Surface MiningSurface Mining(The Concept of Surface Mining)(The Concept of Surface Mining)

Mnge 315Mnge 315

©Dr. B. C. Paul Spring 2003 revisions 2010, 2011©Dr. B. C. Paul Spring 2003 revisions 2010, 2011

These slides contain concepts considered by the Author to be These slides contain concepts considered by the Author to be “common knowledge” to those schooled in Surface Mining. Many of “common knowledge” to those schooled in Surface Mining. Many of

the ideas found in these slides can be found in any of numerous the ideas found in these slides can be found in any of numerous basic texts on Surface Miningbasic texts on Surface Mining

Profile of an IndustryProfile of an Industry

• Basic Divisions of Mining IndustryBasic Divisions of Mining Industry– CoalCoal– HardrockHardrock

•MetalMetal

• Industrial MineralsIndustrial Minerals

•Quarries/ Sand and GravelQuarries/ Sand and Gravel

• Deposit Geology influences how each Deposit Geology influences how each division would apply surface miningdivision would apply surface mining

Surface MiningSurface Mining

• Called Surface Mining because mine Called Surface Mining because mine is open to the sky where mineral of is open to the sky where mineral of interest is being excavatedinterest is being excavated

• Why is being open to sky importantWhy is being open to sky important– Safety (no stone ceiling above you)Safety (no stone ceiling above you)– Health (open exchange of fresh air)Health (open exchange of fresh air)– Mechanization economicsMechanization economics

•Fewer confined space limitations – bigger Fewer confined space limitations – bigger more productive equipmentmore productive equipment

Liabilities of Surface MiningLiabilities of Surface Mining

• Easily seenEasily seen– More likely target for Nimbys and More likely target for Nimbys and

BannanasBannanas

• Not compatible with two things being Not compatible with two things being on exactly the same land at exactly the on exactly the same land at exactly the same timesame time– Underground allows separate use of Underground allows separate use of

surface and mineral resources surface and mineral resources undergroundunderground

Where Deposits are FoundWhere Deposits are Found

• Some deposits just sitting up on the Some deposits just sitting up on the surfacesurface– Sand and GravelSand and Gravel– Placer Deposits (a metal in sand and Placer Deposits (a metal in sand and

gravel)gravel)

• Most of the time the target minerals Most of the time the target minerals are buried under something elseare buried under something else

Ore and OverburdenOre and Overburden

• Ground or rock that contains enough of Ground or rock that contains enough of some mineral of value to pay for some mineral of value to pay for extraction is called “Ore”extraction is called “Ore”– Gets a little awkward with coal because coal Gets a little awkward with coal because coal

miners consider “ore” to be a hardrock termminers consider “ore” to be a hardrock term•They expect that when you call coal “coal” that They expect that when you call coal “coal” that

you will already figure out that it is valuableyou will already figure out that it is valuable

• The stuff that sits on top of ore and has The stuff that sits on top of ore and has to be moved to surface mine the ore is to be moved to surface mine the ore is called “Overburden”called “Overburden”

Good Stuff – Bad StuffGood Stuff – Bad Stuff

• In order to mine a mineral of interest In order to mine a mineral of interest you must pick up rock and take it you must pick up rock and take it somewhere to processsomewhere to process– Costs money to pick up and move rockCosts money to pick up and move rock– ““Ore” has enough good mineral to be Ore” has enough good mineral to be

worth picking up and processingworth picking up and processing– ““Overburden” is the stuff that is in the Overburden” is the stuff that is in the

way and has to be moved to mine the oreway and has to be moved to mine the ore

Good Stuff IdentificationGood Stuff Identification

• Has a market where it can be soldHas a market where it can be sold• Has to sell for more than mining and processing costHas to sell for more than mining and processing cost• Obvious caseObvious case

– CoalCoal• Example – Costs $1.25 a ton to break up dig and deliver the coalExample – 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 coalCosts $2.50 a ton to “clean rock and sulfur” out of the coal• Coal sells for $28/tonCoal sells for $28/ton• $28 - $1.25 - $2.50 = $24.25 >0 Yup that looks like good stuff$28 - $1.25 - $2.50 = $24.25 >0 Yup that looks like good stuff

– LimestoneLimestone• Cost $1.50 to break up dig and deliver limestone from a quarry to Cost $1.50 to break up dig and deliver limestone from a quarry to

processingprocessing• 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 tonCrushed rock sells for $6.50 per ton• $6.50 - $1.50 - $0.75 = $4.25 >0 Yup that looks like good stuff$6.50 - $1.50 - $0.75 = $4.25 >0 Yup that looks like good stuff

Trickier Good StuffTrickier Good Stuff

• Sometimes only a little of the rock is Sometimes only a little of the rock is something valuablesomething valuable– Copper few rocks are pure copperCopper few rocks are pure copper– Copper case is a rock with scattered grains of Copper case is a rock with scattered grains of

minerals that contain copperminerals that contain copper– Need to contain a certain amount of copper Need to contain a certain amount of copper

before the rock is worth whilebefore 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 Minimum amount of a mineral or element that must be present for things to be worth whilebe present for things to be worth while

A Break-Even Cut-Off GradeA Break-Even Cut-Off Grade

• Suppose Copper will bring you $3 per lb Suppose Copper will bring you $3 per lb after smelting, refining, and delivering after smelting, refining, and delivering expensesexpenses

• Suppose a ton of rock costs $2.50 to Suppose a ton of rock costs $2.50 to break-up load and deliver to the millbreak-up load and deliver to the mill

• Suppose a ton of rock costs $4.00 to grind Suppose a ton of rock costs $4.00 to grind up and put through a concentrator (mill)up and put through a concentrator (mill)

• How much recoverable copper has to be in How much recoverable copper has to be in the rock for it to be “Good Stuff”? the rock for it to be “Good Stuff”?

A CalculationA 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 Break Even Cut-Off Value (BECOV) is the

fraction of the rock that is copperfraction of the rock that is copper• Quick AlgebraQuick Algebra

– $6.50/(3*2000) = BECOV = 0.001083$6.50/(3*2000) = BECOV = 0.001083– Often we like to talk in %Often we like to talk in %– Multiply by 100 to convert to parts per hundredMultiply by 100 to convert to parts per hundred

• 0.1083% copper (about 0.11% BECOV)0.1083% 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 Do you think our processes get 100% of the

copper in the rock?copper in the rock?

A Look at ProcessA Look at Process

• Copper ore is mined using drilling and blasting, Copper ore is mined using drilling and blasting, followed by truck and shovel loading and haulage.followed by truck and shovel loading and haulage.

• The ore is taken to a concentrator where it is The ore is taken to a concentrator where it is crushed and ground to powder and then put into crushed and ground to powder and then put into flotation cells.flotation cells.– Flotation cell is a stirred “bubble bath”Flotation cell is a stirred “bubble bath”– Some minerals hate to get wet (sulfide copper and iron Some minerals hate to get wet (sulfide copper and iron

minerals) – they grab onto the air bubbles and form a minerals) – they grab onto the air bubbles and form a dirty froth at the surfacedirty froth at the surface

– Scrape off the froth – it’s a concentrateScrape off the froth – it’s a concentrate– Other minerals like quartz, feldspar and calcium Other minerals like quartz, feldspar and calcium

carbonate (that have no copper in them – called gangue carbonate (that have no copper in them – called gangue minerals) love the water and stay in the bathminerals) love the water and stay in the bath• They get sucked out the bottom with the waterThey get sucked out the bottom with the water

What are the Chances of What are the Chances of Perfect Recovery?Perfect Recovery?

• What if a mineral particle is part What if a mineral particle is part gangue and part copper sulfide?gangue and part copper sulfide?– This is the reason for crushing and This is the reason for crushing and

grinding – to minimize this problem by grinding – to minimize this problem by achieving “liberation” – copper minerals achieving “liberation” – copper minerals and gangue as separate particles.and gangue as separate particles.

– Suppose we recover 90% of the copperSuppose we recover 90% of the copper•Now our break even cut-off grade is Now our break even cut-off grade is

0.001083/0.90.001083/0.9•Result 0.001203 or 0.1203%Result 0.001203 or 0.1203%

More ProcessMore Process

• Copper Concentrates are sent to a smelterCopper Concentrates are sent to a smelter– Copper is still chemically combined with sulfurCopper is still chemically combined with sulfur– Here we will try to get rid of the sulfurHere we will try to get rid of the sulfur– And suck iron up into a rock slagAnd suck iron up into a rock slag– Put the concentrates in a very hot furnace and Put the concentrates in a very hot furnace and

burn off the sulfur to make sulfur dioxideburn off the sulfur to make sulfur dioxide– Iron and impurities go into “Flux” to make slagIron and impurities go into “Flux” to make slag– Copper settles to the bottom to form “blister Copper settles to the bottom to form “blister

copper” – copper metal with some minor metal copper” – copper metal with some minor metal impuritiesimpurities

• Suppose smelter gets 92% of the copperSuppose smelter gets 92% of the copper– 0.001203/.92 = 0.001308 or 0.1308%0.001203/.92 = 0.001308 or 0.1308%

Last of a ProcessLast of a Process

• Refining of Blister CopperRefining of Blister Copper• Blister Copper is placed in electrolytic cellsBlister Copper is placed in electrolytic cells

– Acid baths with electricity going throughAcid baths with electricity going through

• Blister Copper electrode dissolves awayBlister Copper electrode dissolves away• Pure copper plates out on the other electrodePure copper plates out on the other electrode

– Trace metal impurities settle as “slimes” on the bottomTrace metal impurities settle as “slimes” on the bottom

• Suppose this process is 99% efficientSuppose this process is 99% efficient• 0.001308/.99 = 0.0013210.001308/.99 = 0.001321• The Break Even Cut-Off Grade must be at least The Break Even Cut-Off Grade must be at least

0.1321% copper in place for the rock to be “good 0.1321% copper in place for the rock to be “good stuff” or ore.stuff” or ore.

Economic ConsiderationsEconomic Considerations

• When a block or area of mineralization When a block or area of mineralization exists that can be mined at a profit the ore exists that can be mined at a profit the ore can be called “Ore”can be called “Ore”– Ore can be picked up and processed for a profitOre can be picked up and processed for a profit– What about the overburdenWhat about the overburden

• Have to loose money to get it out of the wayHave to loose money to get it out of the way

• Whether an ore deposit can be considered a Whether an ore deposit can be considered a reserve depends on whether it makes reserve depends on whether it makes enough extra to pay for getting the enough extra to pay for getting the “overburden” out of the way“overburden” out of the way

Stripping RatioStripping Ratio

• Becomes important to know how much bad Becomes important to know how much bad stuff has to be moved to get to the good stuffstuff has to be moved to get to the good stuff

• Amount of Bad Stuff that must be moved to Amount of Bad Stuff that must be moved to get to the good stuff is called the “stripping get to the good stuff is called the “stripping ratio”ratio”– For Coal we ask how many cubic yards of For Coal we ask how many cubic yards of

overburden must be moved to get to one ton of coaloverburden must be moved to get to one ton of coal– In metal mining it is usually tons of overburden per In metal mining it is usually tons of overburden per

ton of ore or cubic yards of overburden per cubic ton of ore or cubic yards of overburden per cubic yard of oreyard of ore• Note that for metal a stripping ratio comes up unit-lessNote that for metal a stripping ratio comes up unit-less

Stripping Ratio ImpactStripping Ratio Impact

• While Ore is worth picking up to process the While Ore is worth picking up to process the amount of overburden that must be moved will amount of overburden that must be moved will determine whether the deposit is profitabledetermine whether the deposit is profitable– The stripping ratio is key in determining whether ore The stripping ratio is key in determining whether ore

becomes a “reserve”becomes a “reserve”• If too much overburden is in the wayIf too much overburden is in the way

– You can’t mine the oreYou can’t mine the ore– You consider tunneling around under the You consider tunneling around under the

overburden and mine the ore without moving the overburden and mine the ore without moving the overburdenoverburden• That is underground miningThat is underground mining• We’re too cool in a Surface Mining Course to talk about the We’re too cool in a Surface Mining Course to talk about the

evil competitionevil competition

Bad Stuff LiabilitiesBad Stuff Liabilities

• Overburden has to go somewhereOverburden has to go somewhere– The evil word “waste”The evil word “waste”

• Just plain rock may become a major regulatory targetJust plain rock may become a major regulatory target

• The Mineral of Interest ProblemThe Mineral of Interest Problem– Society tends to use pure concentrations of Society tends to use pure concentrations of

substances – nature seldom makes pure substances – nature seldom makes pure concentrationsconcentrations

– Even ore often leaves substantial other stuff Even ore often leaves substantial other stuff behindbehind• Sometimes “tailings” are not entirely just a natural rockSometimes “tailings” are not entirely just a natural rock

Our ChallengeOur Challenge

• Use the advantages of open surface Use the advantages of open surface assess to safely and economically assess to safely and economically produce minerals our society needsproduce minerals our society needs

• Control and manage any of our Control and manage any of our disturbances into a form that can be disturbances into a form that can be integrated back into a sustainable integrated back into a sustainable environment environment

Some Basic Stripping Ratio Some Basic Stripping Ratio CalculationsCalculations• I have 1800 cubic yards of overburden and I have 1800 cubic yards of overburden and

600 cubic yards of ore600 cubic yards of ore– 1800/600 = 3 (Stripping ratio is 3:1)1800/600 = 3 (Stripping ratio is 3:1)– This is called a volumetric stripping ratio This is called a volumetric stripping ratio

because it is based on volumebecause it is based on volume• We can often get a volumetric stripping ratio from We can often get a volumetric stripping ratio from

the maps or geometry of the ore bodythe maps or geometry of the ore body

• May need to consider density of materialsMay need to consider density of materials– Suppose my overburden is 4000 lbs/cubic yardSuppose my overburden is 4000 lbs/cubic yard– Suppose my ore is 4500 lbs/cubic yardSuppose my ore is 4500 lbs/cubic yard– We can adjust a volumetric stripping ratio to a We can adjust a volumetric stripping ratio to a

weight based stripping ratio (more common in weight based stripping ratio (more common in metal mines)metal mines)

Converting to a Weight Based Converting to a Weight Based Stripping RatioStripping Ratio

• 1800 cubic yards OB * 4000 lbs/cubic 1800 cubic yards OB * 4000 lbs/cubic yard =7,200,000 lbsyard =7,200,000 lbs– Since we usually work in tonsSince we usually work in tons– 7,200,000 lbs/ 2000 lbs/ton = 3,600 tons7,200,000 lbs/ 2000 lbs/ton = 3,600 tons

• 600 cubic yards of Ore * 4500 600 cubic yards of Ore * 4500 lbs/cubic yard / 2000 lbs/ton = 1350 lbs/cubic yard / 2000 lbs/ton = 1350 tonstons

• Weight Based Stripping Ratio isWeight Based Stripping Ratio is– 3,600 / 1350 = 2.67 to 13,600 / 1350 = 2.67 to 1

Looking at an Economic Looking at an Economic LimitLimit• Suppose we have a ton of copper ore with Suppose we have a ton of copper ore with

0.5% copper0.5% copper– Suppose copper sells for 3.30/lbSuppose copper sells for 3.30/lb

• To make copper we have toTo make copper we have to– Mine the oreMine the ore– Crush it and put it through a concentrator to get Crush it and put it through a concentrator to get

just the copper mineralsjust the copper minerals– Smelt the copper minerals to get out just the Smelt the copper minerals to get out just the

coppercopper– Refine and purify the copper – then we can sell itRefine and purify the copper – then we can sell it

More on Economic LimitsMore on Economic Limits

• Suppose it costs 10 cents a lb to refine copperSuppose it costs 10 cents a lb to refine copper• Suppose it costs $85/ton to smelt concentratesSuppose it costs $85/ton to smelt concentrates• Unrefined copper then is worth $3.30 - $0.10 = $3.20Unrefined copper then is worth $3.30 - $0.10 = $3.20

– Adjusting for 99% recoveryAdjusting for 99% recovery– $3.20*.99 = $3.17$3.20*.99 = $3.17

• A typical Cu concentrate is about 30% copperA typical Cu concentrate is about 30% copper– A ton of concentrate containsA ton of concentrate contains

• 2000 * 0.3 = 600 lbs of Cu2000 * 0.3 = 600 lbs of Cu– $85/ton to smelt concentrates costs$85/ton to smelt concentrates costs

• 85/600 = 0.142 or 14.2 cents/lb85/600 = 0.142 or 14.2 cents/lb• Adjust for 92% recovery – 14.2 cents/lb/0.92 = 15.43 cents/lb Adjust for 92% recovery – 14.2 cents/lb/0.92 = 15.43 cents/lb

recoverablerecoverable– Copper in concentrates is thus worthCopper in concentrates is thus worth

• $3.17 – 0.154 = $3.016 per lb$3.17 – 0.154 = $3.016 per lb

Continuing Our Copper Continuing Our Copper EconomicsEconomics

• Suppose the Concentrator costs $3.00 Suppose the Concentrator costs $3.00 per ton to process oreper ton to process ore

• Suppose the mining costs $2.25 per Suppose the mining costs $2.25 per ton to break up, pick up and deliver ton to break up, pick up and deliver the orethe ore

• Our 1 ton of ore will cost $5.25 to get Our 1 ton of ore will cost $5.25 to get ready for the smelterready for the smelter

• Our ton of ore contains 0.5% copperOur ton of ore contains 0.5% copper– 2000 lbs * 0.005 = 10 lbs of copper2000 lbs * 0.005 = 10 lbs of copper

Can we afford to move that Can we afford to move that ore?ore?• 10 lbs of copper in concentrates is worth10 lbs of copper in concentrates is worth

– 10 * 3.016 = $30.1610 * 3.016 = $30.16– But only recover 90% in concentrator 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 To get that ore out of the ground and into concentrate will cost $5.25concentrate will cost $5.25

• $27.14 - $5.25 = $21.98$27.14 - $5.25 = $21.98– Oh Yes its worth while to dig that ore up and Oh Yes its worth while to dig that ore up and

process it!!process it!!

But What if It Has Overburden But What if It Has Overburden Above it – Can I move that?Above it – Can I move that?

• Check the Break Even LimitCheck the Break Even Limit– The ore is worth $21.98The ore is worth $21.98

• Suppose it will cost me $2.50/ton to Suppose it will cost me $2.50/ton to break up, pick up, and dump break up, pick up, and dump overburden materialoverburden material– $21.98/$2.50 = about 8.76 tons of $21.98/$2.50 = about 8.76 tons of

overburden can be moved to get that overburden can be moved to get that ton of oreton of ore

– This is called a break-even stripping This is called a break-even stripping ratioratio

Now Its Your TurnNow Its Your Turn

• Do Homework #1Do Homework #1– The problem has you calculate stripping The problem has you calculate stripping

ratios and break even limits for a copper ratios and break even limits for a copper mine.mine.