Surface Mining (The Concept of Surface Mining) Mnge 315 ©Dr. B. C. Paul Spring 2003 revisions 2010, 2011 These slides contain concepts considered by the Author to be “common knowledge” to those schooled in Surface Mining. Many of the ideas found in these slides can be found in any of numerous basic texts on Surface Mining

Profile of an Industry Basic Divisions of Mining Industry Basic Divisions of Mining Industry –Coal –Hardrock Metal Metal Industrial Minerals Industrial Minerals Quarries/ Sand and Gravel Quarries/ Sand and Gravel Deposit Geology influences how each division would apply surface mining Deposit Geology influences how each division would apply surface mining

Surface Mining Called Surface Mining because mine is open to the sky where mineral of interest is being excavated Called Surface Mining because mine is open to the sky where mineral of interest is being excavated Why is being open to sky important Why is being open to sky important –Safety (no stone ceiling above you) –Health (open exchange of fresh air) –Mechanization economics Fewer confined space limitations – bigger more productive equipment Fewer confined space limitations – bigger more productive equipment

Liabilities of Surface Mining Easily seen Easily seen –More likely target for Nimbys and Bannanas Not compatible with two things being on exactly the same land at exactly the same time Not compatible with two things being on exactly the same land at exactly the same time –Underground allows separate use of surface and mineral resources underground

Where Deposits are Found Some deposits just sitting up on the surface Some deposits just sitting up on the surface –Sand and Gravel –Placer Deposits (a metal in sand and gravel) Most of the time the target minerals are buried under something else Most of the time the target minerals are buried under something else

Ore and Overburden Ground or rock that contains enough of some mineral of value to pay for extraction is called “Ore” Ground or rock that contains enough of some mineral of value to pay for extraction is called “Ore” –Gets a little awkward with coal because coal miners consider “ore” to be a hardrock term They expect that when you call coal “coal” that you will already figure out that it is valuable They expect that when you call coal “coal” that you will already figure out that it is valuable The stuff that sits on top of ore and has to be moved to surface mine the ore is called “Overburden” The stuff that sits on top of ore and has to be moved to surface mine the ore is called “Overburden”

Good Stuff – Bad Stuff In order to mine a mineral of interest you must pick up rock and take it somewhere to process In order to mine a mineral of interest you must pick up rock and take it somewhere to process –Costs money to pick up and move rock –“Ore” has enough good mineral to be worth picking up and processing –“Overburden” is the stuff that is in the way and has to be moved to mine the ore

Good Stuff Identification Has a market where it can be sold Has a market where it can be sold Has to sell for more than mining and processing cost Has to sell for more than mining and processing cost Obvious case Obvious case –Coal Example – Costs $1.25 a ton to break up dig and deliver the coal Example – Costs $1.25 a ton to break up dig and deliver the coal Costs $2.50 a ton to “clean rock and sulfur” out of the coal Costs $2.50 a ton to “clean rock and sulfur” out of the coal Coal sells for $28/ton Coal sells for $28/ton $28 - $ $2.50 = $24.25 >0 Yup that looks like good stuff $28 - $ $2.50 = $24.25 >0 Yup that looks like good stuff –Limestone Cost $1.50 to break up dig and deliver limestone from a quarry to processing Cost $1.50 to break up dig and deliver limestone from a quarry to processing Cost $0.75 a ton to crush a screen it to marketable size Cost $0.75 a ton to crush a screen it to marketable size Crushed rock sells for $6.50 per ton Crushed rock sells for $6.50 per ton $ $ $0.75 = $4.25 >0 Yup that looks like good stuff $ $ $0.75 = $4.25 >0 Yup that looks like good stuff

Trickier Good Stuff Sometimes only a little of the rock is something valuable Sometimes only a little of the rock is something valuable –Copper few rocks are pure copper –Copper case is a rock with scattered grains of minerals that contain copper –Need to contain a certain amount of copper before the rock is worth while We measure with a “Break Even Cut-Off Grade” We measure with a “Break Even Cut-Off Grade” Minimum amount of a mineral or element that must be present for things to be worth while Minimum amount of a mineral or element that must be present for things to be worth while

A Break-Even Cut-Off Grade Suppose Copper will bring you $3 per lb after smelting, refining, and delivering expenses Suppose Copper will bring you $3 per lb after smelting, refining, and delivering expenses Suppose a ton of rock costs $2.50 to break-up load and deliver to the mill Suppose a ton of rock costs $2.50 to break-up load and deliver to the mill Suppose a ton of rock costs $4.00 to grind up and put through a concentrator (mill) Suppose a ton of rock costs $4.00 to grind up and put through a concentrator (mill) How much recoverable copper has to be in the rock for it to be “Good Stuff”? How much recoverable copper has to be in the rock for it to be “Good Stuff”?

A Calculation $3/lb*2000lbs/ton*BECOV = $2.50+$4.00 $3/lb*2000lbs/ton*BECOV = $2.50+$4.00 –Break Even Cut-Off Value (BECOV) is the fraction of the rock that is copper Quick Algebra Quick Algebra –$6.50/(3*2000) = BECOV = –Often we like to talk in % –Multiply by 100 to convert to parts per hundred % copper (about 0.11% BECOV) % copper (about 0.11% BECOV) But Wait – there is a catch! But Wait – there is a catch! –Do you think our processes get 100% of the copper in the rock?

A Look at Process Copper ore is mined using drilling and blasting, followed by truck and shovel loading and haulage. Copper ore is mined using drilling and blasting, followed by truck and shovel loading and haulage. The ore is taken to a concentrator where it is crushed and ground to powder and then put into flotation cells. The ore is taken to a concentrator where it is crushed and ground to powder and then put into flotation cells. –Flotation cell is a stirred “bubble bath” –Some minerals hate to get wet (sulfide copper and iron minerals) – they grab onto the air bubbles and form a dirty froth at the surface –Scrape off the froth – it’s a concentrate –Other minerals like quartz, feldspar and calcium carbonate (that have no copper in them – called gangue minerals) love the water and stay in the bath They get sucked out the bottom with the water They get sucked out the bottom with the water

What are the Chances of Perfect Recovery? What if a mineral particle is part gangue and part copper sulfide? What if a mineral particle is part gangue and part copper sulfide? –This is the reason for crushing and grinding – to minimize this problem by achieving “liberation” – copper minerals and gangue as separate particles. –Suppose we recover 90% of the copper Now our break even cut-off grade is /0.9 Now our break even cut-off grade is /0.9 Result or % Result or %

More Process Copper Concentrates are sent to a smelter Copper Concentrates are sent to a smelter –Copper is still chemically combined with sulfur –Here we will try to get rid of the sulfur –And suck iron up into a rock slag –Put the concentrates in a very hot furnace and burn off the sulfur to make sulfur dioxide –Iron and impurities go into “Flux” to make slag –Copper settles to the bottom to form “blister copper” – copper metal with some minor metal impurities Suppose smelter gets 92% of the copper Suppose smelter gets 92% of the copper – /.92 = or %

Last of a Process Refining of Blister Copper Refining of Blister Copper Blister Copper is placed in electrolytic cells Blister Copper is placed in electrolytic cells –Acid baths with electricity going through Blister Copper electrode dissolves away Blister Copper electrode dissolves away Pure copper plates out on the other electrode Pure copper plates out on the other electrode –Trace metal impurities settle as “slimes” on the bottom Suppose this process is 99% efficient Suppose this process is 99% efficient /.99 = /.99 = The Break Even Cut-Off Grade must be at least % copper in place for the rock to be “good stuff” or ore. The Break Even Cut-Off Grade must be at least % copper in place for the rock to be “good stuff” or ore.

Economic Considerations When a block or area of mineralization exists that can be mined at a profit the ore can be called “Ore” When a block or area of mineralization exists that can be mined at a profit the ore can be called “Ore” –Ore can be picked up and processed for a profit –What about the overburden Have to loose money to get it out of the way Have to loose money to get it out of the way Whether an ore deposit can be considered a reserve depends on whether it makes enough extra to pay for getting the “overburden” out of the way Whether an ore deposit can be considered a reserve depends on whether it makes enough extra to pay for getting the “overburden” out of the way

Stripping Ratio Becomes important to know how much bad stuff has to be moved to get to the good stuff Becomes important to know how much bad stuff has to be moved to get to the good stuff Amount of Bad Stuff that must be moved to get to the good stuff is called the “stripping ratio” Amount of Bad Stuff that must be moved to get to the good stuff is called the “stripping ratio” –For Coal we ask how many cubic yards of overburden must be moved to get to one ton of coal –In metal mining it is usually tons of overburden per ton of ore or cubic yards of overburden per cubic yard of ore Note that for metal a stripping ratio comes up unit-less Note that for metal a stripping ratio comes up unit-less

Stripping Ratio Impact While Ore is worth picking up to process the amount of overburden that must be moved will determine whether the deposit is profitable While Ore is worth picking up to process the amount of overburden that must be moved will determine whether the deposit is profitable –The stripping ratio is key in determining whether ore becomes a “reserve” If too much overburden is in the way If too much overburden is in the way –You can’t mine the ore –You consider tunneling around under the overburden and mine the ore without moving the overburden That is underground mining That is underground mining We’re too cool in a Surface Mining Course to talk about the evil competition We’re too cool in a Surface Mining Course to talk about the evil competition

Bad Stuff Liabilities Overburden has to go somewhere Overburden has to go somewhere –The evil word “waste” Just plain rock may become a major regulatory target Just plain rock may become a major regulatory target The Mineral of Interest Problem The Mineral of Interest Problem –Society tends to use pure concentrations of substances – nature seldom makes pure concentrations –Even ore often leaves substantial other stuff behind Sometimes “tailings” are not entirely just a natural rock Sometimes “tailings” are not entirely just a natural rock

Our Challenge Use the advantages of open surface assess to safely and economically produce minerals our society needs Use the advantages of open surface assess to safely and economically produce minerals our society needs Control and manage any of our disturbances into a form that can be integrated back into a sustainable environment Control and manage any of our disturbances into a form that can be integrated back into a sustainable environment

Some Basic Stripping Ratio Calculations I have 1800 cubic yards of overburden and 600 cubic yards of ore I have 1800 cubic yards of overburden and 600 cubic yards of ore –1800/600 = 3 (Stripping ratio is 3:1) –This is called a volumetric stripping ratio because it is based on volume We can often get a volumetric stripping ratio from the maps or geometry of the ore body We can often get a volumetric stripping ratio from the maps or geometry of the ore body May need to consider density of materials May need to consider density of materials –Suppose my overburden is 4000 lbs/cubic yard –Suppose my ore is 4500 lbs/cubic yard –We can adjust a volumetric stripping ratio to a weight based stripping ratio (more common in metal mines)

Converting to a Weight Based Stripping Ratio 1800 cubic yards OB * 4000 lbs/cubic yard =7,200,000 lbs 1800 cubic yards OB * 4000 lbs/cubic yard =7,200,000 lbs –Since we usually work in tons –7,200,000 lbs/ 2000 lbs/ton = 3,600 tons 600 cubic yards of Ore * 4500 lbs/cubic yard / 2000 lbs/ton = 1350 tons 600 cubic yards of Ore * 4500 lbs/cubic yard / 2000 lbs/ton = 1350 tons Weight Based Stripping Ratio is Weight Based Stripping Ratio is –3,600 / 1350 = 2.67 to 1

Looking at an Economic Limit Suppose we have a ton of copper ore with 0.5% copper Suppose we have a ton of copper ore with 0.5% copper –Suppose copper sells for 3.30/lb To make copper we have to To make copper we have to –Mine the ore –Crush it and put it through a concentrator to get just the copper minerals –Smelt the copper minerals to get out just the copper –Refine and purify the copper – then we can sell it

More on Economic Limits Suppose it costs 10 cents a lb to refine copper Suppose it costs 10 cents a lb to refine copper Suppose it costs $85/ton to smelt concentrates Suppose it costs $85/ton to smelt concentrates Unrefined copper then is worth $ $0.10 = $3.20 Unrefined copper then is worth $ $0.10 = $3.20 –Adjusting for 99% recovery –$3.20*.99 = $3.17 A typical Cu concentrate is about 30% copper A typical Cu concentrate is about 30% copper –A ton of concentrate contains 2000 * 0.3 = 600 lbs of Cu 2000 * 0.3 = 600 lbs of Cu –$85/ton to smelt concentrates costs 85/600 = or 14.2 cents/lb 85/600 = or 14.2 cents/lb Adjust for 92% recovery – 14.2 cents/lb/0.92 = cents/lb recoverable Adjust for 92% recovery – 14.2 cents/lb/0.92 = cents/lb recoverable –Copper in concentrates is thus worth $3.17 – = $3.016 per lb $3.17 – = $3.016 per lb

Continuing Our Copper Economics Suppose the Concentrator costs $3.00 per ton to process ore Suppose the Concentrator costs $3.00 per ton to process ore Suppose the mining costs $2.25 per ton to break up, pick up and deliver the ore Suppose the mining costs $2.25 per ton to break up, pick up and deliver the ore Our 1 ton of ore will cost $5.25 to get ready for the smelter Our 1 ton of ore will cost $5.25 to get ready for the smelter Our ton of ore contains 0.5% copper Our ton of ore contains 0.5% copper –2000 lbs * = 10 lbs of copper

Can we afford to move that ore? 10 lbs of copper in concentrates is worth 10 lbs of copper in concentrates is worth –10 * = $30.16 –But only recover 90% in concentrator $30.16*0.9 = $27.14 $30.16*0.9 = $27.14 To get that ore out of the ground and into concentrate will cost $5.25 To get that ore out of the ground and into concentrate will cost $5.25 $ $5.25 = $21.98 $ $5.25 = $21.98 –Oh Yes its worth while to dig that ore up and process it!!

But What if It Has Overburden Above it – Can I move that? Check the Break Even Limit Check the Break Even Limit –The ore is worth $21.98 Suppose it will cost me $2.50/ton to break up, pick up, and dump overburden material Suppose it will cost me $2.50/ton to break up, pick up, and dump overburden material –$21.98/$2.50 = about 8.76 tons of overburden can be moved to get that ton of ore –This is called a break-even stripping ratio

Now Its Your Turn Do Homework #1 Do Homework #1 –The problem has you calculate stripping ratios and break even limits for a copper mine.