WO2011060483A1

WO2011060483A1 - Method for removing slag from a furnace

Info

Publication number: WO2011060483A1
Application number: PCT/AU2010/001491
Authority: WO
Inventors: Mark Joseph Prince; Mark Kenneth Letchford
Original assignee: Xstrata Technology Pty Ltd
Current assignee: Glencore Technology Pty Ltd
Priority date: 2009-11-18
Filing date: 2010-11-09
Publication date: 2011-05-26
Anticipated expiration: 2012-05-18
Also published as: BR112012011988A2; MX2012005700A; JP5721734B2; AU2010321669B2; AU2010321669A1; BR112012011988B1; EA022456B1; JP2013511690A; EA201290206A1; CN102686966B; CN102686966A; PL218986B1; PL399890A1; MX353976B

Abstract

A method for tapping slag from a stationary furnace comprises providing a stationary furnace having a tap opening for removing slag therefrom, packing said tap opening with clay or mud, drilling a hole through the clay or mud in the tap opening to form a tap hole through which the slag can flow, the hole drilled through the clay or mud having a diameter that is significantly smaller than a width of the tap opening, and controlling a flow of slag through the hole by adjusting the size of the hole.

Description

METHOD FOR REMOVING SLAG FROM A FURNACE

FIELD OF THE INVENTION The present invention relates to an improved method for removing slag from a furnace. More particularly, the present invention relates to a method for removing slag from a stationary furnace, such as a top entrant submerged lance furnace. The present invention also extends to apparatus used in the method. BACKGROUND TO THE INVENTION

Pyrometallurgical processes typically involve the formation of a slag layer. It is frequently necessary to remove slag from furnaces. There have been a number of different methods for removing slag from furnaces. For example, in tilting furnaces or moving furnaces, it is normal to provide an aperture at a position located above the level of the slag in the furnace during normal use of the furnace. The furnace may then be tipped or rotated so that the slag pours out through the aperture.

In stationary furnaces, slag is conventionally removed in one of two ways: i) a tap opening is provided in the side of the furnace. Such tap openings comprise circular openings formed in the furnace wall. The tap opening is normally closed off using clay or mud that is injected into the opening using a clay gun or a mud gun. The clay or mud form an effective barrier that prevents leakage of molten furnace products through the tap opening. When it is necessary or desirable to remove slag from the furnace, a drill is used to drill a tap hole through the clay or mud. The drill normally has a diameter that is only slightly smaller than the diameter of the tap opening. When the drill is removed from the tap hole, the slag can pour out through the tap hole. When sufficient slag has been removed from the furnace, the tap hole is closed off using a mud gun or a clay gun. In this method for removing slag from the furnace, the slag is removed from the furnace on an intermittent or batch basis. ii) in some stationary furnaces, a tap opening is provided in the side wall of the furnace. A slag receiving receptacle is mounted to the outside of the furnace so that any slag that passes through the tap opening is received in the receptacle. When the slag in the receptacle fills the receptacle, the slag overflows a weir in the receptacle and is collected or transported to another location. In these slag tapping operations, the slag may be tapped from the furnace on a continuous basis. It is generally not necessary to close the tap opening with clay or mud.

Top entrant submerged lance furnaces are used in the production of metals by pyrometallurgical processes. Top entrant submerged lance furnaces comprise a stationary furnace vessel, normally lined with refractory material. The fuel and air or oxygen is normally injected into the furnace charge via a lance that is inserted into the furnace charge from above. When air or oxygen is injected through the lance, the air or oxygen causes agitation of the furnace charge. Therefore, the molten furnace contents are violently stirred in top entrant submerged lance furnaces. An example of a top entrant submerged lance furnace is the ISASMELT™ furnace as manufactured and sold by the present applicant.

ISASMELT™ furnace installations are currently used in the production of copper and lead. Other metals may also be produced using ISASMELT™ furnace installations. In our international patent application number PCT/AU2006/001460, the entire contents of which are herein incorporated by cross reference, we describe a method for producing lead in which a lead ore or concentrate is fed to an ISASMELT™ furnace. This results in the formation of lead bullion and a lead slag. The lead slag is removed from the furnace, formed into lumps (such as by feeding the lead slag to a caster) and the lumps of lead slag are then fed as a feed material to a blast furnace. The lead slag is converted into lead bullion and a discard slag in the blast furnace.

The present applicant does not concede that the prior art described in the specification forms part of the common general knowledge in Australia or elsewhere. Throughout the specification, the term "comprising" or its grammatical equivalents shall be taken to have an inclusive meaning unless the context of this indicates otherwise. BRIEF DESCRIPTION OF THE INVENTION

In a first aspect, the present invention provides a method for tapping slag from a stationary furnace comprising:

- providing a stationary furnace having a tap opening for removing slag therefrom;

- packing said tap opening with clay or mud;

- drilling a hole through the clay or mud in the tap opening to form a tap hole through which the slag can flow, the hole drilled through the clay or mud having a diameter that is significantly smaller than a width of the tap opening, and

- controlling a flow of slag through the hole by adjusting the size of the hole.

The stationary furnace may comprise a top entrant submerged lance furnace. The size of the hole in the clay or mud through which the slag flows may be adjusted by widening the opening or by removing solidified slag from the periphery of the opening. In one embodiment the size of the hole in the clay or mud through, which the slag flows may be adjusted by using a jack hammer or a pick tool to widen the opening or to remove solidified slag from the opening. The tap opening in the furnace may comprise a generally rectangular opening. The generally rectangular opening may have rounded corners.

In some embodiments, the size of the tap hole is adjusted by expanding a horizontal width of the tap hole in the clay or mud.

In some embodiments, the method may further comprise the step of monitoring the flow of slag from the furnace and, if the flow of slag is too low, widening the opening, and if the slag flow is too high, allowing slag to build up around the opening to thereby narrow the opening or packing further mud or clay into the opening to thereby narrow the opening. The flow of slag from the furnace may be monitored by visual inspection carried out by an operator of the furnace. Alternatively, the flow of slag from the furnace may be monitored using an automated flow rate monitoring means. In some embodiments of the present invention, the size of the hole through which the slag flows may be enlarged by using a jack hammer or pick tool. Suitably, the jack hammer or pick tool may be operated by an operator located remotely from the jack hammer or pick tool. This enables the operator to be positioned away from the hot and often dangerous environment near the immediate vicinity of the furnace.

The jack hammer or pick tool may be arranged so that it can be moved along a first axis that extends along the longitudinal axis of the jack hammer or pick tool so that the jack hammer or pick tool can move towards and away from the hole. The jack hammer or pick tool may also be arranged so that it can be moved in a lateral direction so that it can move horizontally relative to the hole. There may also a small amount of vertical movement as well, referred to as luff. It will be understood that arranging the jack hammer or pick tool to move in this fashion simplifies movement and mounting of the jack hammer or pick tool. The jack hammer or pick tool may also be arranged so that it can swivel horizontally relative to the hole (this movement is also referred to as yaw). There may also be a small amount of vertical swivel movement as well (this movement is also referred to as pitch).

It is a feature of the present invention that the hole that is drilled through the clay or mud has a diameter that is significantly smaller than the size of the tap opening in the furnace. For example, the diameter of the tap hole drilled through the clay or mud may have a diameter that is less than 50% of the width of the tap opening in the furnace, even more suitably less than 40% of the width of the tap opening in the furnace, suitably less than 30% of the width of the tap opening in the furnace.

By providing a tap opening in the furnace that has a significantly larger opening size than the size of the tap hole drilled through the clay or mud, there remains significant scope for adjusting the flow rate of the slag being removed through the tap hole by removing further clay or mud that is located between the inner wall of the tap hole drilled through the clay or mud and the inner wall of the tap opening.

As mentioned above, in one embodiment, the tap opening comprises a generally rectangular opening. However, other shaped tap openings may also be used. For example, the tap opening may comprise a circular opening, an oval opening, a square opening, a triangular opening, or indeed any other shaped opening. The opening may have rounded corners.

In one embodiment of the method of the present invention, once the molten charge in the furnace has been established and slag has commenced accumulating inside the furnace, a tap hole is drilled through the clay or mud that packs the tap opening. The tap hole may be drilled using any known tap hole drilling equipment. There are numerous suppliers of such drill equipment and the person skilled in the art will readily understand how this drill equipment is constructed and operated. Therefore, it will not be necessary to describe the drill equipment further.

Once the tap hole has been drilled in the clay or mud, an operator of the furnace can visually monitor the flow rate of slag from the furnace. If the operator determines that the flow rate of slag from the furnace is too low, the operator may then operate further equipment (such as a jack hammer or a pick tool) to remove further clay or mud from the periphery of the tap hole. This acts to enlarge the size of the tap hole, thereby allowing an increase in the flow rate of slag through the tap hole to take place. In some embodiments, the size of the tap hole is enlarged by increasing a horizontal width of the tap hole. If the horizontal width of the tap hole is increased, the height of the tap hole in the furnace will remain essentially constant, which is important in ensuring that slag is removed through the tap hole rather than removing molten metal through the tap hole.

In some embodiments, the slag may be removed from the furnace in an essentially continuous manner.

In some embodiments, the tap opening has a height that is significantly larger than the diameter of the tap hole that is drilled through the clay or mud. In these embodiments, the height of the tap hole can be adjusted by simply drilling the tap hole through the clay or mud at a higher level or a lower level, depending upon operational requirements.

In a second aspect, the present invention provides a top entrance submerged lance furnace comprising a furnace vessel lined with refractory material, characterised in that a side wall of the furnace vessel includes a generally rectangular tap opening.

In one embodiment, the generally rectangular tap opening has a width that is significantly larger than a diameter of a drill used to drill a tap hole through clay or mud that is packed into the tap opening. For example, the tap opening may have a width that is from 2 to 10 times larger than the diameter of the drill, more suitably from 2 to 5 times larger than the diameter of the drill, even more suitably from 3 to 5 times larger than the diameter of the drill.

The generally rectangular tap opening may have rounded corners.

The generally rectangular tap opening may have a height that it is significantly larger than the diameter of the drill that is used to drill a tap hole through clay or mud packed into the tap opening. For example, the tap opening may have a height that is from 2 to 15 times larger than the diameter of the drill, more suitably from 2 to 10 times larger than the diameter of the drill, even more suitably from 3 to 5 times larger than the diameter of the drill.

In a third aspect, the present invention provides a top entrance submerged lance furnace comprising a furnace vessel lined with refractory material, characterised in that a side wall of the furnace vessel includes a tap opening having a width substantially greater than a diameter of a hole drilled through mud or clay packed in the tap opening, the hole being used to remove slag from the furnace.

In one embodiment, the tap opening has a width that is significantly larger than a diameter of a drill used to drill a tap hole through clay or mud that is packed into the tap opening. For example, the tap opening may have a width that is from 2 to 10 times larger than the diameter of the drill, more suitably from 2 to 6 times larger than the diameter of the drill, yet more suitably 2 to 5 times larger than the diameter of the drill, even more suitably from 3 to 5 times larger than the diameter of the drill.

Further description of preferred embodiments of the present invention will now be provided with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a side view of a top entrant submerged lance furnace vessel in accordance with an embodiment of the present invention;

Figure 2 shows a front view of a tapping block assembly used in the furnace vessel shown in figure l ; Figure 3 shows a cross sectional plan view of the tapping block assembly shown in figure 2 taken along section line A in Figure 2;

Figure 4 shows a cross sectional side view of the tapping block assembly shown in figure 2 taken along section line B in Figure 2;

Figure 5 shows a front view showing the positioning of a tap hole drill relative to the furnace vessel;

Figure 6 shows a front view showing the positioning of a jack hammer relative to the furnace vessel; and

Figures 7 to 10 show various views of possible tap hole openings used in embodiments of the present invention. DETAILED DESCRIPTION OF THE DRAWINGS

It will be understood that the drawings have been provided for the purposes of illustrating preferred embodiments of the present invention. Therefore, it will be appreciated that the present invention should not be considered to be limited solely to the features as shown in the drawings.

Figure 1 shows a side view of a top entrant submerged lance furnace in accordance with an embodiment of the present invention. The furnace shown in figure 1 may be an ISASMELT™ furnace. Such furnaces are stationary furnaces.

The furnace 10 shown in figure 1 comprises an outer shell, generally referred to by reference numeral 14. The outer shell will typically be constructed from steel or a steel alloy. As will be well understood by persons skilled in the art, the outer shell will normally be lined with refractory material to protect the outer shell from the intense heat experienced inside the furnace during operation. . . .

The furnace 10 has a side wall 16. The upper part 18 of the furnace includes an expanded region 20 that provides an outlet for gaseous products of the smelting reaction. During use of the furnace, a charge of feed materials is added. Fuel and air or oxygen are injected via a top entrant submerged lance. Smelting reactions then take place to form a molten metal or matte layer and a slag layer. Due to the extreme turbulence induced by injection of air or oxygen through the lance, the molten material in the furnace is vigorously agitated. As smelting continues, the amount of slag in the furnace will build up and it will become necessary to remove slag from the furnace. In order to enable tapping or removal of slag from the furnace during operation, the furnace is provided with a tap opening 28 in a side wall thereof. Tap opening 28 is positioned in the sidewall at a height that is above the design height for the slag/metal or slag matte interface inside the furnace. In this manner, predominantly slag will be removed from the furnace via tap. opening 28. Metal or matte is removed elsewhere from the furnace via a separate tapping operation. Figures 2, 3 and 4 showed further detail of the tap opening 28.

As shown in Figure 3 the tap opening 28 is formed in a tapping block assembly 30, which includes a tapping block 44, made from water-cooled copper.

Referring again to figure 3, the copper tapping block (44) is fastened to the furnace shell 48.

The outermost face of tap opening 28 is protected by an outer plate 68. The actual shape of the opening formed in outer plate 68 is best shown in figure 2. The furnace is lined with layers of refractory material 64 and 32. The tapping hole passes through layers 64 and 32 also. Therefore, opening 28, 60 and 66 define an opening from the outside of the tapping block to the interior of the furnace. As can best be seen from figure 2, the tap opening 28 is a generally rectangular opening.

During the operation of the furnace, the opening defined by respective opening 28, 60 and 66 is initially packed with clay or mud in order to seal the opening. The person skilled in the art will readily appreciate that there are a number of commercially available clay or mud compositions that can be purchased for this purpose. Furthermore, there are a number of commercially available clay or mud guns that can be used to apply or pack the clay or mud into the opening to close the opening. Therefore, it is not necessary to discuss these features any further.

In comparison to known tap openings conventionally used for tapping slag from stationary furnaces, the tap opening 28 shown in figures 2, 3 and 4 has a width that it is substantially larger than the diameter of the tapping drill that is used to drill out the clay or mud that is used to pack and close the tap opening. For example, the generally rectangular tap opening 28 may have a width that is about 2 to 6 times wider than the diameter of the tapping drill. Figure 5 shows a front schematic view of a tapping drill used to tap into the tap opening. In figure 5, the tap hole drill 80 is mounted to a secondary frame 82 that, in turn, is mounted to wheels 84, 86. Wheels 84, 86 enable the secondary frame 82 (and hence the tap hole drill 80) to move to the left or to the right. The wheels 84, 86 themselves move on a further frame 88. Further frame 88 may be considered to be a primary frame. Primary frame 88 has wheels 90, 92 mounted thereto. Wheels 90, 92 are mounted for rotation on rails 94, 96 so that the primary frame can move towards and away from the furnace.

When it is desired to tap molten furnace products from the furnace, the tap hole drill 80 is moved so that the drill bit comes into alignment with the tap opening 28 (tap opening 28 is obscured by the drill in figure 5). The drill is activated and the bit is moved into contact with the clay or mud that packs the tap hole opening 28. Lateral adjustment of the position of the tap to the desired position is achieved by moving the secondary frame 82. The depth of the tap hole being drilled in ^"the tap opening is increased by moving the primary frame closer to the furnace as the drill operates. From the above description, it^'will be understood that the tap hole drill can move on both laterally and longitudinally. A launder 98 is positioned so that when the tap hole drill opens the tap hole, molten furnace products run out of the tap hole and down the launder 98.

As mentioned above, the tap hole opening 28 shown in Figures 1 to 4 has a width that is substantially wider than the diameter of the drill bit used to drill the tap hole into the clay or mud that packs the tap hole opening. If it is desired to increase the flow rate of molten furnace products through the tap opening, a jack hammer or a pick tool may be used to increase the size of the tap hole. Figure 6 shows a jack hammer 1 10 that is mounted to a secondary frame 1 12- and a primary frame 1 14 in a manner that is similar to the mounting of secondary frame 82 and primary frame 88 shown in figure 5. This enables the jack hammer 1 10 to move both sideways (in a lateral direction) and longitudinally (in a direction towards and away from the furnace). Therefore, the jack hammer can move in essentially straight lines in two mutually perpendicular directions. Figures 7 to 10 show some possible variations in the shape of the tap hole formed in the tap ) opening. In figure 7, the tap opening 28 is shown. The tap opening is packed with mud or clay 120. A circular tap hole 122 has been freshly drilled in the clay or mud 120 using the tap hole drill. The circular tap hole 122 enables molten furnace products to flow out of the furnace. If the flow rate of molten furnace products out of the furnace through the generally circular tap hole 122 is not sufficiently large, the jack hammer 1 10 may be used to increase the size of. the opening. For example, as shown in figure 8, the generally circular taphole 122 may be widened by extending the sides 124, 126 by use of the jack hammer. This, of course, increases in the area of the opening, thereby enabling a higher flow rate of molten furnace products from the furnace. If the flow rate achieved through the opening shown in figure 8 is still not sufficiently large, the tap hole may be widened even further by further digging away the edges of the tap opening using the jack hammer. This is shown in figure 9, where the side edges 128, 130 of the tap opening have been even further widened, when compared to figure 8.

If the flow rate of molten furnace products becomes too high, it is a simple matter to allow some of the molten furnace products that are flowing through the taphole to solidify on the edges of the tap hole to thereby reduce the size of the taphole. This is shown in figure 10, where the taphole of figure 9 has been allowed to close up to the taphole 132 as shown in figure 10 by allowing solidification of slag on the edges of the taphole.

It will also be appreciated that the taphole 132 shown in figure 10 can be opened up again using the jack hammer to remove some of the solidified slag from the tap opening. The ability to selectively modify the size of the tap hole to vary the flow rate of molten furnace products (such as slag) through the tap hole is a feature that is believed to be unique to present invention. This feature is possible because the size of the tap opening is significantly larger than the size of the tap hole drilled through the clay or mud that packs the tap opening. Prior art tap openings were generally circular and had a diameter that was very similar to the diameter of the drill bit used to drill out the tap hole. Therefore, prior art tap openings did not allow for the possibility of increasing the flow rate beyond the maximum flow rate provided by the size of tap hole formed by the drill bit. In contrast, in the present invention, once the tap hole has been drilled, there exists a significant extent of clay or mud between the wall of the tap hole and the sides of the tap opening. This allows removal of some of this clay or mud between the wall of the tap hole and the sides of the tap opening to thereby increase the size of the tap opening. As a further feature of the present invention, the height or vertical extent of the tap opening is also significantly larger than the diameter of the drill bit used to drill out the tap hole. This enables the Vertical position of the tap hole to be varied so that the tap hole is always positioned correctly relative to the contents of the furnace. For example, if the tap hole is being used for slag tapping, the tap hole should be originally drilled through the clay or mud packing the tap opening at a height that it is above the interface between the slag layer and the metal layer.

The present invention allows the flow rate of molten furnace products through the tap hole to be varied by controlling and modifying (as required) the size of the tap hole. Conventional tools can be used to implement the present invention. The invention provides greater control over the flow rate of molten furnace products through the tap opening. The flow rate can be controlled either automatically or by operator monitoring and intervention.

The present invention is particularly suitable for use in operations where the liquid furnace product that is removed from the furnace is used as a feed material for a further downstream process. This ¹ is desirable in that it allows for the possibility of being able to avoid intermediate storage vessels or to minimise the size required for any intermediate storage vessels that would otherwise be required to store the feed material for the downstream processing vessel in cases where the flow rate of that material from the top entrant submerged lance furnace is intermittent or of variable flow rate.

The present invention minimises the footprint caster section of the furnace, and is ideal for retrofitting on existing furnaces, or implementation on new furnaces.

Those skilled in the art will appreciate that the present invention may be susceptible to variations and modifications other than those specifically described. It will be understood that the present invention encompasses all such variations and modifications that fall within its spirit and scope.

Claims

CLAIMS.

1. A method for tapping slag from a stationary furnace comprising:

- providing a stationar furnace having a tap opening for removing slag therefrom;

- packing said tap opening with clay or mud;

2. A method as claimed in claim 1 wherein the size of the hole in the clay or mud through which the slag flows is adjusted by widening the opening or by removing solidified slag from the periphery of the opening.

3. A method as claimed in claim 1 or claim 2 wherein the tap opening in the furnace comprise a generally rectangular opening, optionally having rounded corners.

4. A method as claimed in any one of the preceding claims wherein the size of the tap hole is adjusted by expanding a horizontal width of the tap hole in the clay or mud.

5. A method as claimed in any one of the preceding claims further comprising monitoring flow of slag from the furnace and, if the flow of slag is too low, widening the opening, and if the flow of slag is too high, allowing slag to build up around the opening to thereby narrow the opening or packing further mud or clay into the opening to thereby narrow the opening.

6. A method as claimed in claim 5 wherein the flow of slag from the furnace is monitored by visual inspection carried out by an operator of the furnace or by using an automated flow rate monitoring means.

7. A method as claimed in any one of the preceding claims wherein the size of the hole through which the slag flows is enlarged by using a jack hammer or pick tool operated by an operator located remotely from the jack hammer or pick tool.

8. A method as claimed in claim 7 wherein the jack hammer or pick tool is arranged so. that the jack hammer or pick tool is movable along a first axis that extends along the longitudinal axis of the jack hammer or pick tool so that the jack hammer or pick tool is movable towards and away from the hole.

9. A method as claimed in claim 7 Or claim 8 wherein the jack hammer or pick tool is arranged so that the jack hammer or pick tool is movable in a lateral direction so that the jack hammer or pick tool is movable horizontally relative to the hole.

10. A method as claimed in any one of claims 7 to 9 wherein the jack hammer or pick tool is arranged so that it swivels horizontally relative to the hole (yaw), and optionally the jack hammer or pick tool also hasa small amount of vertical swivel movement (pitch).

1 1. A method as claimed in any one of the preceding claims wherein the diameter of the tap hole drilled through the clay or mud has a diameter that is less than 50% of the width of the tap opening in the furnace, preferably less than 40% of the width of the . tap opening in the furnace, more preferably less than 30% of the width of the tap opening in the furnace.

12. A method as claimed in any one of the preceding claims wherein the size of the tap hole is enlarged by increasing a horizontal width of the tap hole whilst keeping the height of the tap hole in the furnace essentially constant.

13. A method as claimed in any one of the preceding claims wherein the slag is removed from the furnace in an essentially continuous manner.

14. A method as claimed in any one of the preceding claims wherein the tap opening has a height that is significantly larger than the diameter of the tap hole that is drilled through the clay or mud.

15. A top entrance submerged lance furnace comprising a furnace vessel lined with refractory material, characterised in that a side wall of the furnace vessel includes a generally rectangular tap opening.

16. A furnace as claimed in claim 15 wherein the generally rectangular tap opening has a width that is significantly larger than a diameter of a drill used to drill a tap hole through clay or mud that is packed into the tap opening.

17. A furnace as claimed in claim 16 wherein the tap opening has a width that is from 2 to 10 times larger than the diameter of the drill, more suitably from 2 to 5 times larger than the diameter of the drill, even more suitably from 3 to 5 times larger than the diameter of the drill.

18. A furnace as claimed in any one of claims 15 to 17 wherein the generally rectangular tap opening has rounded corners.

19. A furnace as claimed in any one of claims 15 to 18 wherein the generally rectangular tap opening has a height that it is significantly larger than the diameter of the drill that is used to drill a tap hole through clay or mud packed into the tap opening.

20. A furnace as claimed in claim 19 wherein the tap opening has a height that is from 2 to 15 times larger than, the diameter of the drill, more suitably from 2 to 10 times larger than the diameter 6f the drill, even more suitably from 3 to 5 times larger than the diameter of the drill.

21. A top entrance submerged lance furnace comprising a furnace vessel lined with refractory material, characterised in that a side wall of the furnace vessel includes a tap opening having a width substantially greater than a diameter of a hole drilled through mud or clay packed in the tap opening, the hole being used to remove slag from the furnace.

22. A furnace as claimed in claim 20 wherein the tap opening has a width that is from 2 to 10 times larger than the diameter of the drill, more suitably from 2 to 6 times larger than the diameter of the drill, yet more suitably 2 to 5 times larger than the diameter of the drill, even more suitably / from 3 to 5 times larger than the diameter of the drill.