Tungsten, Tantalum and Tin: Mining in Rwanda, Uses and Dangers – Extraction of Metals (Senior 6 Chemistry) | Ganzaa

Extraction of Metals

Lessons

Senior 6 · Chemistry

Extraction of Metals

4 lessons

Lesson 4 of 4

Tungsten, Tantalum and Tin: Mining in Rwanda, Uses and Dangers

25 min

The three Ts

Inside every mobile phone there is tin in the solder that joins the parts, tantalum in the tiny capacitors that store charge, and tungsten in the small weight that makes the phone vibrate. Rwanda's rocks contain the ores of all three metals, often called the "3Ts". The Rwanda Mines, Petroleum and Gas Board (RMB) lists licence blocks across the country whose target minerals are cassiterite (tin ore), columbite-tantalite, known as coltan (tantalum ore), and wolframite (tungsten ore). Most of these ores occur in pegmatites and quartz veins, and many are worked in small and medium-sized mines.

Table 4.1: Rwanda's three T minerals

Metal	Mineral	Formula	Main uses of the metal
Tin, Sn	cassiterite	$\mathrm{SnO_2}$	solder for electronics; tin plating of steel food cans; bronze
Tantalum, Ta	columbite-tantalite (coltan)	$\mathrm{(Fe,Mn)(Nb,Ta)_2O_6}$	capacitors in phones and computers; surgical implants; alloys for jet engines
Tungsten, W	wolframite	$\mathrm{(Fe,Mn)WO_4}$	tungsten carbide cutting tools and drill bits; very dense alloys; X-ray tube targets; welding electrodes

The formula $\mathrm{(Fe,Mn)WO_4}$ means that iron and manganese can replace each other in any proportion: wolframite is a mixture of $\mathrm{FeWO_4}$ and $\mathrm{MnWO_4}$ . Tin was covered in Lesson 2. Tungsten and tantalum need different methods, shown in Figure 4.1.

Figure 4.1: Extracting tungsten and tantalum. Neither metal is made with carbon, which would form hard carbides.

Tungsten

Fusing the concentrate with sodium carbonate in air turns the tungsten into soluble sodium tungstate and the iron into insoluble iron(III) oxide:

$\mathrm{4FeWO_4(s) + 4Na_2CO_3(s) + O_2(g) \rightarrow 4Na_2WO_4(s) + 2Fe_2O_3(s) + 4CO_2(g)}$

The melt is leached with water and filtered. Acid then precipitates tungstic acid, which is heated to tungsten(VI) oxide:

$\mathrm{Na_2WO_4(aq) + 2HCl(aq) \rightarrow H_2WO_4(s) + 2NaCl(aq)} \qquad \mathrm{H_2WO_4(s) \rightarrow WO_3(s) + H_2O(g)}$

Finally, hydrogen reduces the oxide:

$\mathrm{WO_3(s) + 3H_2(g) \rightarrow W(s) + 3H_2O(g)}$

Hydrogen is used, not carbon, because carbon would combine with the tungsten to form tungsten carbide, WC. The tungsten forms as a grey powder. With the highest melting point of any metal, 3422 °C, it cannot be cast in an ordinary furnace; the powder is pressed into shape and heated just below its melting point (sintered) until the particles bond.

Tantalum

Coltan is dissolved in a mixture of hydrofluoric and sulfuric acids, which turns the tantalum into a soluble fluoride complex:

$\mathrm{Ta_2O_5(s) + 14HF(aq) \rightarrow 2H_2TaF_7(aq) + 5H_2O(l)}$

Niobium, tantalum's chemical twin, is dissolved too. The two are separated by solvent extraction into an organic solvent (you will meet the partition principle in Unit 6). Potassium fluoride then precipitates potassium heptafluoridotantalate(V), which is reduced with molten sodium, a more reactive metal:

$\mathrm{H_2TaF_7(aq) + 2KF(aq) \rightarrow K_2TaF_7(s) + 2HF(aq)}$

$\mathrm{K_2TaF_7(l) + 5Na(l) \rightarrow Ta(s) + 2KF(l) + 5NaF(l)}$

The salts are washed out, leaving tantalum powder. For a capacitor, the powder is pressed and sintered into a tiny porous block, and a very thin layer of tantalum(V) oxide is grown on its huge surface by electrolysis. The oxide is an excellent insulator, so a very small capacitor can store a large charge.

Definition

Sintering: pressing a metal powder into shape and heating it below its melting point until the particles bond together. Example: tungsten wire and tantalum capacitor anodes are made this way.

Rehabilitation (of a mine site): restoring land after mining, by filling pits, levelling, controlling erosion and replanting, so that it can be used again.

Worked example

Problem. RMB reports that coltan from one Rwandan licence block averages 62.68% $\mathrm{Ta_2O_5}$ by mass. What is the maximum mass of tantalum in 1.00 tonne of this coltan, and what mass of sodium would be needed to reduce it all by the $\mathrm{K_2TaF_7}$ route? (Ta = 180.95, O = 16.00, Na = 22.99)

Solution. Mass of $\mathrm{Ta_2O_5}$ = 0.6268 × 1000 kg = 626.8 kg.

$M(\mathrm{Ta_2O_5}) = 2(180.95) + 5(16.00) = 441.9\,\mathrm{g\,mol^{-1}} \qquad \text{fraction of Ta} = \frac{361.9}{441.9} = 0.8190$

$m(\mathrm{Ta}) = 626.8 \times 0.8190 = 513.3\,\mathrm{kg} \qquad n(\mathrm{Ta}) = \frac{513.3 \times 10^{3}}{180.95} = 2837\,\mathrm{mol}$

Each Ta needs 5 Na: $m(\mathrm{Na}) = 5 \times 2837 \times 22.99 = 3.26 \times 10^{5}\,\mathrm{g}$ .

Answer: 513 kg of tantalum; 326 kg of sodium.

Worked example

Problem. RMB describes one block that produced 80 t of a mixed coltan and cassiterite concentrate containing about 4 t of tantalum and 35 t of tin. Calculate the percentage by mass of each metal in the concentrate, and the mass of cassiterite ( $\mathrm{SnO_2}$ ) that contains the tin. (Sn = 118.71, O = 16.00)

Solution.

$\%\,\mathrm{Ta} = \frac{4}{80} \times 100 = 5\,\% \qquad \%\,\mathrm{Sn} = \frac{35}{80} \times 100 = 43.75\,\% \approx 44\,\%$

$m(\mathrm{SnO_2}) = 35 \times \frac{150.71}{118.71} = 44.4\,\mathrm{t}$

Answer: about 5% Ta and 44% Sn; the tin is in about 44 t of cassiterite, so more than half of the concentrate is cassiterite. The figures are approximate, as given.

Tracing minerals from mine to market

Minerals from the Great Lakes region must be shown to come from legal, conflict-free mines. Bags of concentrate are sealed and tagged at the mine so that their origin can be traced, and exporters apply to RMB through its Mineral Certification Information System for a Mineral Export Certificate or an ICGLR (International Conference on the Great Lakes Region) certificate.

The dangers of extracting metals, and how to prevent them

Table 4.2: Problems caused by mining and extraction, and preventive measures

Problem	Cause	Preventive measures
Land degradation and erosion	clearing vegetation and digging pits on hillsides; heaps of waste rock	rehabilitation: filling pits, levelling, terracing and replanting trees
Water pollution and siltation	washing ore releases mud and fine particles into streams	settling ponds; recycling wash water
Acid rain	$\mathrm{SO_2}$ from roasting sulfide ores	capture $\mathrm{SO_2}$ and convert it to sulfuric acid
Climate change	$\mathrm{CO_2}$ from coke, carbon anodes and electricity from fossil fuels	recycling metals; energy efficiency; renewable electricity
Toxic chemicals	HF in tantalum processing; fluorides from aluminium cells; alkaline red mud	closed systems; scrubbing waste gases; lined storage ponds
Injury and disease among workers	collapsing pits and tunnels; rock dust, which causes the lung disease silicosis; noise	timber supports; wet drilling and dust masks; helmets and other protective equipment; training

In Rwanda these duties are law. Under Law N° 072 of 2024 on mining and quarry operations, a licence holder must submit an environmental and social impact assessment before starting work (Article 38); must rehabilitate the land, filling in boreholes and excavations, planting trees, removing buildings and levelling (Article 39); must have a rehabilitation plan with a budget and provide a rehabilitation guarantee (Articles 40 and 41); and must protect the health and safety of workers (Article 42).

Warning

Hydrofluoric acid is one of the most dangerous chemicals in industry. It passes through the skin without much pain at first and attacks the calcium in bones and blood. Anyone splashed must wash with plenty of water at once and get emergency medical care immediately; calcium gluconate gel is applied by trained first-aiders. It is never used in school laboratories.

Tip

For the reducing agent, think "which is strong enough, and does it spoil the metal?" Carbon for Fe, Zn and Sn; electrolysis for Na and Al; hydrogen for W and sodium for Ta, because carbon would form carbides with them.

Periodic Videos (University of Nottingham): Tungsten (2 min). Watch for how dense tungsten is and for its extremely high melting point.

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Practice

Name the ores of tin, tantalum and tungsten found in Rwanda, and give one use of each metal.
Why is tungsten(VI) oxide reduced with hydrogen and not with carbon? Write the equation.
Why is tungsten made into objects by sintering the powder instead of by casting the molten metal?
Write the equation for the reduction of $\mathrm{K_2TaF_7}$ by sodium. What is the oxidation state of tantalum before and after?
Calculate the volume of hydrogen, measured at room temperature and pressure (24.0 dm³ mol⁻¹), needed to make 1.00 kg of tungsten from $\mathrm{WO_3}$ . (W = 183.84)
Explain how a settling pond reduces the harm done to a river by washing ore.
State three duties that Rwanda's Law N° 072 of 2024 places on a mining licence holder to protect the environment.
Evaluate the benefits and costs of small-scale mining of the 3Ts to a rural community in Rwanda.

Show answersHide answers

Cassiterite (tin): solder or tin-plating cans. Coltan (tantalum): capacitors in phones. Wolframite (tungsten): tungsten carbide drill bits and cutting tools.
Carbon would react with the tungsten to form tungsten carbide, WC, so the metal would not be pure. $\mathrm{WO_3(s) + 3H_2(g) \rightarrow W(s) + 3H_2O(g)}$ .
Tungsten melts at 3422 °C, higher than any ordinary furnace can reach, so the powder is pressed into shape and heated below the melting point until the particles bond.
$\mathrm{K_2TaF_7 + 5Na \rightarrow Ta + 2KF + 5NaF}$ . Ta is +5 in $\mathrm{K_2TaF_7}$ ( $2 (+ 1) + x + 7 (- 1) = 0$ ) and 0 in the metal.
$n(\mathrm{W}) = \frac{1000}{183.84} = 5.440$ mol; $n(\mathrm{H_2}) = 3 \times 5.440 = 16.32$ mol; volume = $16.32 \times 24.0 = 392$ dm³.
The muddy wash water is held still in a pond, so the suspended particles settle to the bottom. The clearer water can be reused or released, so less silt reaches the river, which stays clear for fish, farms and drinking water.
Any three: carry out an environmental and social impact assessment before starting; rehabilitate the land (fill pits and boreholes, plant trees, level the ground, remove buildings); have a rehabilitation plan with a budget; provide a rehabilitation guarantee.
Benefits: jobs and income, export earnings, skills, roads and other services. Costs: land lost to farming, erosion and silted streams, dust and accidents that harm health, and damage left behind if the land is not rehabilitated. A balanced answer notes that the benefits last only if the law's rehabilitation and safety duties are actually carried out.

1The three Ts

2Tungsten

3Tantalum

4Tracing minerals from mine to market

5The dangers of extracting metals, and how to prevent them

The three Ts

Tungsten

Tantalum

Tracing minerals from mine to market

The dangers of extracting metals, and how to prevent them