How are the rare earths extracted (and why it pollutes as much)
Three minerals enrichment chemical treatment solvent comments
In Baotou in interior Mongolia (China), a toxic sludge lake recalls the environmental consequences of the electronics industry.But how do you really extract rare earths, and why is it so polluting?Here are the recipes, of course radioactive.
Bastnäsite-Cérium crystal, one of the three main minerals from which the rare earths are extracted.Credit: Buttshark // Wikimedia Commons
Baotou is the largest city in interior Mongolia, a Chinese province in the north of the country.The life of its 2.5 million inhabitants is marked by the extraction of the rare earths, out of the depths the bayan obo hundred kilometers to the north.These atoms, essential to the manufacture of many technological objects, including our smartphones, are transformed there under sanitary and environmental dystopian conditions.
Only a few kilometers from downtown Baotou sits an artificial lake (Google Maps link), made of accumulated waste from rare earth refineries.Little information filters on its exact chemical composition.Artists who made vases from the sludge from the lake reported that they were radioactive and with a high metal content.Residents of the surroundings are often afflicted with diseases, and newborns suffer a lot of malformations.
It must be remembered, the "rare earths" are not uncommon on the globe.But they are most often dispersed and very polluting to extract.If China has, it is true, a certain number of reserves, it is essentially the only one to want to pay the environmental and human cost of the treatment of these metals.95 % of extractable rare earths come from three rocks: the bastnäsite, the monazite, and the xenotime.
The three minerals
Bastnäsite is named after a Swedish mine west of Stockholm and contains carbon and fluorine, coupled with cerium, lanthane or yttrium.It also often contains neodymium and prastodymia.The main deposits are found in Mountain Pass in California, in Bayan Obo in inner Mongolia and in various farms in Sichuan.As we will see in this article, the bastnäsite fluorine forms highly harmful acids when the rock is transformed.
The name of the monazite means "which is lonely" in Greek.It is a radioactive phosphate rock which contains cerium, lanthane, neodymium and samarium, as well as significant quantities of thorium and uranium.Much extracted in Brazil, South Africa and India, monazite has lost ground against the non -radioactive Bastnäsite, but interests for projects of possible nuclear power plants.
Xenotime is also a rarer phosphate rock, mainly containing yttrium and the heaviest rare earths (gadolinium, dysprosium, terbium, erbium, yterbium, etc.).It is recovered in tin mines in Malaysia.Like monazite, it contains thorium and uranium and is therefore more or less radioactive.
Un cristal de bastnäsite-cérium. Crédit : Robert M. Lavinsky // Wikimedia Commons Un cristal de xénotime associé à de la rutile (titane). Crédit : Robert M. Lavinsky // Wikimedia Commons Un cristal de monazite-cérium (orange) dans du quartz (blanc). Crédit : Robert M. Lavinsky // Wikimedia CommonsAs with all minerals, extracting the useful elements from these rocks requires several stages.Here is step by step how it is done in the industry.We promise you, it's not that complicated on the principle, but maybe you have to hang on a bit.
Enrichment
When the ore is extracted, it does not only contain rare earths.It is full of various major impurities of size often.Enrichment (benefit) consists in ensuring that the percentage of rare earths is as high as possible.The general principle consists in grinding the ore in small pieces, then removing the ends that are useless.
The grinding is done in two stages.First, La Roche passed through a jaw crusher (Jaw Crusher).It is a large machine with a V -shaped mechanism in which the ore is reduced in gravel of less than 1 centimeter in diameter.This step generally takes place on the mine website.Then the gravels went to a ball mill (Ball Mill).A drum filled with metal balls takes care of grinding the stone in dust from 40 to 100 micrometers (or 0.04 to 0.1 millimeter, the size of a living plant cell).
A jaw crusher, in this case a LOKOTRACK LT105.Credit: Bob Adams // Flickr
At the end, you get a kind of mud made of damp dust.It is now necessary to separate the good grain from the tares through a process called flotation (Frath Flotation).The principle is the same as soapy water, where soap clings to fat particles and take them during rinsing.
We put the mud in a tank where we make small air bubbles, as in an aquarium.Then we add two types of chemicals.Depressors are attached to unwanted minerals to sink them to the bottom.And the collectors take the pieces containing rare earths to hang them on air bubbles, so as to bring them up to the surface.
When you deal with Bastnäsite, depressants are phosphoric and dicarboxylic acids;The first is used in the food industry to acidify sodas, and the second are a family that includes certain amino acids of the organization.
Collectors include sodium silicate (irritating and used in household products);hexafluorosilosilicate of sodium (toxic, but used to put fluorine in running water);or lignin sulfonate (non -toxic, widespread on the roads to prevent them from doing dust).
Among the alternative processes, magnetic separation is especially useful for monazite and xenotime where many elements react to magnets.The severity separation, a form of centrifugation, is also used but is considered less effective, because the smallest particles of rare earths are generally purged of the mixture.
At the end of enrichment, it is dehydration (Dewatering).The mineral mud has passed into a concentrator so that dust decanses.Then the latter are the subject of thermal drying.
Credit: Mickey // Flickr
Chemical treatment
The resulting dusts are chemically treated, which makes it possible to pass their purity into rare earths at 90 %.Two methods are possible, acid and basic (the opposite of acid).The chemicals used are aggressive but very ordinary, and can buy in any DIY store.We use sulfuric acid (H2SO4), hydrochloric acid (HCL), nitric acid (HNO3), caustic soda (NAOH) and soda crystals (NA2CO3).
If this step is as polluting, it is therefore not so much because of the substances used as elements that they allow to clean in dust.Here we will focus on the two main minerals, monazite and bastnäsite.
Monazite
Take the monazite.The traditional method is to put monazite in very concentrated sulfuric acid and to simmer the oven for several hours, at a temperature that goes from 120 ° C to 300 ° C depending on the recipes.The mixture is vigorously agitated, until it takes the texture of a thick paste.The dough is removed before it hardens and cooled at 70 ° C with moderately hot water.
We let a whole day soak.At the end, we recover the solution that watches (imagine monazite juice) by getting rid of the residues - a mixture of silicon, titanium, zircon and other impurities.This monazite juice is very very acidic, and we will soften it several times with ammoniac.As if by magic, we will successively see at the bottom of the mixture a cake of thorium and phosphate (radioactive, to throw);a concentrate of rare earths (to keep);and a concentrate of uranium (radioactive, to throw too).
A basic method, more recent and effective, consists in cooking monazite at 140 ° C (thermostat 5 on your domestic oven) in a caustic soda bath, then cooling the resulting dough at 100 ° C with water.This will make juice full of phosphate, which can be recovered for commercial purposes.We add hydrochloric acid to reveal Lanthane juice, recovered for its rare earths.On the bottom only sludge of thorium, titanium and zirconium, well toxic and radioactive.
Bastnäsite
Bastnäsite can be toasted with sulfuric acid by being placed at more than 100 ° C.The goal is to get rid of silicon and especially fluorine, evacuated via toxic smoke.This fluorine is mainly present in the form of fluorhydrical acid (HF) - and sometimes as hexafluorosilicic acid (H2SIF6), which returns chemically to the same.
Fluorhyder acid reacts with calcium, an omnipresent element in the human organism.In the form of smoke, it can cause pulmonary edema or make blind.Mixed with water, it easily crosses the skin, disturbs the functioning of the nerves, and can attack bones or cause heart stops.
Separation process
After this toxic drum - which produces mountains of radioactive sludge and whole clouds of fluorine to be treated - we therefore obtain a mixture of rare earths.But these are all chemically alike.When they are extracted during chemical treatment, we collect them all together in the same cocktail.
To separate them, we generally use various solvents.These are not the most polluting chemicals, except when they contain fluorine (PDF), because we can then end up with the terrible fluorithic acid of earlier.YTTRIUM is for example isolated (PDF) with naphthenic acid, the "na" of the incendiary product napalm, whose polluting effects are rather noticed around the extraction of oil.
Olive oil.
By simplifying a little, each rare earth has its favorite solvent.Imagine that you are preparing vinaigrette.You put mustard in vinegar (aqueous mixture), then you add oil.The water and the oil do not mix, so if you let your vinaigrette rest sufficiently, it is set out: you end up with the vinegar and mustard below, and at the top the oil.
In this example, solvents are like oil and rare earths like mustard.Most rare earths will remain at the bottom, except the one who likes the solvent in question and who will go up.By repeating the process several times with different solvents, we can recover each rare land, which can then be reused for commercial purposes.
It is only at the end of this process, radioactive and full of fluorhydrical acid, that yttrium and cerium can take their place in OLED screens, and the neodymium and the prastoodmus in the magnets of the speakers.Suffice to say that it is almost surprising that - unlike Baotou's air - our smartphones do not feel sulfur.
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