Case Talks

Invitation to know and/or review anIssue that for the process people,relatives and alchemists friends andbig Merlin, develop in their work tending to recover some valuablemetal, in our case, mainly copper. Based on responding a series of questions we will develop a theory that allow us to have an idea of each one of the different stages that are part of a concentrator and SW-EW plantDrawing from Omnibook Book by O. Levenspiel

Other aspect of an alchemistThe known goals of alchemists are the transmutation of regular metals into gold or silver and the creation of a panacea, a remedy that allegedly would cure all the diseases and will prolong the life indefinitely. Since Middle Age, European alchemists invested a lot of effort in the search of the philosopher's stone, a mythical substance that it was believed had an essential ingredient for some of these goals or both of them. Alchemists had good reputation and support for centuries, although not because their search of this unreachable goals nor the philosophical and mystical speculation that rules their literature, but because their mundane contributions to the chemical industries of that age: analysis and refining of minerals, metallurgy, production of ink, tints, paintings and make up, leather curried, fabrication of ceramic and crystal, preparation of extracts and liquor, etc.

Source: Wikipedia Online

The great Merlinteaching to a youngdisciple.Iconography from Middle Age

First Day (1 hour)

What is comminution?

Why this process needs to be developed?

How we do it?...or best say, Where we start?

What equipment is used?...and then, How we select it?

How, finally, we put of all of this in a grinding plant? Abril 2006

ComminutionIn any mining ambit it is talked about conminucin but.

this word does not exist on the Spanish language (it is not on the RAE dictionary)It is an adaptation of the English word comminutionIn Spanish is referred as triturate or grind

Crushing / Grinding Why this process needs to be done?

What we are looking for is disseminated When crushing the ore is obtained more easilyNatural size rock with ore we are interested in

Ore that WE are interested in recoveringGangue that we are NOT interested in recoveringParticle with ore that we are interested in recovering

We see that in order to move from a big particle to a smaller one, we necessarily need to apply a certain quantity of energy for this to happen.

On 1952, Fred Bond postulated three grinding principles to enunciate the Third Comminution Law:Applied energy = product energy feeding energyThe applied useful energy is proportional to the length of the new fissures producedThe fracture easiness of a particle is given by the quantity of failures that such particle may have.

Energy consumed to reduce 80% of a material is inversely proportional to the square root of 80% size; being this last one, equal to the grate opening (in microns) that let pass the 80% in weight of the particles. With this, we have:

E = 10 Wi (1/P801/2 1/F801/2)where :

E = Specific energy consumption, kWh/ton grinded. F80 = 80% size passing through feeding, microns. P80 = 80% size passing in the final product, microns. Wi = Bond work Index, indicative of ore hardness, kWh/ton.

The modern grinding theory considers two parameters: Selection Function: represents the speed that a determined size particle fraction changes to another size; it has units of (min)-1 and it is generally denoted by Sij (is a kinetic relationship of each particle).

Fracture Function: represents the fraction in weight of the particles coming from the rupture that were retained on the j+1 grate and that then remains on the inferior i+1 grate; it is denoted by bij (it characterizes the distribution of the particles size after the fracture process). Therefore: i+1 Bij = bkj k=n

Represents the fraction accumulated of the accumulated fragments in the j grate that, as result of the fracture, will be smaller than the i grate

Therefore, it can be established the following balance for each I particle size

[Particles in I fraction at time (t+Dt)] = [Particles in I fraction at t time] - [Particles in I fraction fractured during Dt time interval] + [New particles added to the I fraction as result of the fracture of the particles originally retained in the thicker grates (j = 1, i-1)]

Crushing / GrindingHow grinding is performed? or event better, Where do we start?Blasting is one of the most economic methods to start the crushing / grinding process

Crushing / GrindingHow grinding is performed? Normal sequence after blasting is going through the primary crushing process.It can be a gyratory or a jaw crusherGyratory CrusherJaw Crusher

How you select a gyratory crusher? You need to have the following information:

Data of the ore visible density Data of ore hardness (Wi) Feeding granulometry (it is important to know the quantity of fine ore smaller ore to the one on the crusher discharge) Product medium size (P80)desired value

With this information, you enter to the selection tables developed by the manufacturers, as the one below:

What determine the capacity of a primary crusher? The volume given by the grinding section Density: at greater density, greater capacity Quantity of fine ore arriving with feeding

Important: the ore hardness is the one that mainly determines the power required on the crusher, more than the size of the particles being fed.CSSVolume in grinding area

Why there is a stockpile between the mine ad the plant?

The main objective is to provide a lung capacity between the mine and the plant and ensure with this a continuous flow downstream (normally a grinding circuit)

What is the difference between a silo and a stockpile? Stockpiles don't have vertical walls and the ore is self stocked and supported, forming a rest angle.

It is very important the determination of some ore parameters, such as fluidity and humidity

What are the following steps? SAG Grinding Conventional Grinding: bar mill follow by ball mill Unitary Grinding secondary / tertiary crushing follow by ball millGenerally, a clear trend towards the SAG mills and Ball Mills circuit facilities is found.What is a SAG Mill?It is a semi autogenous mill (Semi Autogenous Grinding) and it is mainly identified by: having a minor quantity of grinding balls compared to the conventional mills (8% to 15% against 28% to 36%) Having a big diameter (L/D > 1.75 meanwhile the ball mills have 0.50

SAG MillMotor ringMotor fansStopTrommel

Grate TrommelFeeding chuteSAG MillUnder sizedOver sizedFeedingWater

SAG Mill Speed and Shells

SAG MillSome facts on the SAG Mills:Its operation is very unstable and therefore a program is required that continuously check the dependent variables (power and supports pressure) and adjust the manipulated variables (feeding in t/h, solid percentage in the mill, rotation speed)Overload conditions are frequent and, if they are not controlled, they can block the millThe motor ring is particularly a constant torque towards the maximum speed condition; once this value is exceeded, the torque decreases proportionallyThe supports pressure is an indirect indication of the mill weight; a way of knowing its real weight is installing load cellsThe lubrication system has a high pressure nitrogen engine back up system (>2000 psi)Even it doesn't look like, it is an equipment that can easily break with the balls that have (it can be of 5 to 6)

SAG MillHow you dimension a SAG Mill?Generally we have found that the Bond relationship with Wi does not represent the real operating conditionThere are several companies that have dedicated themselves to determine parameters that can be correlated:In Australia, the JK Institute, using pendulum tests has developed correlations very close to realityIn Canada, MinovEx, using tests that determine the SPI parameter (SAG Power Index) that correlate it with the required powerIn USA the Modern Grinding Theory has been followed, using the Bij and Sij parametersThe CIMM in Chile has a SAGA that can be used as semi industrial test equipment

How efficient is a SAG Mill?

Generally it is a very inefficient equipment, with respect to power consumption..Why?The mill mass needs to be rotatedThere is a low probability of fracture (a low quantity of balls in the mill)Part of the energy is dissipated as heat and noiseApproximately only 10% of the energy installed is converted in energy really used to crush the rock

El SAG reduces the size of a maximum of 200.000 (8) up to 4000 - 8000 microns.

But this product is still too coarse, therefore it needs another grinding stage.BALL MILLS.

1. UNITARY MILL DIMENSIONING

Calculation Method

The dimensioning of the new unitary mill is done using the Bond method, considering a design factor of 1,1; according to the expression below:

(2)

Where:

W:Power required (kWh/tc)

WI:Work Index (kW/Tc)

EF1:Correction Factor by Dry Grinding

EF1 = 1,0 for Wet Grinding

EF2:Correction factor by Open Circuit (only ball mill)

EF2 = 1,0 for closed circuit

EF3:Correction factor by mill diameter (DINT)

for

(3)

for

(4)

As per Mular recommendation, a value of 1,00 will be used for EF3.

EF4:Correction factor by feeding size

(5)

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, where RR corresponds to the reduction reason according to the following equation,

= 55,56

(6)

and FO corresponds to the optimum feeding size

FO =

= 3.755

(7)

Therefore, replacing (6) and (7) in (5), you have:

EF4 =

= 1,23

EF5:correction factor by fines overgrinding (only ball mill)

(8)

This correction factor is applied to P80 less than 75 m, for bigger sizes:

EF5 = 1,00

EF7:correction factor by reduction reason (RR)

for ball grinding

(9)

EF7 =

=1,00

The EF6 and EF8 parameters are correction factors for the Bar Mill, therefore, for the Ball Mill it is assumed a unitary value for calculation purposes.

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Like this, the equation (2) is:

W = 10 14,75

1 1 1 1,23 1 1 1 1

W = 11,73 kWh/tc

With this, the power required (W) in kWh/tm is:

W =

= 12,93 kWh/tm

The tonnage per hour is obtained from the design load, availability and design factor, according to the following relationship,

F =

= 975 tm/h

Then for 1 073 tm/h we have an installed power of

P = 12,93 975 = 12.606 kW

In hp,

P =

= 16.905 hp

It is considered a motor ring transmission efficiency of 97%, therefore, the motor selected is

P =

= 17.428 hp = 12.996 kW

Like this, the power installed is 17.500 hp.

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How efficient is a Ball Mill?

Generally, it is a very inefficient equipment, with respect to the power consumption.Why?The mill mass needs to be rotatedThe fracture probability is higher than the SAG one, but it still low if it is considered that the circulating loads is in the order of 350%Part of the energy is dissipated as heat and noiseApproximately only 25% of the energy installed is converted in energy really used to break the rock

Therefore, the particles fraction that have not reached the optimum size need to be re-circulatedbut, how I separate them from the total mineral mass? Using equipment such as the hidrocyclones

HidrociclonesAllows the separation of particles by size using the centrifugal and gravitational forceApex(Discharge)Vortex(Overflow)Feeding

Calculation of number of cyclones required in the process

On the Krebs cyclones graphic (see Exhibit A) it is observed that for a pressure of 12 psi and a cyclone diameter of 26 in, the capacity of each cyclone is 450 m3/h approximately. The diameter and capacity of each cyclone will be confirmed with the Krebs[2] method

(Ec. 9)

where:

dcut

: cut size of 95%, defined as the separation size that guarantees a maximum of 5% of the retained material in such grate on the cyclone overflow product, (m)

Dc

:cyclone diameter (in)

f1: correction for density of the fed solid ()

f1 =

= 0,96

f2: correction for solid % in volume (Cv) of the fed pulp

.. f2 =

= 9,32

f3: correction for feed pressure to the cyclone (P in psi)

f3=

=0,96

Like this, the equation (9) is simplified to:

dcut = 16,098 0,96 9,32 0,96 260,467 . dcut = 628 m

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On the other hand, the processing capacity of each cyclone is estimated in accordance to:

(Ec. 13)

where,

f4: correction for feed pressure to the cyclone (P in psi)

.. f4 =

=1,09

f5: correction for solid % in volume (Cv) of the fed pulp

.. f5 =1 + 0,004461 38,51,28 =1,48

Of equation 13,

QCY = 0,408 1,09 1,48 262,047 . QCY = 518 m3/h

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Grinding Circuit What you need to look at?

Grinding Circuit What you need to continue looking at? Is there access with a bridge crane? Are there overflows evacuation? Am I minimizing the head height of the pumps? Do I have space for the electrical rooms of the ring motors? Have I minimized the trommel-screener height? Are the lubrication rooms in a height spot? ..etc.

Known open gratet = 0P80