I will be sharing knowledge on most important aspect of Mining Engineering, which is Drilling and blasting.
Actually, I always considered this as the most important aspect because as a Mining Engineer this is our tool, this is the only aspect that other Engineers can never share with us as a Miner.
This aspect is wide and extend to various other Fields.
Some of which are:
1.Petroleum in terms of casing perforation, Exploration (seismic survey) among other applications
2. Civil engineering works in the aspect of road construction ( road obstacle rock fragmentation), subsurface liquidification/compaction to mention few.
3. Borehole drilling: most time during Borehole drilling bed rock encountered are fragmented by blasting.
This is just few out of the application of drilling and blasting in other Fields.
What is Drilling?
Drilling is the creation of artificial hole into rock by man controlled or automated machines without the admittance of man in the hole. Most mining Engineers define drilling from their field perspective as: "Creation of artificial cavity into insitu rock outcrop or Boulders mainly for the placement of explosive and other blasting agents". This definition as I had earlier said is from the perspective of a Mining Engineer, because generally speaking drilling is not only carried out for blasting operation only. Drilling can be done for other purposes like; Tunnel boring using TBM, shaft boring using SBM, Raise boring using RBM, drilling can be done for mechanical purposes(riveting, bolting among others), drilling can be done for underground wall and roof support purposes,to mention few.
There are basically four method of drilling but not limited to these four.
1. Rotary drilling
2. Percussive drilling
3. Rotary percussive drilling
4. Thermal drilling.
We have other special drilling methods like seismic, chemical drilling methods
Drilling for Blasting is base on the rock drillability.
This is the ability of the rock to allow the ease penetration of the bit during Drilling. This drillability is a function of the rock strength, grain size, grain arrangement, rock type. Drilling operation by rotary method involve the advancement of a rotating bit mounted on a string into the rock. The rotating bit cut the rock grains and crush it as the bit advance. In percussive drilling, the drilling principle is base on hammering of the bit by a heavy load.This method is use for soft rock drilling, it slow and energy consuming. This method was earlier uses to drill the first oil cat well and is also employed by the old men in drilling water well in sedimentary environment for water. The combination of the principle of both percussive and rotary drilling techniques is know as rotary percussive Drilling method. This method tend to be more efficient, as the drill bit rotate down the hole, the hammer impact compressive load on it.
This will takes me to explain the principle behind rock drilling.
How is Drilling achieve in a rock?
Before I explain this, I will like to mention some type of drilling machines and also mention some of the most important part of a drilling machine. Examples of a Drilling machine are; wagon drilling machine, jumbo (underground), jack hammer among other machines.
The important part of a Drilling machine are;
1. Drill: This supply energy to perform the drilling (called energy supplier or source)
2. Drill string/pipe: usually hollow pipes made of high strength metal. It perform three main function, 1. Transmit the energy from the rotary table to the drill bit, 2. Transport drilling fluid to the hole bottom, 3. Help in advancing the drill bit as the drilling depth increases
3. Drill bit: This is the energy applicator, this ultilize the energy transmitted by the drill string against the stability and the initia of rock grains to bore hole of define circumference in the rock in contact. This so called bit is design such that it has all the strength, shape, structure and other properties required to download negative energy into the rock grains and weaken it for hole formation. Bit is usually of different type base on cost, material use, shape & orientation e.t.c.Some examples of drill bit are the; cone bit, drag bit, to mention few.
The mechanism behind drilling is simply explained in three steps:
1. The drill bit rotate and impact energy on the rock matrix,
2. The rock grains matrix loss strength and become weaken as soon as the impacted energy is greater than their grain internal resistance strength.
3. As soon as the rotating/impacting energy of the bit increases to the rock breaking point, Micro fractures develop within the grain matrix structure either intergranularly or intraganularly.
This energy level at which the rock grains fractures and loss stability in mineral processing is called the *work index*. Remember, I only mentioned three important part of the drilling machine, there are many other parts like, rotary table, collar, control board, air hose, among others. In rotary percussive drilling machine the drill system can have it hammer mounted in two ways:
1. Surface mounted
2. Down the hole DTH hammer
Surface mounted, the hammer is located above the drill rod. The percussion load is transmit through the drill string to the bit. The main disadvantage of this is that, most of the energy supplied by the hammer are been lost during transmission, only less amount is delivered to the drill bit to do work. The energy can be lost by vibration, heat, among other ways. For DTH, the hammer is place directly above the drill bit the energy is supply to the bit directly as the hammer hit the bit. DTH is also called ITH (in the hole).
Before drilling operation is carried out, drilling is planned. The structure and the arrangement of the hole is design. the hole parameters design are; burden, spacing, hole depth, drill pattern to mention few.This design is very important, as it affects the blasting output. Poor drill design result into production of oversize, and less optimum size. For the seek of year one students who will be resing this article, I will give definition to some terms now.
Note most of my definition are from my practical view, they tend to make you understand what I am talking about clearly.
Note them all, you may not find such in any textbook.
Some important definition use in drilling And blasting
I will give some on Drilling now, and for the blasting lecture I will give that related to Blasting
1. Burden: this is the distance between two rows of hole. It also define technically as the distance between the drill hole row to the free face.
2. Spacing is the distance between two adjacent holes in the same row
3. Stemming length: the length of the drill hole filled with cutting materials to confine explosive charge in a hole.
4. Subdrill: the extra hole length drill beyond the bench level to provide base for the bench floor
5. Bench height: is the maximum drill height per blasting. Is also the vertical distance between bench Crest and toe
6. Drill pattern: is the arrangement of holes on the bench floor.
7. Free face: this is the open surface that form the pit or mine face.
8. Drill cuttings: this is the rock particule cut by the bit as it advance into the rock.
9. Drill fluid: This is liquid, gas or mud use during Drilling to ease penetration, lubricate drill system and also remove cutting among other functions like hole wall stabilization.
There are other terms, but with this you have the required basic knowledge needed to know about Drilling.
Drilling pattern use depend on the purpose of drilling.
Drill done for developmental purpose are carried out to:
1. Collect sample
2. Reserve estimation
3. Grade determination
4. Exploration generally.
Has different pattern compared to that carried out for blasting purpose. Developmental Drilling pattern can be systematic, regular, irregular pattern among others. But for Drilling carried out for blasting the pattern can be:
1. Square pattern: the pattern in which the burden and spacing is the same.
2. Rectangular pattern: in this pattern, the spacing is greater than the burden by 1.45-1.55 depending on the rock strength.
3. Staggered pattern: in this the spacing is greater than the burden by 1.1-1.25
The type of drilling pattern use depend on the rock strength and the rock blastability. The drill parameters affect the blast output so much as the fragmentation depend vividly on this parameters.As we all know that, the main aim of Mining is to make profit. The downstream operations like crushing, grinding, separation/screening depend on the fragmentation of the rock. Poor fragmentation apart from affecting g downstream operation, it also affect loading operation and haulage operation/haulage cycle also know as fleet design. Fleet design is mainly on the modification of haulage operation in the mine so as to achieve maximum efficiency, also to avoid haulage and loading equipment under usage and over usage. But before I start the next lecture, I will talk on how to design a Drilling operation for specific hole diameter.
As a Mining Engineer, knowing this design alone is a tool for you, during job interview as a blasting Engineer. I will also, Mention some software you can learn on which are use for Drilling and blasting optimization.
I will talk on Kuznetsov, rammler and Cunningham fragmentation model both modified and original model.
more in the next post
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