Rare Metal Detecting

Article by Martin Pitt CEng FIChemE

The conversion of rocks to metals is quintessential chemical engineering which created the technological society. Martin Pitt recalls some of the less common metals he has known

TITANIUM was discovered in 1791 by Cornish vicar and mineralogist William Gregor (1761–1817) who named it manaccanite after the parish in which he found the rock. It was later discovered that the element that was named titanium by German chemist Martin Heinrich Klaproth (1743–1817) in 1795 was the same.

The metal was first isolated in 1910 by New Zealand metallurgist William Hunter (1878–1961) but did not appear very promising in its mechanical properties. It was only when very pure samples were obtained in the lab in 1925 that its true nature began to be understood. Someone who did was Luxembourg metallurgist William Justin Kroll (1889–1973), developing a process in the 1930s. In 1938 he filed a patent in the US and showed samples to American industry, who were not interested. Nevertheless, in 1940 he wisely fled his home country for the US before the Nazi occupation, going on to apply for US citizenship.

The metal is a sort of super aluminium: light, strong, and corrosion-resistant. It has the highest strength to weight ratio of all the metals and a high melting point of 1,668°C. It is used in high-temperature alloy applications in aerospace and the chemical industry, as well as some corrosive situations such as seawater-cooled condensers in the oil industry with sulfide on one side and chloride on the other.

The industrial process starts with the purest available titanium oxide TiO2 from the mineral rutile. Heated with carbon and chlorine it gives the tetrachloride TiCl4 as a vapour which is condensed and then distilled. This is then reduced with another metal of lower melting point which can therefore be separated from the titanium sponge. The Kroll process used magnesium, beginning production in the US in 1948, once the potential was realised (and after a seven-year battle by Kroll to get his patent back, having seen it seized under the Alien Property Act when the US joined World War Two).

Meanwhile back in the UK, IMI (originally Imperial Metal Industries) was a non-ferrous metal company who made a lot of heat exchangers and became interested in the new metal. It developed its own process using sodium (as Hunter had done) instead of magnesium, and the UK titanium industry began in 1955.

I worked for a while at the special metals division of IMI Birmingham, on what was actually a historic site. The company made a lot of tubing in different alloys for heat exchangers. There was therefore no surprise when the government sponsored a tube alloys project there during World War Two. In fact, it was where membranes were developed and manufactured for gaseous diffusion enrichment of uranium for the atom bomb, which was a key part of what became the Manhattan Project (see TCE 994).

When I arrived, I was impressed by the technology used both to make titanium and to convert it into alloy products such as heat exchanger tubes with fins to increase surface area – the high melting point and its chemical properties had to be considered.

My main work was concerned with surface treatment and coating of titanium. This involved preparing the surface by etching and controlling the growth of the oxide layer which formed on exposure to air or could be created in an oxidising solution. Thin films of oxide are coloured due to interference, and I grew quite skilled in producing different effects, even making some small pendants from offcuts for members of staff. This led to the local art college requesting instruction from me, which the company agreed to. Coloured titanium is now an established form of jewellery.

IMI produced platinum-coated titanium electrodes for chlorine manufacture by electrolysis of brine. My first task was to try to replicate an oxide film which had been produced by a student on summer placement, which was nearly as good as a platinised one in preliminary tests – potentially a huge saving. I took his lab notes and tried to work out what errors he might have made. Eventually I was permitted to do what I had asked for in the beginning and cut off a small sample from the tiny test electrode for analysis. His mistake had been to take a platinised one instead of a plain one.

Titanium was first discovered by William Gregor and originally named manaccanite, coloured titanium is now an established form of jewellery

Zirconium and hafnium

At IMI I had been amused to see workshop bins marked “waste zirconium” and “waste hafnium”, elements I had barely heard of. As they were expensive, they were recycled, but they had to be kept apart to avoid contamination, which could ruin some products.

Zirconium was also identified by Klaproth in 1789 from the mineral zircon, and its production devised by Kroll in 1945, using ZrCl4 instead of TiCl4. Its existence and properties were previously and correctly predicted in 1879 by Russian chemical genius Dimitri Mendeleev (1834–1907) by a missing space in his Periodic Table of the Elements in the same group as titanium. Mendeleev also predicted another element, now called hafnium, after Hafnia, the Latin name for Copenhagen. It was in the Danish capital that it was identified by X-ray spectroscopy in 1923 by Dutch physicist Dirk Coster (1889–1950) and Hungarian chemist Georg von Hevesy (1885–1966), and isolated in the lab in 1924.

Zirconium is used in alloys in the process industries for situations where it shows superior corrosion resistance to titanium. Hafnium also occurs in zircon, at around 2%. For process use, there is no need to separate them as their properties are so similar.

However, key applications are in nuclear reactors. Hafnium is relatively opaque to neutrons, so is used in moderator rods to slow down the reactor. Zirconium is relatively transparent so is used as fuel container and needs to be free of hafnium. In addition, hafnium’s scarcity means it is much more expensive, so it is vital that these two elements are not mixed. It is also used as a minor component in alloys and in microelectronics.

Separation is difficult, mainly achieved by liquid-liquid extraction of complex salts. More nuclear reactors will mean more demand for hafnium-free zirconium in the future.

AdelaChalupova, CC BY-SA 4.0
Zirconium’s key application is in nuclear reactors. It is relatively transparent so used as a fuel container

Ruthenium

This was named after Ruthenia (Latin word for Russia), and isolated in 1844 by Russian chemist Karl Ernst Klaus (1796–1864) after earlier inconclusive suggestions of its existence. It is hard, grey, and brittle with a high melting point of 2,450°C. It is one of the platinum group metals, found in the same ore. Basically, gold, platinum, and palladium from smelted ore are dissolved in aqua regia, leaving rhodium, iridium, ruthenium and osmium, which are then separated in several stages.

Being a lot rarer than platinum it was not going to be thrown away but did not appear very useful in the quantities available. In 1941, the revolutionary Parker 51 fountain pen was introduced, the first in which the ink dried by soaking into the paper, binding with it and evaporating isopropanol. From 1943, the nib had the very tip coated with ruthenium, resisting both abrasive wear and the corrosive ink. There were few other applications.

By the 1960s, economic uses were being sought for accumulated world stocks. The precious metals company Johnson Matthey offered chemists 100 g of ruthenium chloride RuCl3 to play with, in the hope of finding new worthwhile reactions, so long as they returned waste for recycling.

This is where IMI came in, noting a feature of its electrochemistry which meant it should be energy efficient in a chlorine-generating anode and therefore even better than platinum in electrolysis of brine. I worked on techniques for getting a thin, adherent, and stable (under electrolysis) ruthenium coating on titanium, what is now known as a mixed metal oxide (MMO). The final technique used a solution of a ruthenium compound in an organic solvent, applied by paintbrush to the etched electrode before baking in a furnace.

I was known as an amateur artist, so when I left, I was kindly provided with a quality set of art paints. Inside was an additional tube of ruthenium paint.Decades later, as an academic, I was approached by someone from industry to act as a consultant on the flow design for a new hospital device to generate fresh hypochlorite sterilising fluid at the touch of a button by electrolysing salt water. He told me the electrodes were secret, so I said I guessed (correctly) the anode was ruthenium oxide on titanium, which somewhat surprised him. Instead of being a consultant I got him to take on a student, who joined the company at the end of successful development.

Chemists did indeed find many interesting reactions with ruthenium compounds as a catalyst. In 1992, American chemist Robert Howard Grubbs (1942–2021) published a paper on the first of a series of organoruthenium compounds, which allowed a variety of olefin compounds to be produced by green chemistry (low waste, low energy processes). He formed a spinoff company to market what became known as Grubbs catalysts in 1998, going on to receive the Nobel prize for chemistry in 2005.

Today, ruthenium is used in catalysts, as a hardening agent for platinum, and in various roles in electronics, but a major use is still in electrodes for chlorine production and some other electrochemical processes. MMOs are an extensive technology.

Top to bottom: 1948 Parker pen advertisement – from 1943 the nibs had the tip coated with ruthenium to resist abrasion and the corrosive ink; 99.99 fine ruthenium crystal

Niobium

English chemist Charles Hatchett (1765–1847) was sent a sample of a new mineral from America which he therefore named columbite (after Columbia, the poetic name for the US). In 1801 he identified a new element similar to tantalum which he named columbium. However, others claimed that it was the same as tantalum, and tantalite ore was claimed to have new elements niobium, polonium, ilmenium, and dianium. In 1866, Swiss chemist Jean Charles Galissard de Marignac (1817–1894) demonstrated that columbium was the same as niobium and the others were mixtures of niobium and tantalum. In 1951, it was internationally agreed that the name niobium and symbol Nb should be used. However, American chemists and metallurgists insisted on using columbium and Cb for decades after.

It is a hard, high-melting (2,477°C) metal, and the first use was in 1906 in incandescent lamp filaments, where it was quickly replaced by higher-melting tungsten. In the 1920s it was found to be a useful hardener for steel, and this is still its primary use. Newer ores enabled larger quantities at lower cost, and a range of “superalloys” were developed including for arduous chemical or temperature environments such as rocket motors, which is its next most common use today. During the Space Race and Cold War, the US considered it a vital resource, so it was flown rather than travelling by ship. In the 1960s, it was discovered that it could make superconducting alloys.

My last project at IMI was to trial electroplating niobium onto titanium in molten salt. Having built the apparatus, my supervisor rejected and returned the NbF3 I had ordered from the US because it was labelled columbium fluoride. I left shortly after, so did not see the result.

Top to bottom: The hard, high-melting metal niobium; The distillation of quicksilver, an illustration taken from De re metallica (Latin edition) published in 1621

Mercury

As quicksilver it was known to ancient civilisations, because it is found as the metal in certain rocks and is relatively easy to extract by heating its ore cinnabar HgS in the absence of air and condensing the vapour. However, the vapour is a cumulative poison – slaves and criminals sent to Roman mercury mines were being sentenced to death. Mercury nitrate (Hg(NO3)2) was used to felt cloth in hat manufacture, giving rise to mental problems, referenced by the Mad Hatter in Alice in Wonderland by Lewis Carroll (Charles L Dodgson, 1832–1898).

Mercury dissolves precious metals, so is used to collect tiny particles of gold, silver, or platinum from ore. The mercury is then distilled off, leaving the metal. Unfortunately, this is often done in a crude way, so thousands of people still suffer health damage today from its use in extracting gold, either directly or from its passage into the environment.

It reacts to make compounds called amalgams, and one containing silver and tin has been used to fill teeth since the 7th century in China. Modern dental amalgams provide very little risk to the patient but are a significant pollution issue in crematoria.

The electrodes I was developing were to be used as the anode in the Castner-Kellner brine cell where the cathode was a pool of mercury, which could attack platinum. Chlorine would be released from my electrode, sodium would go into the mercury, which would be pumped contacted with water in another vessel to give NaOH and recycled. As a result, my electrodes had to survive a mercury dip test, which they did much better than platinum. At the time this was the major industrial use of mercury, but the process has since been replaced by membrane cells.

As children we played with a toy called a mercury maze, but such things have been banned because of the health risks, along with lead soldiers (See TCE 990/991). My brother was very keen on American superhero comics. One was Metal Men: six intelligent shape-changing robots with characteristics according to the metals they represented. The artist made Mercury Man red like the liquid in many thermometers, which of course contained coloured alcohol, not mercury, but are cheaper and safer.

I was once called to deal with an incident when two technicians carrying what they thought was a heavy chart recorder tilted it and spilled several kilograms of mercury from the inside. It was a process gas analyser using mercury to move a sample from the process to the detector. It took us a long time to clear up.

A sad experience was when a friend showed me samples of handwriting from a university technician: first when applying for a job; and years later very shaky, applying for compensation because of mercury vapour poisoning. He had worked on a wind tunnel which had many mercury manometers and there were spills in a closed dark area which he often went through. In my academic career I did all I could to eliminate mercury manometers and thermometers from laboratories, and to persuade people of the danger of spillages. Fortunately, such items are now banned from sale to the public and are not sold by laboratory suppliers in the UK and most of Europe.


Martin Pitt CEng FIChemE is a regular contributor. Read other articles in his history series: https://www.thechemicalengineer.com/tags/chemicalengineering-history

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