Essent Meas
Elemental measurements in mineral processing
THEORETICAL PART
Mineral processing engineers as well as mining and environmental specialists
should be familiar with essential properties of various elements of surrounding nature.
It includes such properties as acidity or alkalinity of aqueous solutions, oxidationreduction property of solution, salt and solids concentration in water, density and other
properties of solids. The goal of this exercise is to learn mention above properties.
Density
It is a property characterizing the concentration of matter measured as the mass
per volume:
=m/V
(1)
where
 - density (g/cm3, or kg/dm3, or Mg/m3, etc.)
m- mass (g, or kg, or Mg, etc.)
V - volume (cm3, or dm3, or m3 etc.)
Since the volume depends on temperature, density also depends on temperature.
Usually the density is given at 20oC. The densities of materials are well known and
their numerical values can be found in handbooks and monographs. A good source of
density of solid is the CRC Handbook of Chemistry and Physics. Densities of selected
substances are given in Table 1.
Table l. Densities of common materials (g/cm3) at 20oC
Asphalt
1.1-1.5
Magnetite
4.9-5.2
Sugar
1.59
Butter
0.86-0.87
Diamond
3.01-3.52
Pyrite
4.95-5.1
Dolomite
2.84
Mercury
13.59
Galena
7.3-7.6
Glass
2.4-2.8
Cork
0.22-0.26
Coal
1.2-1.8
Quartz
2.65
Gold
19.31
1
A precise determination of density of materials can be accomplished applying
devices called picknometers, and the measurement should be performed according to
specific standard procedures [European (ISO), Polish (PN), American (ASTM),
German (DIN). etc.] standards. To measure density one can also use other
apparatuses, for instance helium densimeters.
A determination of the density of a material can be accomplished by
determining the mass of the sample and its volume. To accomplish that a piece of
material is weighted to determine its mass and next its volume is determined by
measuring the volume of water displaced when the object is completely immersed in
water.
pH
Immersion of any material in water changes the properties of resulting
suspension or solution due to dissolution or sorption processes. The aqueous solution
assumes certain acidity or alkalinity characterize by a so-called pH factor. pH is a
negative logarithm of concentration of hydrogen ions in aqueous solutions:
pH = -log (H+)
(2)
The term pH consists of letter p, which in mathematics stands for negative logarithm
and letter H denoting concentration of hydrogen ions (H+). Concentration should be
expressed in mol/dm3 or kmol/m3. pH of aqueous solutions slightly depends on
temperature. Usually pH is expressed at room temperature.
pH indicates acidity of alkalinity of the solution. When pH is lower than 7 the
solution in acidic. When pH is greater than 7, the solution is alkaline. The values of pH
can be from about –1 to about 15. In the vicinity of 7 the solution in neutral because
the concentration of H+ ions (which determine acidity) and OH- ions (which determine
alkalinity) is about equal. It results from a basic relationship between concentration of
hydrogen (H+)ions and hydroxyl (OH-) ions in water:
2
[H+] . [OH-] = 10-14
or pH + pOH = 14
at pH=7 pH = pOH or [H+] = [OH-]
(3)
(at 21oC)
The measurement of pH can be accomplished with special devices called pH-meters or
using special indicating solution or paper bands (pH indicators).
The pH of clean waters is between 5.6 –7. This value is a result of the presence
of CO2 in water leading to formation carbonic acid. Natural water can be neutral or
acidic due to presence of humid acids, or slightly alkaline due to the presence of
alkaline ions such as Mg++ or Ca++ coming from surrounding materials, for instance
CaCO3.
Eh
Another parameter, which can characterize the property of aqueous solution is
the oxidation-reduction potential, also called Eh or redox potential. Eh indicates the
ratio between concentration of a species able to undergo oxidation and another species
able to undergo reduction reaction. Eh is defined as:
0
Eh = E h -(RT/nF) ln(red/oxy)
where
Eh - redox potential, in mV or V
R - gas constant
T - absolute temperature
n - number of electrons taking part in the reduction-oxidation (redox) reaction
F - Faraday constant
ln - symbol of natural logarithm
red – concentration of reduced species, kmol/m3
oxy- concentration of oxidation species, kmol/m3
3
(3)
E0h – standard potential (when the red and oxy are the same), mV or V
When the solution contains more than one redox substance, then the measured
redox potential is called mixed redox potential. Potential Eh is measured in relation to
a standard electrode, which is the standard hydrogen electrode also called standard
normal electrode (SHE). In practice the potential is measured against a special
electrode (for instance saturated calomel electrode or SCE), and then recalculated into
Eh (SHE) using a simple formula.
Eh is a much more complex parameter than pH and it depends on the pH of the
solution. Highly oxidative solutions (for instance when the solution contains dissolved
oxygen) provide positive values of Eh while highly reducing solutions (for instance
0
sulfide ions) negative. It strongly depends on the species forming the redox pair or E h .
0
Selected values of E h are given in Table 2.
0
Table 2. Standard potentials ( E h ) for selected materials in relation to standard
hydrogen electrode
Normal potential
Electrode reaction
Short notation
E h0 (V)
S2O82  + 2e = 2 SO 24 
S2O82  / SO 24 
2,050
ClO– + 2H+ + 2e = Cl– + H2O
ClO–/Cl–
1,640
1,510
MnO4
MnO4
+ 8H+ +5e = Mn2+ + 12H2O
2Cl–
/Mn2+
Cl2 +2e =
O2 + 4H+ + 4e = 2H2O
Fe3+ + e = Fe2+
O2 + 2e + 2H+ = H2O2
(CN)2 + 2H+ + 2e = 2HCN
Cl2/2Cl–
O2/O2–
Fe3+/Fe2+
O2/H2O2
(CN)2/HCN
Fe(CN)36 + e = Fe(CN)64 
Fe(CN)36 / Fe(CN)64 
Cu2+ + e = Cu+
2H+ + 2e = H2
Cu2+/Cu+
H+/H2
SO 24  + 2H+ + 2e = SO 32  + H2O
SO 24  / SO 32 
N2 + 4H+ + 4e = N2H4 (hydrazyne)
S + 2e = S2–
Zn2+ + 2e = Zn
N2 /N2–
S/S2–
Zn2+/Zn
4
1,360
1,228
0,771
0,680
0,370
0,363
0,167
0,000
–0,103
–0,333
–0,510
–0,763
Conductivity
Dissolution of soluble salts in water leads to creation of ions. The ions present
in the solution transmit electrical current. Such solutions accelerate redox reactions for
instance corrosion of metals. The ability of aqueous solutions to conduct electrical
current can be determine by measuring conductivity. Table 3 shows electric
conductivity of aqueous solution containing sodium chloride.
Table 3. Conductivity of diluted NaCl solutions at 18oC
Relative conductivity
Concentration (weight %)
-1 . cm-1]
0,00003
0,0000539
0,00015
0,000265
0,001
0,00165
0,005
0,0078
0,05
0,0642
Hydrophobicity
Most inorganic materials is hydrophilic. It means that water easily wets the
surface of these materials. Exception is sulfur, talc and graphite which are
hydrophobic. Organic substances (for instance crude and fuel oils and their
derivatives) are usually hydrophobic. The presence of hydrophobic substances in the
environment usually indicates its pollution. The simplest way of measuring
hydrophobicity of a material is determination its so-called contact angle. There are
many ways of measuring contact angle. The most frequently used, and simplest, is the
sessile drop method. The hydrophilic materials display and zero or small contact
angles while highly hydrophobic up to 110o.
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EXPERIMENTAL PART
Exercise 1. Evaluation of density
Take a coal lump about 2-3 cm in size. Determine its mass by weighing coal by
means of an electronic balance. Next, determine the volume of the same piece of coal
by measuring the volume of water displaced when the coal is immersed in water.
Calculate density of the investigated coal. Make similar measurement for quartz and
pyrite. Calculate the densities of the investigated materials and compare them with the
literature values.
Exercise 2. Determination of the density of suspension and determination of solids
concentration in the suspension
Weigh precisely certain amount of ground coal and make coal suspension in
water using precisely pre-measured amount of water. For instance, mix together 1 part
of coal and two parts of water by weight. Calculate the density of the created
suspension paying attention to the final volume of the suspension. Check the
calculated density of the suspension again density determined directly by means of
balance and a graduated cylinder. Figure out how to accomplish that and discuss it
with the instructor. Take three samples of the suspension and put them on watch
glasses and determine the mass of the sample. Then, dry up the sample at 105o C in a
oven and determine the mass. Give the final results as an average of three
measurements. Compare the obtained solids densities. Save the dried coal for next
experiments (exercise no. 3-5).
Exercise 3. pH measurement
Create a suspension of coal in water by suspending in a test tube 2 grams of
coal and 25 cm3 of water. Make a similar suspension with oxidized coal. Stir the
mixture well and measure the pH of suspension. Compare both measurements and
interpret the observed differences taking into account the fact that coal and pyrite
present in the coal undergo oxidation with the formation: carboxylic acids (coal) and
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sulfuric acid (pyrite). Write down the chemical reactions between pyrite and coal as
well as between pyrite and oxygen. What was the concentration of the hydrogen ions
in the oxidized coal suspension?
Exercise 4. Eh measurement
Measure Eh of water, 30% solution of hydrogen peroxide, and an Eh standard.
Measure also Eh of suspension of coal and suspension of oxidized coal after bringing
the samples to the same pH. Compare the obtained values of Eh for both coals. Which
suspension has more oxidizing power?
Exercise 5. Conductivity measurement
Measure the conductivity of coal and oxidized coal. Why the resulting
conductivities are different? On the bases of the measured conductivity of NaCl
solutions read from the calibration curves equivalent concentration of NaCl in both
suspensions.
Exercise 6. Hydrophobicity measurement
Take a piece of coal. Polish it with sandpaper. Put a drop of water on the
polished surface of coal, look carefully at the shape of the drop and draw the shape of
the drop on a sheet of paper. Evaluate the contact angle. Next, remove the drop and
smear a drop of fuel oil on the surface of the coal. Put a drop of water on the
contaminated with fuel oil surface and determine the contact angle. Describe the
measured results.
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