proportions, according to the requirements of the iron reducing process, at least a portion of said process gas is mixed with additional gas supplied from a treatment circuit (44) arranged outside said furnace (10) to obtain said reducing gas, the device being characterized in that dividing means (28, V₈, 46, V₉) are available for dividing said process gas into two portions, a first portion of which is suitable for being supplied to a preheater (36) for subsequent refeeding to said furnace (10), and a second portion which is suitable for being supplied to said treatment circuit (44) to be reconditioned and subsequently supplied to said furnace (10), and said first mixing means (38, 50) are provided for mixing the gas supplied from said preheater (36) and to mix the gas escaping from the treatment circuit (44) to form a gas mixture before it is fed back into said furnace (10). 15. Apparatus according to claim 14, characterized in that washing means and cooling means (21, 24) are provided upstream of said dividing means, so that said process gas is capable of being washed, cooled and subjected to a substantial reduction in the water content therein. 16. Apparatus as in claim 14, characterized in that said preheater (36) is adapted to preheat said process gas to a temperature between 650 and 950ºC before it is mixed with said additional gas. 17. Device as in claim 14, characterized in that the second mixing means (V₂, V₄) are provided for the further mixing of said gas mixture with O₂ or air enriched with O₂ for the purpose of achieving a partial combustion of H₂ and CO present in the reducing gas and thus raising the temperature in the reduction zone of the furnace to values between 800ºC and 1150ºC, advantageously between 1000 and 1150ºC. 18. Apparatus as in claim 14, characterized in that means are provided for supplying at least one hydrocarbon into said reducing gas or together therewith into the reduction zone of the furnace. 19. Apparatus as in claim 18, characterized in that said hydrocarbon consists of CH₄ and is suitable for contributing to the reduction of the iron oxide in iron metal and simultaneously producing further H₂ and CO. 20. Apparatus as in claim 14, characterized in that means (17, 18) are provided for supplying said reducing gas into at least two reduction zones (12, 14) of said furnace (10), one arranged above the other, so that in a controlled manner a first stage of preheating and prereduction takes place in the upper part (12) of the furnace (10) and a second stage of final reduction in the lower part (14) of said furnace (10). 21. Apparatus as in claim 20, characterized in that regulating means (55, 56) are provided to regulate the supply and the different percentages of said reducing gas in the different reducing zones (12, 14) along the length of said furnace (10). 22. Device as in claims 18 and 20, characterized in that at least two elements are provided for mixing the reducing gas and the hydrocarbon upstream of the inlet of said furnace (10) in correspondence with said reducing zones (12, 14), for the purpose of providing a mixture, the hydrocarbon being supplied in a proportioned and controlled manner independently and autonomously to each of said zones (12, 14). 23. Device as claimed in any one of claims 14 to 22, characterized in that the second extraction means (15) has at least two ends (15a to 15c) shaped like a cone or a truncated cone. 24. Apparatus as in claim 23, characterized in that said ends (15a to 15c) are shaped like a cone or a truncated cone which taper downwards and each is provided with a corresponding lower opening (16a to 16c) through which said reduced ferrous metal can be selectively tapped in a controlled and independent manner. 25. Apparatus as claimed in any one of claims 20 to 24, characterized in that the second supply means (81) are provided for supplying at least partially CH₄ into said furnace (10) in a zone between said second outlet means (15) and the lower of said reaction zone (14). 26. Device as in claims 23 and 25, characterized in that said second feed means (81) are arranged in a zone in the intersection between said ends (15a to 15c) which are shaped like a cone or a truncated cone.