dewatering , hydrometallurgy and plant flowsheet

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BE MINING ENGINEERING V SEMR MINERAL PROCESSING CODE 21MN52 MODULE 5- 8 HOURS
Dewatering: Thickener and filter.
Hydro-metallurgical methods of recovery: Leaching principle, various methods and applications.
Flow Sheets: Simplified flow sheets for the beneficiation of beach sand, coal and typical ores of copper, lead,
zinc and manganese with special reference to Indian deposits.
Teaching-Learning Process
Chalk and talk, PowerPoint Presentation & Videos
DEWATERING
INTRODUCTION, IMPORTANCE AND TYPES
Dewatering is defined as a process separating water from slurry pulp to obtain clarified clean water with out solids
and solids without water or minimum permissible water.
Water is essential in most mineral processing operations in handling, and separation of minerals. But dewatering is
essential in mineral processing due to ease of storage, low storage space requirement, requirements for the next
utilization of products, and conservation of water and energy
The dewatering can be obtained by mechanical and thermal methods The mechanical dewatering methods consists
of screening- centrifuging, gravity sedimentation. Filtration [ pressure – vacuum] The thermal dewatering consist of
drying [ indirect rotary kilns or gas suspended flash/ fluidized driers Hence Dewatering methods can be broadly
classified into three groups: (1) sedimentation; (2) filtration; (3) thermal drying
The dewatering is an expensive process both from capital and operating cost view point as most of the pulps to be
dewatered are very dilute with dilution ratio of 10. A several combinations of above mentioned dewatering methods
are used in series to achieve the results economically.
Sedimentation is most efficient when there is a large density difference between liquid and solid. This is always the
case in mineral processing where the carder liquid is water. Sedimentation cannot always be applied in
hydrometallurgical processes, however, because in some cases the carrier liquid may be a high-grade leach liquor
having a density approaching that of the solid. The bulk of the water is first removed by sedimentation, or thickening,
which produces a thickened pulp of perhaps 55-65% solids by weight. Up to 80% of the water can be separated at
this stage. Filtration of the thick pulp then produces a moist filter cake of between 80 and 90% solids, which may
require thermal drying to produce a final product of about 95% solids by weight.[ refer figure 1]
Figure 1 Effect of dilution on different dewatering process with cost
Effect of dilution water on different dewatering units with cost
100
80
60
40
20
y = -0.9035x + 72.028
R² = 0.7483
0
-20
0 10 20 30 40 50 60 70 80 90 100
% Water

2. Thickening
Gravity sedimentation or thickening is the most widely applied dewatering technique in mineral processing, and it is a
relatively cheap, highcapacity process, which involves very low shear forces, thus providing good conditions for
flocculation of fine particles. The thickener is used to increase the concentration of the suspension by sedimentation,
accompanied by the formation of a clear liquid. In most cases the concentration of the suspension is high and
hindered settling takes place. Thickeners may be batch or continuous units, and consist of relatively shallow tanks
from which the clear liquid is taken off at the top, and the thickened suspension at the bottom The clarifier is similar in
design, but is less robust, handling suspensions of much lower solid content than the thickener The continuous
thickener consists of a cylindrical tank, the diameter ranging from about 2 to 200m in diameter, and of depth 1-7 m.
Pulp is fed into the centre via a feed-well placed up to 1 m below the surface, in order to cause as little disturbance
as possible (Figure 3). The clarified liquid overflows a peripheral launder, while the solids which settle over the entire
bottom of the tank are withdrawn as a thickened pulp from an outlet at the centre. Within the tank are one or more
rotating radial arms, from each of which are suspended a series of blades, shaped so as to rake the settled solids
towards the central outlet. On most modern thickeners these arms rise automatically if the torque exceeds a certain
value, thus preventing damage due to overloading. The blades also assist the compaction of the settled particles and
produce a thicker underflow than can be achieved by simple settling. The solids in the thickener move continuously
downwards, and then inwards towards the thickened underflow outlet, while the liquid moves upwards and radially
outwards. In general, there is no region of constant composition in the thickener. Thickener tanks are constructed of
steel, concrete, or a combination of both, steel being most economical in sizes of less than 25 rn in diameter. The
tank bottom is often fiat, while the mechanism arms are sloped towards the central discharge. With this design,
settled solids must "bed-in" to form a false sloping floor. Steel floors are rarely sloped to conform with the arms
because of expense. Concrete bases and sides become more common in the larger-sized tanks. In many cases the
settled solids, because of particle size, tend to slump and will not form a false bottom. In these cases the floor should
be concrete and poured to match the slope of the arms. Tanks may also be constructed with sloping concrete floors
and steel sides, and earth bottom thickeners are in use, which are generally considered to be the lowest cost solution
for thickener bottom construction.
The method of supporting the mechanism depends primarily on the tank diameter. In relatively small thickeners, of
diameter less than about 45 m, the drive head is usually supported on a superstructure spanning the tank, with the
arms being attached to the drive shaft. Such machines are referred to as bridge or beam thickeners (Figure 4). The
underflow is usually drawn from the apex of a cone located at the centre of the sloping bottom. A common
arrangement for larger thickeners, of up to about 180m in diameter, is to support the drive mechanism on a stationary
steel or concrete centre column. In most cases, the rake arms are attached to a drive cage, surrounding the central
column, which is connected to the drive mechanism. The thickened solids are discharged through an annular trench
encircling the centre column (Figure 5).. The two primary functions of the thickener are the production of a clarified
overflow and a thickened underflow of the required concentration. For a given throughput the clarifying capacity is
determined by the thickener diameter, since the surface area must be large enough so that the upward velocity of
liquid is at all times lower than the settling velocity of the slowest-settling particle which is to be recovered. The
degree of thickening produced is controlled by the residence time of the particles and hence by the thickener depth.
The solids concentration in a thickener varies from that of the clear overflow to that of the thickened underflow being
discharged. Although the variation in concentration is continuous, the concentrations at various depths may be
grouped into four zones, as shown in Figure 3. The satisfactory operation of the thickener as a clarifier depends upon
the existence of a clearliquid overflow at the top. If the clarification zone is too shallow, some of the smaller particles
may escape in the overflow. The volumetric rate of flow upwards is equal to the difference between the rate of feed of
liquid and the rate of removal in the undertow. Hence the required concentration of solids in the underflow, as well as
the throughput, determines the conditions in the clarification zone
The continuous thickener consists of a cylindrical tank, the diameter ranging from about 2 to 200m in diameter, and
of depth 1-7 m. Pulp is fed into the centre via a feed-well placed up to 1 m below the surface, in order to cause as
little disturbance as possible (Figure 3). The clarified liquid overflows a peripheral launder, while the solids which
settle over the entire bottom of the tank are withdrawn as a thickened pulp from an outlet at the centre. Within the
tank are one or more rotating radial arms, from each of which are suspended a series of blades, shaped so as to
rake the settled solids towards the central outlet. On most modern thickeners these arms rise automatically if the
torque exceeds a certain value, thus preventing damage due to overloading. The blades also assist the compaction
of the settled particles and produce a thicker underflow than can be achieved by simple settling. The solids in the
thickener move continuously downwards, and then inwards towards the thickened underflow outlet, while the liquid
moves upwards and radially outwards. In general, there is no region of constant composition in the thickener.
Thickener tanks are constructed of steel, concrete, , steel being most economical in sizes of < 25 rn

3. Figure 4 Bridge thickener Figure 5 Centre Pier supported thickener
Theory of thickening
The method developed by Coe and Clevenger (1916) is commonly employed to determine surface area when the
material settles with a definite interface. If F is the liquid-to-solids ratio by weight at any region within the thickener, D
is the liquid-to-solids ratio of the thickener discharge, and W t h -1 of dry solids are fed to the thickener, then (F- D)W t
h -1 of liquid moves upwards to the region from the discharge. The velocity of this liquid current R is thus
R= (F-D)W AS [1]
(where A is the thickener area (m2) and S is the specific gravity of the liquid (kg l-1). Because this upward velocity
must not exceed the settling rate of the solids in this region, at equilibrium
(F-D)W AS =R [2]
The required thickener area Ais therefore
A= [(F-D)W / RS] [3]
where R is the settling rate (mh-1). F is the liquid-to-solids ratio by weight at any region within the thickener, D is the
liquid-to-solids ratio of the thickener discharge, and W t h -1 of dry solids are fed to the thickener A is area of thickener
The Coe and Clevenger method requires multiple batch tests at different arbitrary pulp densities before an
acceptable unit area can be selected
UA CC= [(F-D) / RS] [4]
The Kynch model (1952) offers a way of obtaining the required area from a single batch-settling curve, and
is the basis of several thickening theories The results of a batch-settling test are plotted linearly as
mudline (interface between settled pulp and clear water) height against time (Figure6 ] UAK= CoHo]/ tu
[

4. Types of thickeners
1. Conventional thickeners
2. Tray thickeners
3. High capacity/ rate thickeners
4. Paste thickener
5. Lamella thickeners
1. Conventional thickeners; The continuous thickener consists of a cylindrical tank, the diameter ranging from
about 2 to 200m in diameter, and of depth 1-7 m. Pulp is fed into the centre via a feed-well placed up to 1 m below
the surface, in order to cause as little disturbance as possible (Figure 3). The clarified liquid overflows a peripheral
launder, while the solids which settle over the entire bottom of the tank are withdrawn as a thickened pulp from an
outlet at the centre. Within the tank are one or more rotating radial arms, from each of which are suspended a series
of blades, shaped so as to rake the settled solids towards the central outlet. On most modern thickeners these arms
rise automatically if the torque exceeds a certain value, thus preventing damage due to overloading. The blades also
assist the compaction of the settled particles and produce a thicker underflow than can be achieved by simple
settling. The solids in the thickener move continuously downwards, and then inwards towards the thickened
underflow outlet, while the liquid moves upwards and radially outwards
2 Tray thickeners’; They (Figure 7) are sometimes installed to save space. In essence, a tray thickener is a
series of unit thickeners mounted. vertically above one another. They operate as separate units, but a common
central shaft is utilised to drive the sets of rakes
Figure 7 Tray thickener
3 High rate thickeners; The feed enters via a hollow drive shaft where flocculant is added and is rapidly
dispersed by staged mechanical mixing. This staged mixing action is said to improve thickening since it makes most
effective use of the flocculant. The flocculated feed leaves the mixing chambers and is injected into a blanket of slurry

5. where the feed solids are further flocculated by contacting previously flocculated material. Direct contact between
rising fluid and settling solids, which is common to most thickeners, is averted with slurry blanket injection. Radially
mounted inclined plates are partially submerged in the slurry blanket; the settling solids in the slurry blanket slide
downwards along the inclined plates, producing faster and more effective thickening than vertical descent. The height
of the slurry blanket is automated through the use of a level sensor.[Figure 8]
Figure 8 High rate thickener
4 High density/ paste thickener High density thickeners (or high compression thickeners) are an extension of
high capacity thickening utilising a deeper mud bed to increase capacity and underflow density. High rate rakeless
thickeners use a deep tank and a steep bottom cone to maximise underflow density while eliminating the rake and
rake drive. In some applications underflow with the consistency of paste can be produced from high density and
rakeless thickeners. However for consistent paste underflow several manufacturers offer deep cone thickeners in
applications where surface tailings. disposal by wet stacking or underground paste backfill is required. The tank
height to diameter ratio is often 1:1 or greater [figure 9]
Figure 9 Paste thickener
5 Lamella thickener
Theoritcally lamella thickner is projected total inclined plates area Ac
Ac = N A 1.4 Cosφ [14]
Figurre 10 Lamella thickener
data iron ore tails
HRT 35 3.3 30-35 6 40 15.7
549.5 584.5 10.6 560.10 1.04 1.5 52.5 87.5 10.6 63.1
1.38 497 0.05 0.025 0. 01 1.2535 45.126 1.29 11.01 1.3
22.02 6 8
12116 36348 HD 300 19200 726926
M12 -1.5KW RR3200 0.31 2.36 3.7 11
3

6. FILTRATION
Filtration is separation of finelydivided solids from water by driving the pulp through a memberane porous to f water
but impervious to the solids through which water passes and by removal of cake over the memberatne. The
mechanism of filtration consists of a small bore tube through which water is sucked or pressurized while the particle
accumulate at the entrance as the device is operated they bridge across the opening allowing onlywater and
forming a cake of bridged material.
The rate of filtration dV/dt is proportional to filtering area, pressure difference between filter medium, average cross
section area of pores in the filter cake, the number of pores per unit area of memberane, viscosity of filtrate and
thickness of filter cake
Types of filters
Pressure filters; Pressure is applied to push the liquid through he memberane.and cake. Ex. Larox pressure filter
Suction or vaccue filter; Suction by vaccum is applied to pull water through the memberane and cake Ex disc and
drum vacuum filers
Vacuum filtration is the simplest form of “through blow” dewatering. A pressure differential created by a vacuum
pplied to the inside of the filter drum causes air to flow through the filter cake thereby displacing the contained water.
The solids are retained on a filter cloth and are carried to discharge point by the rotation of the drum.
Drum Vacuum filter
1. Drum – filter cloth mounted on segmentgrids. Internal drainage pipes.
2. Drum drive – variable speed
3. Support frame
4. Tank
5. Vacuum head – seal arrangement to connect rotating drum to stationary vacuum piping
6. Agitator – to suspend solid particles in tank
The cycle time is 4 times pickup time as angle is 90 degrees or 2 times dry time as angle is 180 degrees and ratio of
dry to pick up time is 2 Drum filters can have cake wash and may be used in hydrometallurgy industries also.
Dewatering of medium Cu concentrate 10 t/h
1. Application needs internal flow drum vacuum filter
2. Filtration rate from page 16 is 500 kg/m 2 eff. and hour. 3. Filter area is 10000 / 500 = 20 m2 Drum filter BF 2436
has an effective filter area of 20.3 m2 a total area of 27 m2
Drum vacuum filter

7. Disc vacuum filter
Most commonly used filter in mineral processing
1. Disc– filter cloth mounted on segment grids. Internal drainage pipes.
2. disc drive – variable speed
3. Support frame
4. Tank
5. Vacuum head – seal arrangement to connect rotating drum to stationary vacuum piping
6. Agitator – to suspend solid particles in tank
The cycle time is 4.5 times pickup time as angle is 80 degrees or 2.5 times dry time as angle is 144 degrees and
ratio of dry to pick up time is 1.8 Disc fiters are used in mineral prcessing where no cake wash is required and
occupies less space.
Disc Filter
Dewatering of medium Cu concentrate 10 t/h
1. Application needs internal flow disc vacuum filter
2. Filtration rate from is 500 kg/m 2 eff. and hour. 3. Filter area is 10000 / 500 = 20 m2Disc filter BF 24-4 2 m dia 4
nos disc has an effective filter area of 24m2 a total area of 36 m2
Pressure filter
Pressure is applied to push the liquid through he memberane.and cake. Ex. Larox pressure filter
Suction or vaccue filter; Suction by vaccum is applied to pull water through the memberane and cake Ex disc and
drum vacuum filers
The four main components of a filter press include the frame, filter plates, manifold (piping and valves), and filter
cloth, a key ingredient for optimizing filter press operations
The working principle of filter presses is that slurry is pumped into the machine such that solids are distributed evenly
during the fill cycle. Solids build up on the filter cloth, forming the filter cake; the filtrate exits the filter plates through
the corner ports into the manifold, yielding clean filtered water.Filter presses are a pressure filtration method and as
such, as the filter press feed pump builds pressure, solids build within the chambers until they are completely chock-
full of solids, forming the cake.Provision for cake drying by air blow is also there Once the chambers are full, the
cycle is complete and the filter cakes are ready to be released. In many higher capacity filter presses, fast action
automatic plate shifters are employed, speeding cycle time.
Pressure filter

8. Thermal drying
Indirect Heat Rotary Dryer (Kiln)
• Controlled environment interior excludes products of combustion
• Heat transfer by conduction and radiation
• Pulse-fired burners available
• Facilitates recovery of off-gases and product vapours
• Diameter 0,5m - 4,5 m Length 2.5 m to 30 m
• Applications in hazardous-, ultra fine- and combustible materials
Indirect Heat Rotary Dryer (Kiln)
Fluid Bed Dryer[FBD]
Important components ;Combustion -Windbox --Expansion Chamber
For drying of most granular and powdery materials
Capacity up to 300 ton/h.
Particle size minus 6 mm and • Size range 6:1 (optimal)
Fluid Bed Dryer[FBD]

9. Hydro-metallurgical methods of recovery: Leaching principle, various methods and applications.
Principles of hydrometallurgy
When metals are chemically processed in an aqueous environment, the technology used is known as
hydrometallurgy, and it consists of three separate stages:
• The metal of interest must first be transported from the solid feed material (ores, concentrates, etc.) into an
aqueous solution.
• This metal-containing solution (or solutions produced from it) must subsequently be concentrated and
purified.
• The metal must subsequently be retrieved from the purified solution in solid form.
Different processes involved in hydrometallurgy
Leaching
Leaching is a fundamental extractive operation in hydrometallurgical processing that involves the transfer of
a metal of interest from naturally occurring minerals into an aqueous solution. In basic terms, it involves the selective
dissolution of precious minerals by exposing the ore to an active chemical solution known as a leach solution.
Leaching is the process of extracting metal from metal-bearing materials that come into contact with a valuable metal
using aqueous solutions. It is the process of bringing aqueous solutions containing a lixiviant into contact with a
substance containing precious metal. The lixiviant in the solution might be acidic or basic. The lixiviant type and
concentration are generally regulated to allow some degree of selectivity for the metal or metals to be recovered.
To optimize the rate, extent, and selectivity of dissolution of the desired metal component into the aqueous phase,
the lixiviant solution conditions vary in terms of pH, oxidation-reduction potential, the presence of chelating agents,
and temperature. Certain metals can be extracted selectively using chelating agents.
Factors affecting the choice of leaching agents
The choice of leaching agents depends on:
• The chemical and physical properties of the substance to be leached
• The price of the reagent.
• The corrosive effect of the reagent and the construction materials
• The leaching agent’s selectivity for the intended ingredient to be leached.
• The ability to be renewed.
Common leaching agents/ lixiviants
Water; Some compounds, such as CuSO4, ZnSO4, and the majority of alkali metal compounds, dissolve quickly in
water. Some low-grade copper sulfide ores progressively convert into water-soluble sulfate.
Acids; Mineral acids, particularly sulphuric acids, are the most commonly used leaching agents.
Bases; Several bases, such as NaOH solution or NH4OH, are commonly used in numerous leaching procedures.
Bauxite is leached under pressure using a hot concentrated NaOH solution, whereas ammonia solution is utilized in
the leaching of native copper, copper ores, NiS, and Cu2S.
Aqueous salt solutions; The dissolution of gold during its extraction from veins in silica rock is the most prominent
example of a salt solution acting as a leaching agent. A solution of NaCN dissolves gold. The reaction is,
4 Au(s) + 8 NaCN(aq.) + O2(g) + 2 H2O(aq.) ⇋ 4 NaAu(CN)2 (aq.) + NaOH(aq.)
Separation of leach liquors/ mother solution/ pregnant solution
Almost all hydrometallurgical processes include the leaching of particles in order to dissolve valuable
components. Before generating the final product, solid-liquid separation is frequently performed. In this stage, the
leach liquor solution is separated from the solid residues using one or more methods. Common methods of
separation of leach liquors involve:

10. Washing; This process is based on the density difference between metallic ore and contaminants. When the ore is
processed with a stream of running water, lighter impurities are washed away, leaving heavier ore particles behind.
Filtration; The separation of a suspension into a solid filter cake and a liquid filtrate by passing it through a
permeable filtering material is known as filtration. The qualities of the suspension (e.g., size distribution,
concentration), filtering materials (for example, the width and shape of pores), as well as the forces, applied to the
suspension are important factors to be considered during this process.
Thickening;In leaching operations, thickening is the separation of a slurry or solid-liquid mixture into a dense slurry
containing the majority of the solids and an overflow of liquor. The solids in a suspension settle in a tank due to
gravity and produce a thick pulp.The pulp and clear liquid at the tank’s top can be removed continuously or
occasionally.
Settling; In this method of solid-liquid separation, the leachate obtained after leaching is neutralized, and the
neutralized slurry is separated and removed by adding flocculants
(A flocculant is a chemical that can be added to water to help colloids and other suspended particles combine to
generate heavier particles)
Important factors to be considered during the leaching process
Particle size;The ore or concentrate particles must be tiny enough for the valuable metals contained within them to
be physically exposed to the leach solution. The rate of leaching is affected by the degree of exposure. In general,
the rate of leaching increases as particle size decreases.Rates of diffusion The rate of diffusion of reactants or
products can influence the rate of a leaching mechanism. When a species in the solution phase diffuses slowly to
and from the mineral surface, enhancing the degree of agitation of the solution, it increases the frequency at which
the species diffuses.
The rate of a chemical reaction.; In some cases, the rate at which leaching processes occur at the mineral surface
dictates leaching kinetics. The degree of exposure of the valuable metal can be increased, the leaching system’s
temperature or pressure can be raised, or a catalyst can be used to speed up the chemical reaction rate.
The concentration of the leaching agent ;The rate of leaching increases as the concentration of the leaching agent
increases.
Insoluble substances;If an insoluble reaction product is generated during leaching, its rate will be determined by the
nature of the product. The rate of leaching will be considerably reduced if it forms a non-porous layer. If the solid
product is porous, the rate will be affected only slightly or not at all.
Types of leaching metod
1 IInsitu leaching 2 Heap leaching 3 Dump leaching 4 Agitation leaching, 5Vat leaching,6 Autoclave leaching
In situ leaching It is also referred to as solution mining. It is a mining method that involves drilling boreholes into a
deposit to recover minerals such as copper and uranium. The procedure begins with the drilling of holes into the ore
deposit. The leaching solution is poured into the deposit and comes into contact with the ore. The dissolved ore-
containing fluid is then pumped to the surface and treated. The solvent is fed into the ore via a series of pipelines
bored into the ground. The resulting liquor is extracted using a variety of pipe shape drills. The solvent flows down
and is penetrated into the ore body by these pipe-shaped drills for leaching. The solute is leached as the solvent
flows through the pipe-shaped drilling and ore body.

11. Heap leaching; Crushed (and often agglomerated) ore is placed in a heap lined with an impermeable layer in heap
leaching procedures. The leach solution is sprayed on top of the heap and allowed to trickle downhill through it. The
heap design typically includes collection sumps that allow the “pregnant” leach solution (solution containing dissolved
valuable metals) to be supplied for further processing. Gold cyanidation is one example, in which pulverized ores are
extracted with a solution of sodium cyanide, which dissolves the gold in the presence of air, leaving behind the
nonprecious residue.
Dump leaching; Dump leaching combines heap and in-situ leaching properties. Depending on the dump location, an
impermeable layer may or may not be employed in a dump leach. The ore is dumped to allow similar processing to
heap leaching, but the geological conditions of the area allow a valley or pit to act as the sump.
Agitation leaching;A time-consuming procedure in which soil is slurried with fluid extraction. When equilibrium is
reached between the metals on the soil surface and the metal contained in the solution, the solubilization of metal in
the soil slows and extraction is considered complete.
Vat leaching;Vat leaching is the process of interacting material with leach solution in big tanks or vats after it has
been size reduced and classified. Agitators are frequently used in vats to maintain solids suspended in the vats and
increase solid-liquid interaction. Prior to further processing, the leached particles and pregnant solution are normally
separated after vat leaching.
Autoclave leaching;Autoclave reactors are used for reactions at higher temperatures, which might increase the rate
of the reaction. Similarly, autoclaved allow the use of gaseous reagents in the system.

12. Factors affecting the viability of leaching operation
1. The degree of dissolution.
2. The dissolution of the value metals must be as complete as possible for a high degree of metal extraction
from an ore, which is the fundamental consideration to take into account in any leaching process.
3. Leaching selectivity
4. The selectivity of the dissolving reactions is the second aspect that influences the viability of leaching. This
is significant since all ores contain minerals other than those containing the metal of interest.
5. The selectivity of the leaching reactions dictates the extent to which this occurs and, as a result, the purity of
the metal-bearing solution produced by the leach.
6. The capital cost of the leach apparatus
7. The capital cost of leaching equipment can vary greatly, especially when the materials of construction are
considered, which is dependent on the needed working conditions.
8. The price of leach solutions
9. The unit costs of the various chemicals and the quantities utilized influence the cost of the leaching process.
Solution concentration and purification
Following leaching, the leaching liquid must generally be concentrated to recover the metal ions.
Furthermore, some unwanted metals may have been dissolved during the leaching process. To remove the
unwanted components, the solution is frequently filtered. The following processes are used for solution concentration
and purification:
Precipitation
Precipitation is the selective elimination of a specific metal compound or the removal of a significant contaminant by
precipitation of one of its components. Chemical precipitation is the primary method for recovering or removing
metals from a solution. Chemical precipitation is one of the most extensively used techniques for heavy metal
removal from inorganic effluents in industry. It primarily involves the use of chemicals (precipitants) to convert a
soluble substance into an insoluble form (insoluble precipitates of heavy metals like hydroxide, sulfide, carbonate,
and phosphate). Heavy metals can be easily removed once they have precipitated to form solids.
Cementation
A form of precipitation, cementation is a heterogeneous process in which ions are reduced to zero valence at a solid
metallic contact. It is the extraction of metals from a solution based on an electrochemical reaction between the
cementing metal and the precipitated metal’s ion. The precipitation of the metal is followed by a change in its
concentration in the solution, and hence by a change in its potential. When the equilibrium values are reached, the
process comes to an end.
Electrowinning
Electrowinning is the electrodeposition of metals from their ores that have been immersed in a solution. It’s
sometimes referred to as electro-extraction. Because of their high electro-potential values, it is most typically
employed to recover metals such as gold, silver, copper, and zinc.
Current is carried from an inert anode through a liquid leach solution containing the metal in the electrowinning
process, allowing the metal to be recovered as it deposits on the cathode.
Electrowinning cells provide companies with a low-cost choice with improved efficiency. Furthermore, electrowinning
yields exceedingly clean goods.
Solvent extraction
Metal is extracted from one phase to another using an extractant and a diluent combination. Because the major
element (diluent) is some form of oil, this mixture is typically referred to as “organic” in solvent extraction.
The pregnant leach solution (PLS) is emulsified with the stripped organic and then separated. Metal will be
transferred from the PLS to the organic. The end outcome will be a loaded organic and a raffinate stream. When
electrowinning is used, the loaded organic is emulsified with a lean electrolyte and allowed to separate. Metal will be
transferred from the organic to the electrolyte.
The streams that arise will be a stripped organic and a rich electrolyte. The organic stream is recycled via the solvent
extraction process, whereas the aqueous streams are recycled via the leaching and electrowinning processes.

13. Ion exchange
Ion exchange has traditionally been used to purify water and remove metal pollutants from dilute waste streams. Its
application in eliminating trace metallic contaminants from hydrometallurgical process streams has recently
expanded significantly. It is also employed as a primary recovery and concentration unit operation for particular
commodities, where both technical and cost advantages become apparent. The solution is exchanged for cations or
anions using chelating agents, natural zeolite, activated carbon, resins, and liquid organics coated with chelating
agents. The chemicals used and the impurities present influence selectivity and recovery.
Metal recovery
The final step in a hydrometallurgical process is metal recovery. Metals suitable for sale as raw materials
are frequently produced directly during the metal recovery stage. However, if ultra-high purity metals are to be
produced, additional refining may be required. Electrolysis, gaseous reduction, and precipitation are the three main
types of metal recovery techniques. Copper, for example, is a primary target of hydrometallurgy since it is easily
obtained through electrolysis. Cu2+ ions decrease at low potentials, leaving other contaminating metals like Fe2+
and Zn2+ behind.
Electrolysis
Electrowinning and electrorefining are two processes that use electrodeposition of metals at the cathode
and either metal dissolution or a competitive oxidation reaction at the anode to recover and purify metals.
Precipitation
In hydrometallurgy, precipitation is the chemical precipitation of metals and their compounds or impurities
from aqueous solutions. Precipitation occurs when a particular species’ limit of solubility is exceeded through reagent
addition, evaporation, pH change, or temperature manipulation.
Advantages of hydrometallurgy
1. Hydrometallurgy is presently employed to create over 70 metallic elements, and it involves selective
separation in battery recycling, which results in salt extraction. After desorption of the adsorbed ions, ion
exchange leads to a variety of separation and recovery procedures for metals at low concentrations.
2. Hydrometallurgy is capable of extracting complicated and low-grade ores.
3. Hydrometallurgy has a modest investment cost.
4. Hydrometallurgy is significantly more environmentally friendly.
5. Greater control over each stage of the process leads to the recovery of valuable by-products. Material
handling is also simplified.
6. A hydrometallurgical method eliminates the need for coke, an increasingly expensive reducing agent.
7. Liquor waste from the last recovery process may be recycled.
8. Low-temperature processing, low handling cost of leaching products, and the ability to treat low-grade ores
in hydrometallurgy make leaching superior to high-temperature smelting.
9. Sulfides are burned off in traditional pyrometallurgical smelting, releasing SO2 gas into the atmosphere. In
comparison to pyrometallurgy, hydrometallurgy releases only a fraction of the gases into the atmosphere.
10. Hydrometallurgy is more environmentally friendly than pyrometallurgy.
11. Hydrometallurgy is suitable for lean and complicated ores. With the steady depletion of rich ore resources,
traditional pyrometallurgical processes for metal extraction are becoming increasingly challenging in many
instances.
12. A hydrometallurgical process can begin on a modest scale and scale increase as needed. Because of the
need for process economy, pyrometallurgical processes must typically be planned as large-scale
operations.
Disadvantages of hydrometallurgy
1. Hydrometallurgy uses a huge amount of water, which has a greater potential for contamination.
2. There are difficulties in solid-liquid separation;
3. impurities problems may arise throughout the purification process during hydrometallurgy.
4. More time is required for high metal recovery