Chemical Processing Of Minerals

Published Date: 02 Nov 2017

ITS ORES

Chemical Processing of Minerals.

PEME 3621

Pablo Miguel Martin Soladana

(ID:200742341)

March 14, 2013

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Contents

1 Uranium 2

1.1 Physical and Chemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Currents uses and trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Deposits 3

2.1 Uranium Ores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.2 Deposits: Types and location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

3 Mining techniques 5

3.1 Conventional underground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2 Conventional open pit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Leaching 6

4.1 Leaching chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.2 In-situ Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4.3 Heap Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.4 Atmospheric agitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4.5 Pressure Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

4.6 Strong acid pugging and curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5 Solid-Liquid extraction 11

6 Puri_cation 11

7 Final Product Recovery 12

8 Leaching behavior depending on the gange mineral 12

1

Abstract

1 Uranium

1.1 Physical and Chemical properties

Since properties of the element are important for its extraction they are to be explained in this section:

Uranium is a natural element classi_ed as actinism in the periodic system. As many other elements

in this group, uranium have metal appearance (silvery white). The atomic weight of this element is

238 g/mol and its density is 19:1g=cm3 which is 70% denser than lead. Its melting point is 1132_C

which is 200_ less than that of iron (1538_C).

Mechanical properties are similar to lead in terms of malleable and ductile. Thermal and electrical

conductivity are poorer than lead so that: 27:5Wmô€€€1Kô€€€1 and 3:47Kmô€€€1ô€€€1. Since uranium behaves

as a metal it got several oxidation stages. Those are 6, 5 4 and 3 although 2 and 1 can be reached

under certain conditions. Uranium is reactive on contact with acids in such a way that it is solved in

them, and is easily oxidized in air so that o layer of oxide is created. An other chemical property is

pyrophoricity which means that a self-ignition reaction occurs. [5]

Uranium have mainly three isotopes which occurs naturally. They are 238U,235U and 234U which

are present in 99.27%, 0.72% and 0.005% respectively in earth's crust. However there are three other

isotopes created arti_cially in nuclear reactions (232U,233U and 236U). While 238U is stable and unlikely

to be _ssile , 235U and 234U decay releasing alpha particles. The half-live of this isotopes are 7:04_108

and 2:455 _ 105 years respectively.[5]

1.2 Currents uses and trends

Due due to high concentration in certain parts of the crust and the presence of a _ssile isotope in

su_cient concentration, Uranium is a valuable mining product. However is not only appreciated as

nuclear fuel but also because its properties as colorant and because its hardness for high-density pen-

etrators.

It is know that uranium oxide was _rstly used around 79A.D. as yellow colorant for glasses vessels.

It is currently used with the same propose although several other have been found in civil applica-

tion such as Inertial navigation systems for aircrafts. Uranium nitrate is used as photographic toner.[5]

The most well know use for Uranium is nuclear fuel. Uranium enriched in 235U by enrichment

process can be used in self sustaining _ssion chain reaction where neutron are released and absorbed

by uranium so that great amount of energy can be recovered. One pound of uranium 0.2% enriched

have the same energy potential than 150 tons of coal.

n +235 U !236 U !144 Ba +89 Kr + 3n + 117MeV (1)

Enrichment process are to be explained in the last section of this report.

In Table 1 we can appreciate the production of uranium from 2004 and 2011 as well as the main

producers.

Kazakhstan, Canada and Australia are three main producers of the mineral. Kazakhstan owes the

36% of the global production, Canada and Australia produce 17 and 11% respectively.[1]

2

Figure 1: Uranium World Production (2004-2011)

The history about extraction methods starts in 1990, by this time 55% of world production come

from underground mining. along 90's the extraction methods changed in such a way that in 1999

underground extraction was about 33%. After 2000, due to Canadian new mines and Australian's

Olympic Dam the underground production have increase up to 37%. Table 2 shows current extraction

methods.

Figure 2: Extraction Methods in 2011

2 Deposits

Understanding uranium minerals requires further knowledge about its formation and how can the be

found in nature stage. The aim of this section is to set the extraction context in terms of what and

where uranium minerals can be found.

2.1 Uranium Ores

The uranium ores are generally produced by uranium precipitation from uranium-bearing uids. De-

pending on pH the presence of anions (such as Fô€€€, Clô€€€, SO2ô€€€

4 , CO2ô€€€

3 , PO3ô€€€

4 ) and oxidation potential

of the bearing uid, Uranium is more likely to be disolved.When conditions change the uranium pre-

cipitates in a broad range of minerals shown in Table 3[2].

Uranite is the most common uranium mineral. The likelihood of _nd impurities such as heavy

metals or Rare earths is high. In the other hand pitchblende have the same chemical structure than

uranite but this term is related with uranite with less amount of impurities in the ore[2].

Figure 4[2] shows the relation between Uranite and other minerals. They are call secondary

minerals.

3

Figure 3: Uranium Minerals

Figure 4: Mineral formed from Uranite

2.2 Deposits: Types and location

Fayek et al. [2] classi_es the uranium deposits in several categories. They are to be commented and

illustrated with and example:

_ unconformity-related deposits They are created from the precipitation of brines. The most

common uranium minerals are uraninite and pitchblende. Tera are some examples as: Eagle

Point, Millennium, Rabbit Lake and McClean Lake, Collins Bay and Cigar Lake.

_ Sandstone deposits They are produced in sedimentary area with low consolidated grains of

sandstone. Uranium in groundwater had precipitated at low temperatures. The most com-

mon minerals in this deposit are co_nite and carnotite. Examples this deposits are Inkay

and Mynkuduk (Kazakhstan)), Smith Ranch (USA), Khiagdinskoye ((Russian Federation),Mas

Laveyre (France);

_ Hematite Breccia complex deposits This deposit is associated with the present of metals such as

iron, copper, gold and rare earths in between uranium . The deposit is in hematite- magnesite

breccia. The most important deposit is Olympic Dam deposit in South Australia. Mine deserves

special mention since is one of the largest mines and integrated metallurgical processing plant in

the world. It ranks the seventh position Contains also the greatest uranium body in the world[6].

In this mine uranium is produced as a by product in the extraction of copper (explained in Sec-

tion 3). Figure 5 shows an aerial view of the plant[2].

4

Figure 5: Olympic Dam (South Australia)

_ Quartz-pebble conglomerate deposits One example of this deposit is Witwatersrand.

_ Vein deposits (granite related deposits) It consist on carbonates and quartz minerals where uran-

ite or pitchblende can be found. Gunnar(Canada), Mina Fe (Spain), Schlema-Alberoda(Germany)

are examples orthis systems.

_ Intensive deposits Formed by hight temperatures due to magma, so granite and other metamor-

phic are related with them. Uranite, thorianite and uranothorite. Rossing deposit (Namibia),

Bingham Canyon and Twin Butte (USA).

_ Volcanic and caldera related deposits Streltsovsk caldera in the Russian Federation, Nopal deposit

(Mexico).

_ Metasomatic deposits Zheltorechenskoye and Pervomayskoye deposits (Ukraine),

_ Sur_cial deposits Calcrete is the main mineral in this deposit, They are relatively new formed

deposits. Examples are Lake Maitland (Australia) and Langer Heinrich deposit (Namibia).

_ There are several other kind of deposits. There are to be named but no explained: Collapse

breccia pipe deposits, Phosphorite deposits, Metamorphic deposits, Limestones and paleokarst

deposits, uranium bearing coal deposits, Rock types with elevated uranium contents.

Fayek et al. [2] have created the Map 6 were several kinds of deposits are located around the

world.

3 Mining techniques

3.1 Conventional underground

The mining areas are located several meter underground. This areas are connected by passages to

aboveground entrance. The situation of the deposit determines the shape and direction of the passages.

a general distribution of a mining pit is shown in Figure 7. The main sections of the mine are: Vehicles

ramp, extraction area, exploration area, ventilation stack and elevators.

Depending on the body shape the mining methods di_ers. If the deposit is horizontal and at the

extraction area can be picture as a room supported by columns as shown in Figure 8. Other way of

_nding the deposit is in a seam, the system of extraction is call longwall mining and it can be shown

in Figure 9. In last instance, Figure 10 shows cut and _ll mining where the deposit is deep and can

not be acceded by the previous methods. Extraction is performed upwards by lifting the oor with a

_lling material.

5

Figure 6: Location of the deposits and its type

Figure 7: Structure of a underground mine

3.2 Conventional open pit

Open pit extraction is a suitable technique in cases where the deposit is close to the surface and its

extension is large enough to get pro_t of it. A lot of earth have to be moved and treated, It must

be economically viable. The technique consist of digging in a selected area starting from the surface

heading the deposit. The along the prospection, benches are created in order to easy the extraction

operation and prevent the pit from collapsing. The maximum slope is 45_ but it depends on the earth.

The _gure 11 shows how open pit looks like.

4 Leaching

Since leaching is the widely used extraction technique[1], it will be explained d in more detail. There

are large number of leaching techniques. Depending of the ore characteristics or mineral properties

there are more or less suitable methods. Diagram ?? was taken from from OECD technical report [9].

It summarizes the leaching methods sorted by its suitability depending or ore features.

6

Figure 8: Extraction from Flat

body

Figure 9: Longwall extraction Figure 10: Cut-and-_ll mining

Figure 11: Open pit mine, Ernest Henry mine, Australia

4.1 Leaching chemistry

Uranium can be found naturally in to valence states. Hexavalent form occurs as Uranium trioxide

UO3 and tetravalent form is found as UO2. The _rst one is soluble and its solubility reaction is as

follows:

UO3 + 2H+ ! UO2+

2 + H2O (2)

Uranyl ions UO2+

2 further reacts with sulphate ion, in the case of sulphuric acid leaching, and with

carbonate ion in case of alkali leaching. Reactions are as follows:

UO2+

2 + 2(SO4)4ô€€€ ! UO2(SO4)4ô€€€

3 (3)

UO2+

2 + 2(CO3)4ô€€€ ! UO2(CO3)4ô€€€

3 (4)

For tetravalent state of uranium further treatment is required in order to increase recovery. And

oxidation agent combined with electron acceptor (ferric ion) is added which leads to a oxidation.

UO2 + 2Fe3+ ! UO2+

2 + 2Fe2+ (5)

Fe3+ ions are consumed. In order to mantain ferric ion concentrarion and other oxidant (magnese

dioxide) is added So that Fe2+ is oxidized back to Fe3+ form. Normally Fe3+ concentration is between

1 and 2 g/L. This allows a good extraction of uranium. For further inchease the kinetics of the porcess

temperature is rised to 70_C.

4.2 In-situ Leaching

Kazakhstan and USA are the countries where this method is more extended. Instead of mining, an

arrangement of pipes are installed underground in such a way that a ow of leaching uid is created

through the deposit(Figure 13). Due to the properties of the uid, uranium is dissolved (Chemistry

7

Figure 12: Leaching techniques shorted by kind of ore

of the process is to be explained in Section 4.1)

Compared with underground and open pit mining, in-situ leaching have little impact in the surface.

On the contrary, there are few requirements that the deposit must meet. It must be a porous body so

uids can pass through it. In addition the deposit must be contained in between impermeable layers

in such a way that uid can be recovered and does not contaminate nearby aquifers[1] Uranium in situ

leaching involves greater enviromental manegment since hazard liquids are in risk to be in contact with

aquifers. Leach liquid must be controled along mining operation and ground water must be restored

at previous levels after operations

4.3 Heap Leaching

Unlike In-situ leaching, Heap leaching requires from extensive ming techniques such as underground

mining of open pit mining to extract the ores. After its extraction they are grained. Heaping surface

must sloped and insulated from the ground by using impermeable plastic layers. Pierced pipes are

lied on the insulating surface. The grained ore is heaped over the pipe system (20-25 ft). Hoses with

nozzles are instaled on the heap so leaching uid can be sprayed from them. The heap must have

a speci_c percolation rate which allows the leaching uid good contact with the ore and also pass

through it easily. Liquid is recover at the bottom of the heap by the pipes and then storage in a tank

to be fed into further treatment process. Figure 14 shows the ow diagram for the process [8].

4.4 Atmospheric agitation

For acid leaching sulphuric acid is the preferred liquid for agitated leaching. The process is commonly

run in series tanks with overow devices. Working continuously could lead to a bypass problems which

reduce residence time and hence, the e_cient of the process. Chemwes plant in South Africa runs

the process bach wise in Pachuca tanks avoiding bypass problems. [9] Since, acid is a steel corrosive

8

Figure 13: In-Situ leaching diagram.

liquid, tanks must be lined with ribber or wood stave. the temperature must be mantain around

70_C. In addition, tanks are provided with covers in order to reduce emission of fumes. As shown in

_gure 15 agitation systen can be supplied mechanically or by using air lift (Pachuca) depending on

size distribution, particle size and ore density but the most common system is mechanical agitation

with rubber lined propeller or turbine.

Five or four tanks in series is the minimum number of tanks although plants are normally equipped

with twelve. Acid concentration is kept hight enough to avoid uranium precipitation. Temperature,

as well as acid must be maintained constant. Oxidant concentration is not critical in the process but

is required in order to maintain high ferric ion concentration and to oxidant the U(IV). As shown in

Figure 15 most used oxidants are pyrolusite (MnO2) and sodium chlorate[9].

Alkali leaching process performed in Pachuca tanks where a leaching liquid is added. This liquid

contains 40-50 g/L Na2CO3 and 10-20 g/L NaHCO3 and a pH value within the range 9 to 10.5. The

ore must be grained at 75_m. In addition the e_ciency of the process will depends enormously on the

temperature in such a way that 70 to 5 hours can be saved increasing the temperature from 80 to 120_C.

The critical points in atmospheric alkali leaching are: oxidation, temperature and concentration

of leaching solution.

4.5 Pressure Leaching

There are some kinds of ores that due to its complexity are not economically viable at atmospheric acid

leaching. Because of that pressure is applied in order to enhance recovery. Due to the new conditions,

oxygen concentration is larger and oxidation os ferrous sulphates is possible leading to ferric sulphate

which is the best oxidant for U(IV). Chemistry is as follows:

2FeS2 + 7O2 + H2O ! 2FeSO4 + 2H2SO4 (6)

2FeSO4 + H2SO4 +

1

2

O2 ! Fe2(SO4)3 + H2O (7)

There are two sorts of autoclaves normally used: four chamber horizontal, and vertiacal tank with

pneumatic stirring. They can be lined with lead and acid proof bricks. Even providing the vessel with

anti corrosion lining there are still corrosion problems in the propeller. An example of acid pressure

leaching is Key Lake in Canada.

9

Figure 14: Heap Leaching ow diagram.

Alkali leaching can also be performed at high pressure, in adition, it allows higher temperatures

which will enhance recovery. Thus residence can be e_ectively reduced untill 6 or 8 hours. Lodeve

mill in France is to be taken as an example. The mineral contains 7% of CO2. Leaching is performed

in two stages at 600kPa. Other parameters a showed in the Table 1.

Working in two stages lead to a certain advantages such as: due to high carbonate concentration

Parameter First Stage Second Stage

Leach Time (h) 3 6

Temperature (C) 145 140

Pulp density (% solids) 50 50

Carbonate concentration (g/L) 30 90

Table 1: Pressure Leaching typical parameters

the second step high recovery rate can be achieve, short residence time, high recovery rates and low

operation cost and capital investment

4.6 Strong acid pugging and curing

1-3 mm grained ore is treated with small amount of concentrate sulphuric acid. The quantity of liquid

is 10% en volume. the mud is then cured at 65-100_C for 12-24 hours. Then solubilized uranium can be

extracted by washing the ore in a pan _lter or shallow bed. The process is suitable for refractory ores.

In addition, The addition of strong oxidant is avoid since sulphuric acid is enough to oxidize tetravalent

uranium. Less amount of pregnant solution is created, this makes the process more economically in

terms of equipment size and also makes the process appropriate for areas with water scarcity. Figure

16 shows the pugging and curing process:

10

Figure 15: New construction Plants equipped with agitated leaching.

Figure 16: Flow diagram of Pugging and curing process.

5 Solid-Liquid extraction

The reason why solids must be removed is that produces operation problems in further operations.

The liquid containing uranium solution is a valuable product that must be separated as more e_ciently

as posible since solids can carry uranium salts within them. The overall cost of this operation involves

40% of total capital costs. There are two units operation widely used in the separation process, they

are thickening and _ltration. Thickening equipment is shown in _gure ??:

6 Puri_cation

Since liquid from solid-liquid extraction contains may impurities such as metals, it must be puri_ed.

There many methods available and there are chosen coupled or separably depending on variables such

as, impurities concentration, uranium ratio and target purity.

_ Precipitation in two stages.

_ Ions exchange. (Suitable for low uranium concentration liquids)

_ Impurities removal by ions exchange, precipitation.()

_ Solvent extraction.(Suitable for ores with metal as nickel and cobalt)

11

Figure 17: Thickening equipment.

_ Solvent extraction of impurities.

7 Final Product Recovery

In all nuclear fuel plants Uranium yellow cake is the starting raw material and since nuclear industry

requires high standards, this last step in the process is important.

For this propose precipitation presents the proper features to get the highest products requirements.

However, sometimes previous puri_cation step was no enough to get the desired product, hence and

additional solvent extraction or ion exchange step (or both of them in series) is necessary.

After puri_cation the main precipitation process is performed buy adding agents which make the

uranium precipitate. Depending the solution pH the purity of the feed, the product speci_cation,

precipitation agent price and environmental management the precipitation agent is to be selected

among the following ones: Hydrogen peroxide, ammonia, sodium hydroxide, magnesium hydroxide

and magnesia.

Since ammonia solution precipitation is the most common one is to be explained in little more detail.

It only occurs st certain rage of pH, also slightly changes in the conditions leads to a better recovery

(_ltration): Chemistry is as follows:

2UO2(SO4)4ô€€€

2 + 6NH3 ! (NH4)2U2O7 + 4SO2ô€€€

4 (8)

Other precipitation agents leads to other reactions:

Magnesia:

UO2SO4 +MgO + xH2O ! UO3 _ xH2O +MgSO4 (9)

Hydrogen peroxide:

UO2+

2 + H2O2 + 2H2O ! UO4 _ 2H2O + 2H+ (10)

Precipitation from alkali solitions presents other kind of chemistry presented here below:

2NaUO2(CO3)+6NaOH ! Na2U2O7 + 6Na2CO3 + 3H2O (11)

The pH value is around 12 and reaction taeks 2-4 hours to complete

Lost step of the process is drying and calcination. Table 18 presents temperature values and ratios

for calcination and drying:

The _nal consist on yellow cake which is to be storage in tanks and delivered to enrichment plats

to obtain nuclear fuel.

8 Leaching behavior depending on the gange mineral

Since uranium is found all around the world the range of gange material present with the metal is

wide. The sort of gange with the metal will be critical in leaching conditions selection:

12

Figure 18: Temperature, ratio and retention time for calcination and drying operation.

_ Quartz: Does not a_ect seriously to leaching process since it is a non reactive mineral

_ Carbonates: They are acid consumer although magnesite(MgCO3) and dolomite((CaMg)(CO3)2

have slow kinetics. Alkali leaching is recomended in general

_ Phosphate: apatite (Ca5(PO4)+3

) is less reactive than carbonate although it takes acid from

leaching.

_ Silicates: Since there are a wide range of silicates there are classi_ed in several groups depending

on its reactivity with acids: Insoluble in acid; such as kaolinite (Al2Si2O5(OH)4) or those

with electronegative cations such as Zr or Be. Those with soluble cations but insoluble silica

(chrysocolla((Cu; Al)2H2Si2O5(OH)4nH2O)) And the last kind are those with cations and silica

soluble in acid(willemite). Whether acid is suitable for the extraction depends on the properties

of each ore

_ Iron oxides: pH of 1 is adequate for uranium extraction in the presence of hematite and mag-

netite. But other minerasl such as goethite consumes large amount of acid.

_ Sulphides: Its solubility can not be predicted. Chalcopyrite, for instance is insoluble in acids

but chalcocite is not.

_ Fluorite: This mineral is soluble, hence is acid consumer.

_ Calcium sulphate: Does no react with acid but it does with alkali solutions. Just acid leaching

is a suitable technique.