Life cycle analysis of titanium

Life Cycle Analysis of Titanium

Charles Bronson MDS14M003

CONTENTS 1. Introduction

2. Goal and Scope Definition

3. Process Description and System Boundaries

4. Unit Process Description

5. Conclusion

6. References

Introduction

Titanium is corrosion resistant, very strong and has a high melting point. It has a

relatively low density (about 60% that of iron). It is also the tenth most commonly

occurring element in the Earth's crust. That all means that titanium is a really

important metal for all sorts of engineering applications.

In fact, it is very expensive and only used for rather specialised purposes.

Titanium is used, for example:

in the aerospace industry - for example in aircraft engines and air

frames;

in orthopaedics - for replacement hip joints etc.,

For pipes, etc, in the nuclear, oil and chemical industries where

corrosion is likely to occur.

Titanium is mainly extracted from rutile ores through chlorination and reduction

using Manganese (known as Kroll Process). This Report has been prepared with

the view of collecting all significant Life Cycle Inventory data for titanium

extraction (i.e. raw materials and energy use, air and water emissions, solid waste

generated). This report summarizes the cumulative inputs and outputs of

environmental significance (air emissions, waste generation, and resource

consumption) associated with producing Titanium from Rutile (TiO2)

Goal and Scope Definition

The intended purpose of this Inventory report is to characterize resource

consumption and significant environmental aspects associated with the

production of Titanium from Rutile (TiO2).

Process Description and System Boundaries

The primary aluminium production covered by this study includes the following

unit processes:

Rutile Mining

Chlorination of Titanium ore to form Titanium Tetrachloride

Reduction/Distillation

Electrolysis

Titanium Ingot Melting

The relationships between the unit process is given below in the block diagram

Unit Process descriptions

1) Rutile Mining

The operations associated with this unit process include:

• the extraction of bauxite rich minerals from the site; sand is pumped in slurry

form to concentrator

• the concentrator uses spiral circuit or wet high intensity magnetic

separators to extract magnetic limonite from non magnetic Rutile and Zircon

• Beneficiation activities such as washing, screening, or drying and minerals are

further refined and separated and then kiln-dried.

• Site restoration activities such as grading, dressing and replanting.

Rutile mining activities mainly take place in tropical and subtropical areas of

the earth. Rutile is also present in beach sand in low concentration which is

subjected to pre-concentration for obtaining a composite heavy mineral

concentrate which is subsequently subjected to physical separation into

different factions like monazites, ilmenite, Rutile, leucoxene, sillimanite,

kyanite, garnet zircon etc.

There is a significant amount of energy use in wet high intensity magnetic

separators. Solid wastes also occur in the form of residue while washing and

screening.

2) Chlorination/Chloride Process

The chloride process is used to separate titanium from its ores. In his process,

the feedstock is chlorinated at 1000 c with carbon and chlorine gas, giving

titanium tetrachloride.

2FeTiO3 + 7Cl2 + 6C→2TiCl4+2FeCl2+6CO

The process is a variant of a carbothermic reaction, which exploits the reducing

power of Carbon.

Under steady state conditions the chloride process is a continuous cycle in which

chlorine changes perpetually from the oxidized state to the reduced state and

reverse. The oxidized form of the chlorine is molecular chlorine Cl2, the reduced

form is titanium tetrachloride (TiCl4). The oxidizing agent is molecular oxygen

(O2), the reducing agent is coke. Both must be into the process. Titanium ore can

be understood as a mixture of oxides from various metals mainly titanium.

The rutile or titanium slag and coke are mixed and continuously fed into the

fluidized-bed chlorinator and are reacted with chlorine gas introduced through

the bottom of the chlorinator. The temperature of the chlorinator is maintained

at or above 1000 C and the resulting formed TiCl4 is a high temperature gas.

Impurity metals contained in the titanium slag, such as iron and aluminium are

simultaneously chlorinated and are carried by the TiCl4 gas into the cooler in the

next step.

The high temperature TiCl4 is cooled in next stages, the impurities like iron

chlorides are condensed and removed as solid wastes. The added oxygen leaves

the process with the product TiO2, the added coke leaves the process together

with the added oxygen from the titanium in the form of CO & CO2. The other fed

metals leave the process in the form of metal chlorides.

The consumption of various factors per tonne of TiO2 is as follows

Media Consumption Unit Media Consumption Unit Electricity 360 kWh Fuel Gas 2.3 GJ

Steam 0.5 t NaCl 8 kg

Oxygen 350 Nm^3 NaOH 25 kg

Nitrogen 100 Nm^3 Aluminium 6.5 kg

Compressed Air 40 Nm^3 KCl 0.05 kg

Clean Compressed Air

2 Nm^3 Mineral Oil 3.6 kg

Chlorine 350 Nm^3 Scrubbing

Agent 2 kg

Fuel Gas 2.3 GJ Ca(OH)2 500 kg

Refrigerant 0.6 Nm^3 Water 2.5 m^3

Coke 370 Kg D-I Water 2.5 m^3

Toluene 13 kg Makeup Cooling Water

4.5 m^3

NaCl 8 kg

The reacted metal is put into large distillation tanks and heated. Impurities are

separated using fractional distillation and precipitation. Distillation is one of the

most energy intensive processes. Water is used in the cooling during distillation.

There is a significant amount of by-products in the form of metal chlorides and

inert solid wastes which cannot be further processed into any by product. Air

emissions are in the form of CO, CO2 released during the reaction and NOx and

SOx gases released during fuel burning.

3. Reduction using Mg and Electrolysis

The world’s supply of titanium metal, about 250k tons per year is made from

Titanium Tetrachloride (TiCl4). The conversion takes place by the reduction of the

chloride with magnesium metal and yields titanium metal and magnesium

chloride. This procedure is known as the Kroll process. This is done at 800-850 C in

a stainless steel retort to ensure complete reduction

2Mg + TiCl4→2MgCl2 + Ti

Argon is then pumped into the container. This removes the air and prevents

contamination with oxygen or nitrogen. The magnesium reacts with the chlorine to

produce liquid magnesium chloride. This leaves a titanium solid, which is removed

by boring from the reactor and treated with water and hydrochloric acid to

remove excess magnesium and magnesium chloride.

The energy requirement is more as an 8-ton batch takes about four days. The next

step is vacuum distillation where Mg(L) and MgCl2 (L) are trapped in the titanium

are vaporized and removed by heating the titanium to about 1000 C. The resulting

solid is a porous metal called sponge. This is further purified using leaching or

distillation which adds to the energy requirement of extraction.

Vacuum Distillation

Initially in the vacuum distillation step, heat transfer occurs through Mg (L)

and MgCl2 (L). As Mg and MgCl2 diminish, the titanium sponge alone must take

care of the heat transfer. In addition to the poor thermal conductivity of titanium

itself, the presence of pores restricts the heat transfer and the vacuum distillation

rate which results in longer distillation process and greater energy requirements.

The reduction reaction generates a huge amount of heat. The industrial reaction

rate is controlled by how to remove this heat which uses significant amount of

water. The tapped Magnesium Chloride which is processed in an electrolysis plant

where it is regenerated into Mg and Cl2. The MgCl2 electrolysis accounts for 60 -

70% of the entire titanium smelting process. Electricity is a major component of the

titanium smelting cost. The power consumption of today’s electrolytic cells is

10000kwh/ton-of-Mg which is half of 1978’s level.

4. Ingot Melting Process

Titanium has a strong affinity for oxygen and nitrogen and has a high melting point

of 1670 C. Thus conventional refractories cannot be used for melting titanium; it

must be melted in a water-cooled copper crucible in a vacuum or an inert

atmosphere

Vacuum arc remelting (VAR) process

Titanium sponge, titanium scrap, additives and master alloy are press formed into

blocks weighing a few tens of kilograms. These blocks are welded under an inert

gas atmosphere to form a primary electrode of a columnar shape. The primary

electrode consists of a few tens to a few hundreds of blocks, depending on the

ingot size. The raw materials and additives are equally weighed and mixed in each

briquette.

The primary electrode is melted by the direct arc current produced between the

primary electrode and the water-cooled copper crucible connected to the anode

of the furnace. The molten titanium is solidified in the water-cooled copper

crucible to form an ingot. The ingot is melted one or two more times to produce a

homogeneous ingot. VAR process ingots generally weigh 4 to 8 tons.

Titanium readily reacts with metals to form alloys hence a temporary steel liner is

used for coating the refractory while melting titanium, this temporary refractory

lining also ends as solid waste. The general gas emissions (NOx, SOx) are due to the

fuel combustion.

Conclusion

Titanium is the ninth-most abundant element in Earth’s crust and the seventh-most

abundant metal. Titanium occurs within a number of mineral deposits, principally

rutile and ilmenite, which are widely distributed in Earth’s crust and lithosphere,

and it is found in almost all living things, rocks, water bodies and soils. Hence it is

costly due to the number of stages and the energy required in extracting Titanium

from its ore. Titanium being more reactive than carbon cannot be reduced using

carbon and hence the usage of Mg adds to the cost as well as increases the

environmental impact on the environment.

The extraction of Titanium has a strong environmental impact during mining as

well as in processing the TiO2. The majority of raw materials like Mg and Chlorine

are reused but the electrolysis is an energy intensive process. It accounts for about

60-70% of the Titanium smelting process’s energy requirements.

There has been research going around the world to find alternate method of

extracting Titanium which would be both less energy intensive and has less

environmental impact.

The majority of air emissions are from fuel combustion and also majority of solid

waste is reused or recycled with small amount of untreatable inert solids and metal

compounds like Iron Chlorides etc.,. Thus it is imperative that the major focus

should be on finding alternate methods of extracting Titanium which reduces the

energy cost as well as indirectly impacting the sustainability.

References:

1.) Technology Trend of Titanium Sponge and Ingot Production by Toho Titanium

co. ltd