Titel Bar Fluoride Evolution Model
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Introduction

Fluoride evolution is often classified as gaseous and particulate. Gaseous fluorides are those that continue to be gases at ambient temperature: HF, CF4, SiF4, and C2F6. Entrained and volatilized bath become particulate at the lower temperatures above the cell. In addition, alumina and carbon dust will sorb HF and therefore contribute to particulate fluoride.

We follow here the Fluoride Evolution Model that W. Haupin and H. Kvande have developed and published in several papers (Lit. 1984, 1993 and 2002). These papers define Fluoride Evolution as the fluorine (F) content (kgF/tonneAl or gF/kgAl) of the hydrogen fluoride (HF) and of the particulate that leaves the the cell with the cell gas. Most of the evolution will be captured by the cell’s hooding and fume treatment system. That which escapes capture will be called Fluoride Emission. Hence, fluoride emissions can be calculated from the hooding efficiency (fhood: 0.95 – 0.98 for modern cells) according to Equ. 1.

Emissions Meaning of Symbols

(1)

Other sources of fluoride emissions include fluorides from bath adhering to spent anodes, fluorides from tapping ladles and fluorides from anode carbon baking furnaces. This other sources of fluorides and the silicon fluoride (SiF4) are not included in the model. Only a small amount of SiF4 is produced. It is hydrolyzed to silicon oxide (SiO2) and and hydrogen fluoride (HF) by the humidity (H2O) of the air. The aqmount produced depends on the silica content of the anodes.

Schema Fluorde Evoltion

Schema Fluoride Evoltion.

The cell gas, CO2 and CO produced by electrolysis, is saturated with gaseous fluoride species (NaAlF4, its dimer Na2Al2F8 and NaF). When the cell gas cools down these gaseous species condensate to solid fluoride particels. The humidity (H2O) of the alumina ore and the hydrogen of the anodes react with the electrolyte fluorides to gaseous hydrogen fluoride (Primary HF Formation). Liquid droplets of the electrolyte are entrained to solidify when they are cooling down. And finally water of the moist air hydrolyzes partly the solid fluoride particles to create hydrogen fluoride (Secondary HF Formation).

Fluoride Evolution

Fluoride evolution consists of particulate and gaseous fluorides. The particulate i.e. solid fluoride particles are formed by the condensations of gaseous electrolyte species namely natrium aluminum fluoride (NaAlF4), its dimer [(NaAlF4)2 = Na2Al2F8] and sodium fluoride (NaF) as well as solidified entrained liquid bath droplets. Gaseous fluoride is hydrogen fluoride (HF) that is generated when fluorides present in the bath or in the vapour phase react with moisture i.e. water (H2O) of the alumina or the atmosphere (Lit. 2011).

HF Formation

(2)

Another source of hydrogen fluoride is from the hydrogen content of the anode, either from adsorbed hydrogen or hydrocarbon (Lit. 2001).

HF from H

(3)

It was suggested that the hydrogen was initially oxidized by the anode carbon dioxide to form water, and this water subsequently hydrolysed the sodium tetrafluoroaluminate to form gaseous hydrogen fluoride. However one can not distinguish between any of the proposed mechanisms, because the anode is at a potential that enables both electrochemical oxidation of the hydrogen to water and the direct formation of hydrogen fluoride.

HF generation from hydrogen within the anode and from water introduced to the electrolyte is associated with the Primary Generation of hydrogen fluoride. The HF generation outside of the cell electrolyte i. e. the thermal hydrolysis of some of the particulate fluoride is termed as Secondary Generation of hydrogen fluoride.

Volatilization of Bath

Most of the particulate fluoride evolved from cells results from vaporization of the electrolyte. The Haupin - Kvande Fluoride Evolution model assumes that the gases produced by electrolysis (CO2 and CO) leave the bath carrying an equlibrium partial pressure of the bath species. As the the cell gas temperature falls the vapour condenses to form particulate.

The Haupin - Kvande Model of 2002 uses a relation for the total vapor pressure of the bath (VP) according to Lit. 2001:

VP of Gustavsen

(4)

with

A, B of 2002

(4A)

and the model of 1993 applied the following equation:

VP of 1993

(5)

with

A, B of 1993 Meaning of Symbols

(5A)

The electrolyte vapor contains as major species NaAlF4, its dimer NaAlF4)2 = Na2Al2F8) and sodium fluoride (NaF). The partial vapor pressure of sodium fluoride (PNaF) is given with:

Partial Pressure NaF Meaning of Symbols

(6)

For the equilibrium constant (Kp) of the dimerisation reaction

HF Formation

(7)

one finds

Kp Meaning of Symbols

(8)

and solves the quadratic equation for the partial pressure of the monomer (PM):

monomer Meaning of Symbols

(9)

and finally get the partial pressure of the dimer (PD):

dimer Meaning of Symbols

(10)

To determine the specific masses of the gaseous fluoride species that leave the cell with the carbon dioxide and monoxide gase we determine how many moles of CO2 and CO (ΣnkgAl) are produced by electrolysis. Using the expressions for the specific electrolytic productions of CO2 and CO (SEPCO2 Equ. 2.20 and SEPCO Equ. 2.21) we write:

sum moles CO2, CO Meaning of Symbols

(11)

By using the ideal gas equation we continue to determine the specific moles (nM) and specific mass (mM) of the monomer NaAlF4 species

specific mass of monomer Meaning of Symbols

(12)

and finally the specific masses of the dimer Na2Al2F8 (mD), sodium fluoride NaF (mNaF) and volatilized bath (FFP):

specific mass dimer, NaF, F Meaning of Symbols

(13)

Entrained Bath

The cell gas entrains liquid bath as droplets. As the gas cools these droplets freeze and become particulate. The crust acts both as a filter to remove entrained liquid and provide a long exit path for entrained droplets to settle out. The following equation takes care of this "catching" action with the factor fcatch. Haupin and Kvande (1993 and 2002) use fcatch = 0.9.

Entrained Bath Meaning of Symbols

(14)

Haupin and Kvande developed an equation for the surface tension (σ) from data of Lit. 1983.

Surface Tension Meaning of Symbols

(15)

Formation of Hydrogen Fluoride (HF)

As already indicated in Equ. 2 the hydrolysis of aluminum fluoride (AlF3) produces gaseous fluoride as hydrogen fluoride (HF). The moisture (H2O) reacts with AlF3 rather than cryolite (Na3AlF6) or any other bath component because the equilibrium constant for reaction of Equ. 16 is much higher.

HF from AlF3

(16)

The equilibrium constant for the reaction of Equ. 16 leads to the following expression for the partial pressure of hydrogen fluoride (PHF).

PHF from AlF3 Meaning of Symbols

(17)

Gibbs Free Energy

is expressed as linear function of temperature over the range of 1200 to 1300 K.

Delta G Fn of T Meaning of Symbols

(18)

Partial Pressure of Water

We calculate the partial pressure of water (PH2O) according to the Principal Equation (Equ. 19A) by considering the water content in the alumina ore and the hydrogen content of the anodes (Equ. 19B):

Modified Principal Equation Meaning of Symbols

(19A)

(19B)

Simplifying and solving for the mole fraction of water gives the following expression for the partial pressure of water (PH2O) in the cell gas:

PH2O Meaning of Symbols

(20)

Activity of Alumina

For the activity of alumina we use Equ. 4.3 of the chapter Reversible Decomposition Voltage.

Activity of Aluminum Fluoride

Activity data for AlF3 in the electrolyte were fitted by the equation:

Activity AlF3 Meaning of Symbols

(21)

HF Generated by Hydrolysis of Electrolyte

To determine the gaseous fluoride generated by hydrolysis of bath (FGB in gF/kgAl) we could multiply like in Equ. 12 the partial pressure of hydrogen fluoride (PHF Equ. 17) with the atomic weigth of fluorine and ΣnkgAl. However the calculated values showed to be larger than measured data. The reason is that hydrolysis does not proceed toward thermodynamic equilibrium. Equation 21 takes care of this kinetic behaviour.
Also a variable ffeed was added to take care of the alumina feeding technique. ffeed = 1 is used for point fed cells and ffeed = 0.5 for break and feed technique. With the break and feed technique ore is dumped on the crust and later the crust is broken to add the ore. Part of the humidity is driven off as the ore lies on the crust. With point fed cells more of humidity enters the bath.

HF by Hydrolysis of Electrolyte Meaning of Symbols

(22)

Hydrolysis of Cell Fume

Another source of gaseous fluordie arises from hydrolysis of NaAlF4 vapor by moisture in the air brought in by the cell's exhaust draft:

HF from NaAlF4

(23)

Treating this reaction similar to the previus reaction (Equ. 17) gives:

PHF from NaAlF4 Meaning of Symbols

(24)

The incoming air supplies moisture but also cools the cell gas, limiting hydrolysis. fhydr is an adjustable factor between 0 and 3 allow for variations in the kinetics resulting from changes in ore cover. fhydr = 1 represents average conditions.

HF by Hydrolysis of Cell Fume Meaning of Symbols

(25)

Summary

The next relations determines the total particulate fluoride (FP), the total gaseous fluoride (FD) and finally the total fluoride (FT). An adjustabe factor fcover accounts for poorly covered cells. For a good cover fcover = 1 for a poor cover fcover = 2 to 7 depending on how large the hole or holes are on the crust.

Summary Meaning of Symbols

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