comparison of surface chemistry conditions between oxide and sulphide

Comparison of Surface Chemistry Conditions Between Oxide and Sulphide Minerals During Dissolution

( In Acidic and Alkalic Solutions )

Supervisor

Dr.Karimi

By

Reyhane Mazahernasab

Nov2012

Content

IntroductionComposition and structure of oxide surfaces in

aqueous solutionSurface Chemistry of Oxides and Adsorption

ReactionsDissolution of oxide minerals

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Introduction Our work in this area includes:

the description of mineral structures in solution

chemical reactions that take place on a surface

charge effects

dissolution process

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IntroductionSurface chemistry: a fundamental part of aqueous geochemistry there is no such thing as rock-water interactions, there are only

water-surface interactions. Everything is really happening on a mineral surface, including :[3]

Dissolution and Precipitation

Adsorption

Ion Exchange

Mineral redox

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Introduction

Therefore, once we know the surface composition, we can predict:

how pH will modify it what surfactants might will adsorb on

it how ions in solution will affect its

behavior.[2]

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Dissolution is a heterogeneous reaction that takes place at the interface between a solid and liquid phase and sometimes also a gaseous phase. At the boundary between the two phases a diffusion layer is formed. In the case of a solid in an aqueous phase this layer consists of a stationary aqueous layer. The diffusion layer can be thinned by vigorous stirring but never be completely removed. Typical thickness of the diffusion layer in a well stirred system is in the range of 1-10 μm [9]

Introduction

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Composition and structure of oxide surfaces in aqueous solutionOxides often hydrolyze in the presence of water to

form hydroxide layers at the surface, so the two classes of material behave similarly.[2]

Reason: 1. When an oxide mineral is cleaved, it exposes dangling

bonds to metals and oxygen wherever the fracture cut a chemical bond.

2. Water reacts with this surface, adding hydrogen to the oxygen atoms, and hydroxyls to the metals, resulting in a surface of metal-hydroxides, and bridging oxygens. [3]

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Composition and structure of oxide surfaces in aqueous solution

Each of these hydroxides can accept or donate a proton similar to the behavior of water, for example with quartz or silica: [3]

SiOH SiO + H

SiOH + H SiOH2+

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Composition and structure of oxide surfaces in aqueous solution

These sites are amphoteric, and each of these reactions can be characterized by an equilibrium expression, e.g.:

Where K0 is the dissociation equilibrium constant analogous to the Ka used in acid base chemistry.[3]

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Composition and structure of oxide surfaces in aqueous solution The process of hydration of the oxide surface is illustrated in Fig.1

Fig. 1 : hydroxylation of oxide surface

(a)metal oxide lattice surface with unbalanced bonds. The metal ions in the surface layer have a reduced coordination number.

(b)in the presence of water molecules, the surface metallic ions attract water molecules to restore previous coordination number in the lattice.

(c) Illustrates the fact that the attracted surface water molecules hydrate metallic ions and then , the process produces a hydroxyl surface layer. [7]

oxygen metal

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Surface Chemistry of Oxides and Adsorption Reactions

These factors largely determine the reactivity of hydrous oxides:

The combination of mineral surface type and number of hydroxyl groups and the environment in which the mineral resides valence and coordination of the structural metal

atoms. [4]

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The chemistry that occurs at the surfaces of amphoteric oxides is dominated by acid–base interactions. The acid–base behavior of surface hydroxyl groups can be illustrated in the following equation [2]:

the equilibrium in acidic conditions the equilibrium in basic conditions

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Important features of the oxide–water interface include the electric double layer, adsorption of ions and which as discussed below is a function of pH [5]

Electrical Double Layer The double layer model reflects an unequal distribution of charges at the

mineral-water boundary, and depends on the distribution of ions at the surface: [3]

1) The inner layer is the fixed charge on the mineral surface. 2) The outer layer can be composed of two types of sorbing ions:

Inner Sphere Complexes: These are solution complexes that closely associate with the charged mineral surface (chemisorption), often forming specific bonds with the mineral surface.[3] often described as specific adsorption [5]

Outer Sphere Complexes: Indicates a weaker, longer distances interaction with the surface with perhaps solvent molecules between the surface and the adsorbate[3]. as non-specific adsorption [5]


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Fig. 2: The schematic showsthree different types of adsorption processes:1) a non-specific outer-sphere

complex2) two different inner-sphere

complexes 3) the formation of a precipitate on

the surface. [5]

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STERN-GRAHAME DOUBLE-LAYER

MODEL

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The nature of the oxide surface will depend on pH according to the following equation:

In general, anion adsorption will be favored when the pH is at or below the pzc whereas cation adsorption will become more favorable at or above the pzc.

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Ion exchange:In every double layer one type of counterion can

be exchanged for another type of the same sign. Surface protons or metal ions may exchange

places for cations in solution, especially if the solution species are multiply charged. Surface hydroxyls can exchange with anions in solution in a parallel manner. If the charge on the adsorbed species is larger than the original site's charge, the original surface charge can be reversed. [2]

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Dissolution of oxide minerals

In the dissolution process which involves detachment of a surface unit into the solution, the metal complex at the interface serves as the precursor of the activated complex in transition state theory. The release of the metal from the dissolving surface is similar to the ligand exchange around solute metal complexes.

Other models such as the electric double layer have been instrumental in understanding the interaction and reactivity at the mineral–water interface[5]

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The strength of surface complexes, and thus degrees to which the bonds between surface metal centers and crystal lattice are weakened can determine the dissolution rate of metal oxides.[4]

The dissolution of metal oxides is typically the combination of several reactions,including ligand-promoted, reductive, and proton-promoted dissolutions.[4]

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Ligand-Promoted Dissolution: dissolution mechanism is usually expressed by the

following three steps: (1) fast adsorption of ligands on oxide surface by ligand

exchange of surface hydroxyl groups and formation of surface complexes,

(2) detachment of surface metal species, usually facilitated by polarization of metal-oxygen bonds within the mineral structure

(3) regeneration of the surface site and transport of the detached metal into bulk solution. [4]

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Reductive Dissolution :

Reductive dissolution involves electron

transfer between ligands and surface metal,

which is a surface-controlled process.

Adsorption reactions produce inner-sphere

and outersphere surface complexes that

could increase the density of reductant

molecules and facilitate electron transfer.[4]


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Dissolution of oxide minerals Proton-Promoted Dissolution:Mineral dissolution may also occur via a proton-

promoted pathway. Proton-promoted dissolution occurs when H+ attacks the solid surface, destabilizing the metal–oxygen bond.

The protonation of surface sites polarizes the critical bonds between metal centers and lattice, promoting the dissolution of metal oxides.[4]

. Proton-promoted dissolution is only important at low pH, but the dissolution rate is relatively low.[4]

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Dissolution Mechanisms [7] :


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The Effect of PH:mineral oxide dissolution is highly dependent

on pH. Dissolution rates of oxides increase below

pHpzc with decreasing pH and increase with increasing pH in alkaline media.[7]


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During the leaching of copper minerals, sulphuric acid and ammonia are generally the most used leaching mediums.

Typical reactions of oxidized copper ores with sulphuric acid can be given as follows:[5]


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Effect of solution pH in chalcopyrite leaching[10]

The role of the pH of are shown in Fig.3:The leaching rate was zero when the pH of the solution was less than 5.2.The leaching rate increased with the pH of the leaching solution from pH 5.3to 10.5. When the pH of the leaching solution increased from 10.5 to 11.2, the leaching rate dropped sharply.


Fig. 4: The effect of pH of the leaching solution on the leaching rate of chalcopyrite

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at pH 5.3 the ratio of [NH3] to [NH4+] is about 10-

4 .This indicates that when the pH of the solution was less than 5.3, almost all total ammonia (NH3+NH4

+) would present as ammonium ion [NH4

+];thus, there was not enough free ammonia available to leach copper from chalcopyrite.

From pH 5.3 to 10.5, the free ammonia concentration increased with the increase in pH.

Also an increase of pH or concentration of OH-

would result in an increase in leaching rate. Therefore, the leaching rate increased with the increase of pH from 5.3 to 10.5.


comparison of surface chemistry conditions between oxide and sulphide

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