Southern Nevada Section

Field Trip to Rare Earths Mine

Molycorp Mine Tour Report - ACS Las Vegas Section

Mountain Pass CA - February 7, 2014

Tour Organizer:  Tom Gill

Molycorp Host:  Kelton Smith

Report:  David Batchelor, John Gerlach

 

The mine produces a rare earth element (REE) ore from the Sulfide Queen ore body.  The REE's occur as the mineral bastnasite, a REE carbonate fluoride (MCO3F).  The ore is crushed and ground to liberate the mineral grains.  The ground ore slurry is heated, a collector and a frother are added and the bastnasite recovered by froth flotation.  The bastnasite is then dissolved in hydrochloric acid and the REE's separated from each other by a combination of solvent extraction and selective precipitation.

 

Tom Gill, Chair, Kelton Smith, Molycorp host.

Geology:  The Sulfide Queen is a 1.4 b.y.o. igneous carbonatite intrusion into a complex set of gneisses.  The ore consists of 60% carbonate (calcite, dolomite, siderite and ankerite), 20% sulfate (barite, celestite), 10% bastnasite (REE fluorcarbonate), and 10% silicate (mostly quartz).  Supergene hematite deposited along fractures gives the ore it predominate rust color.  The ore body has 750 m of known strike, is roughly 75 m wide, and dips to the SW at 45o.  Mining is via an open pit, which can be seen on Google Earth at 35o 28' 40" N and 115o 31' 57" W.  The ore was discovered in 1949 by two uranium prospectors from Goodsprings, one of whom was an employee of Molycorp, who noticed the elevated radiation levels.  When he submitted a sample of what he hoped was uranium ore to the Boulder City, NV US Bureau of Mines office, he had instead found the best REE ore they'd seen.  REE ores generally contain either abundant light end REE (La - Eu 96 to 99%)  with little (1 to 4%) of the heavy REE or with a relatively abundant Y (65%) component the light REE (10%) and a more abundant heavy end REE (25%).  The natural abundances for the rare earths follow the odd/even rule La 33%, Ce 49%, Pr 5%, Nd 12% Pm 0%, Sm 1%, Eu 0.1% ....Lu 0.0001%.  The Molycorp ore is of the more common variety with large amounts of light end REE and almost no heavy end REE.  The Mt. Pass ore averages 8% to 12% REE oxide equivalent, with some places over 60%.  This is a very high grade ore.  It is said that the mine is the highest REE deposit in the world and that the tailings and waste rock dumps are the second and third highest REE deposits.  Bastnasite as a mineral has another advantage over most of the other REE mineral forms which are phosphates.  Because it is not a phosphate it has a much lower thorium and uranium content.  Once the current standards for controlling radioactive waste were adopted (1980's) non-bastnasite mines closed.

 

Heavy end REE ores all contain large amounts of yttrium (Y).  This is a result of the lanthanide contraction.  The REE ion sizes for the plus three oxidation state found in nature steadily decrease with atomic number.  The result is that Y3+ has an ionic radius similar to Dy3+.  Hence, the heavy end REE's are enriched in the mineral deposition process relative to the light end REE's.  A typical heavy REE ore element ratios would be 65% Y, 5% each Gd, Dy, Er, Yb, 1% each Tb, Ho, Tm, Lu, and 4% to 2% each La, Ce, Pr, Nd.  This rarer type of enrichment in heavy REE due to co-deposition with Y is seen in both the common phosphate deposits (monazite, apatite, and xenotine) as well as the rarer carbonate deposits (bastnasite).

 

Open Pit:  Bottom is about 400 feet below photographer.

Mine History:  One of the two discoverers was an employee of Molybdenum Corporation of America and suggested that the company stake claims to the deposit.  Production of cerium for ferrocerium lighter flints began in 1952.  Cerium was also used in ceramic and glass as a color additive.  Then (1960's) europrium production started to supply red phosphor for color televisions.  With a cheap source of Ln2(CO3)3 it was now possible to produce high refractive index lens for eyeglasses and cameras.  In the 1970's fluid catalytic cracking (FCC) using lanthanum and to a lesser extent cerium were used in combination with reforming to convert low grade feedstock into no-lead gasoline.  Later (1980's) cerium oxide use in automobile catalytic converters would greatly reduce the use of precious metals like platinum and palladium.  As a result of these petroleum production and destruction uses Union Oil of California bought Molycorp in 1977.  In response to the "Cobalt Crises" in the 1970's an alternative high strength low weight magnet was discovered NdFeB.  The production of neodymium and to a lesser extend praseodymium, samarium and dysprosium stated in the 1980's.  Neodymium became the major source of revenue.  These magnets use 2 tons Nd for a giant wind turbine generator, a few grams for a power door lock or power window in a car, and only a fraction of a gram for the vibrator in your cell phone.

 

The 1990's arrived and events were mostly positive.  Expanded production at Mt. Pass benefited from the economics of scale.  A consolidation of downstream processing from Pennsylvania, Colorado and elsewhere further reduced production costs.  Production costs could be further reduced if a natural gas pipeline were built to the site.  It would be used to replace the use of electricity and fuel oil for process heating.  Most of the technology was obsolete as was the equipment design.  These needs could be met by doing a little research and replacing equipment as it wore out.  Everything seemed normal.  Most of the non-bastnasite mines were closed.  Only the Chinese remained.  They were held back by using a primitive sulfuric acid leach process.  Then massive amounts of barium chloride were used to make the switch to a chloride system prior to solvent extraction.  There were a few problems on the horizon.  The tailings pond had enough space to last until 2000+.  Building a new one at a new location would take a year.  In 1992 an environmental impact study was started and filed with the government along with the design for the new tailings pond.  The permit was approved in 2005.  More serious were the Chinese.  In the late 1990's the Chinese decided on a different marketing strategy.  In a classic economics textbook style for creating a monopoly they would offer customers a very low price if they would buy all of their supply from them.  At first Molycorp offered to meet the lower price.  Selling ceric oxide at $3.50/lb and making a couple of dimes was one thing.  Selling it for more than $1/lb less than the production cost was insane.  Lanthanum was already being dumped in a surface pit because there were no buyers at $1.00/lb.  Most customers were committed to having at least two suppliers.  Some customers, such as, General Electric wanted to invest in the mine if it could have the neodymium production to make magnets for MRI machines.  It was a very dynamic time.

 

The mine was not to survive these exciting times.  First overburden mining was stopped.  Then in 1998 individual metal separation was stopped.  The only product left was the 25% of the bastnasite that was acid washed and sold as an abrasive for glass polishing.  Operating staff dropped from the 350 level to the 50 level.  Eventually the exposed ore would run out.  The tailings pond would run out of space just before the last of the ore was processed.  Who would have thought that the new tailings pond project started 10 years before it was needed would not even receive a construction permit until 13 years had passed (2005).  The mine closed completely in 2002.

 

A change of owners occurred at the end of 2005 when Chevron bought Unocal.  In 2008 Chevron sold the mine to Molycorp Minerals LLC.  The new company was founded solely to buy and operate the Mt. Pass mine.  Chevron retained ownership of off-site structures, i.e., primarily the 15 mile industrial sewer line from the plant to the evaporation ponds at Ivanpah dry lake.  These structures are all being dug up and removed.  Surprisingly there is relatively little problem with all the wastewater discharged to this facility or the evaporite deposits.  The bulk of the salt is sodium chloride.  The REE salts are plus three metals and behave in the environment like other plus three metals such as aluminum and iron.  (LaCl3 LD50 4.2 g/kg, NaCl LD50 3.75 g/kg).  Prior to discharge the plant wastewater used to be neutralized with lime.  This step also scavenged REE's as a hydroxide precipitate at neutral pH.  This water is saturated with calcium and strontium.  As the water flows 2700 feet vertical downhill it picks up carbon dioxide from the atmosphere and the slowly forms a layer of Ca and Sr carbonate scale on the inside of the pipe.  So far so good.  However, there is a trace amount of radium in the water (not enough to be actionable amounts according to environmental regulations).  A small fraction of this radium co-precipitates with the other carbonates to form pipe scale.  It is this pipe scale that has actionable levels of radioactivity from the radium.  To prevent plugging this pipe scale needs to be removed periodically.  The scale is discharged to the evaporation pond and settles near the discharge pipe.  Scale has also been discharged to the desert when the pipeline ruptures during pigging.  If the pipeline is not pigged regularly the scale pushed ahead of the pig builds up to the point that it blocks the forward movement of the pig.  This scale accumulation ahead of the pig blocks the flow of water.  The pressure builds up to the bursting pressure of the pipeline joints (300 psi) and a water/scale slurry is discharged to the desert.  The pipeline is not strong enough to withstand being filled completely full of water from the 4700 foot level of the mine to the 2000 foot level of the evap. ponds.  It isn't pigged often enough.  There are no sensors to detect water pressure build up during pigging.  The water disappears into the ground and is gone forever.  The flake like scale stays on the surface like so many cornflakes.  Scale removal is pretty simple, just vacuum it up.  Scale detection is easy, just use a Geiger counter.  The alluvial material in the valley has little radioactivity relative to scale.  The scale radioactivity is more similar to that of the ore zones on the mountain.

 

 

  Bottom of Open Pit .  Shovel, Drill and Water Truck.

Mining:  The current mine operates in a typical drill, blast, load, haul method.  The mine is a small but typical open pit operation with 30 foot benches.  It is roughly 400 feet deep and 1000 feet across.  Blast holes are drilled at the rate of about 1 foot per minute to a depth of 35 feet.  Blasting uses ANFO (ammonium nitrate fuel oil).  ANFO is ignited in each hole by two burster charges, one at the bottom and the other about half way up.  Very little rock is cracked on the outward compression shock wave.  It is the tension wave reflecting from the face of a bench that cracks the rock.  Then the developing gas pressure pushes the rock slightly up and sideways.  Fly rock is not desired.  After drilling a blast pattern, cuttings from every blast hole are analyzed to determine REE grade and the ore or waste classification.  Ore grade and position are entered into the master mine layout in a computer.  The GPS unit on the shovel tells the computer the location of each scoop it takes.  A few calculations by the computer are then used to tell the operator if it is waste or ore and the grade of the ore.   Hauling is by either 70 ton or 100 ton trucks.  

Concentration:  The mill is a crush, grind, float operation that separates the bastnasite mineral from the gangue.  A three stage crushing circuit turns everything into road gravel.  Water is added and a ball mill grinds the ore to flour like consistency.  The goal is to separate the individual mineral grains from each other.  The ground ore slurry is heated.  A collector (fatty acid) changes the bastnasite surface to hydrophobic.  A frother (2-ethylhexyl alcohol or similar) generates a foam.  Air is sparged into the bottom of each float cell.  Bastnasite grains attach to rising bubbles and form a bastnasite rich foam that continuously overflows the rougher cells.  This rough concentrate is reground and further purified in the cleaner and scavenger cells.  The gangue does not float and exits the circuit to the tailings impoundment.  The bastnasite concentrate is filtered and sent to the cracking and metal separation plant.

 

Chlor-Alkali Plant.  Salt generated in the process is recovered, purified and used to produce chlorine, sodium hydroxide and hydrochloric acid.  An interesting fact about this plant is that its capacity limits the mine size.  At 50 megawatts of electricity the natural gas fired power generation plant is the maximum that is regulated at a co-generation plant.  Above that the plant would be regulated as a power plant.  This difference is large enough economic disadvantage to preclude not only a larger plant but even connecting this plant to the power grid.  

Primary Metal Separation:  In the new process the bastnasite mineral is reacted directly with hydrochloric acid to dissolve it.  This process is only partially effective as insoluble REE fluorides with form.  The REE fluorides can be converted to hydroxides with sodium hydroxide.  These hydroxides are then dissolved with the residual acid from the first step.  The Mt. Pass mine is the largest user of hydrochloric acid west of the Mississippi.  Once the REE's are in neutral solution as plus three chlorides the real chemistry begins.   There is little in the way of differential chemical properties as one goes across the REE row in the periodic table.  The f orbital series is buried deep in the plus three ion.  Cerium can be oxidized to Ce4+ and precipitated as CeO2 to remove it from the other plus three ions.  Europrium can be reduced to Eu2+ with a Jones reductor and then selectively precipitated.  The primary method for separating each metal from the other is solvent extraction (SX).  The old plant had 7 SX circuits for separating groups or individual metals from the others. 

 

The new plants primary SX circuit uses a new extractant that is like DEPHA on steroids.  What could that be?  DEPHA is Bis (2-ethylhexyl)phosphoric acid.  From HSAB theory the REE plus three ions are all on the hard acid end of the spectrum.  The organo-phosphorus hard acid characteristic decreases in the following manner phosphinic (R2PO(OH), phosphoric (RO)2P)O(OH), thiophosphoric (RO)2PO(OH), dithiophosphoric (RO)2PO(OH).   DEPHA is a phosphoric acid compound.  If the new reagent is DEPHA on steroids then a phosphinic acid would seem to be logical.  A google search reveals that Bis (2, 4, 4 trimethylpentyl)phosphinic acid is being sold commercially as Cyanex 272 for purposes of solvent extraction.  The solvent is a specially prepared product from the naphtha refinery stream (other products are diesel, kerosine, jet fuel, etc.).  Aromatic and volatile components are removed to produce an aliphatic (low water solubility, fast phase disengagement) high flash point solvent.  In any event, the first separation consists of 48 solvent extraction cells.  That many cells are needed because the separation factors between adjacent REE's are only 1.5 to 2.5.  Roughly the aqueous phase and the organic phase flow counter current to each other.  In SX-1 the light ends are separated from the heavy ends.  A second circuit SX-2 is used to separate the light ends.  The heavy's Gd + are not separated.  Each cell has a mixer to establish an equilibrium (10 to 15 min.) between the two phases and a settler to separate them after mixing.  The actual design and operation is very complex relative to a single metal (copper) or a double metal (nickel-cobalt) SX circuit.  To get a visual idea of what happens imagine 48 cells operated at total reflux (batch mode).  Start by loading all cells with the same REE in the organic and aqueous phases.  Eventually after many cycles bands of concentrated individual REE develop across the cells.  Lanthanum would be on the aqueous raffinate end and Lutetium would be on the pregnant liquor end.  The Chinese are supposed to have a 300 cell circuit that does this on a continuous basis.  More practical is to operate individual circuits with fewer cells and focus on one particular aspect of the separation.  At Mt. Pass SX-1 has as a primary function the separation of the light REE from the Heavy REE.  The SX-2 circuit separates La from Ce and from Pr/Nd. 

 

All of the primary products have to be purified to various degrees and converted into final products.  For instance, the initial refined product might be a REE carbonate precipitate.  Customers might desire variations such as the acetate, chloride, fluoride, hydroxide, nitrate, oxide, oxalate, etc.  

 

Bastnasite Concentrate Cracking Plant.  Hydrochloric acid at 6N is added for the initial step in converting to a soluble REE chloride solution. 

 

Primary Solvent Extraction Plant:  Two SX circuits of 48 cells each are used for the primary separation of the individual elements from each other.

REE Uses:  On a weight basis, cerium is produced in the largest amount.  Cerium oxide is used as a top coat on alumina in automobile exhaust catalysts.  This greatly reduces the amount of Pd and Pt used in the catalysts.  Almost as much Lanthanum is produced.  Lanthanum carbonate is used for making glass.  The lanthanum addition results in a glass with high refractive index.  This means that thin light weight lens can be made where weight is important such as in eyeglasses and camera lens.  Lanthanum and to a lesser extent cerium are also used as a catalysts for petroleum refining.  For instance, fluid catalytic cracking (FCC) breaks down high boiling fractions, the pieces of which can be reformed into more desirable products in the gasoline range.  Lanthanum is used in the anode of NiMH rechargeable batteries.

 

In the 1960's the samarium cobalt magnet was developed. Two common formulations are SmCo5 and Sm2Co17.  They replaced alnico magnets where high magnetic strength was desirable.  In 1983 the lighter, cheaper, stronger neodymium iron boron magnets (Nd2Fe14B) were invented.  Neodymium is more than 10 times as abundant as samarium.  These magnets have roughly twice the magnetic energy product as the SmCo magnets.  However, they had less corrosion resistant and had a lower operating temperature, under 100 C vs 300 C.  They were plated (usually nickel) for corrosion resistance.  Later it was found that replacing some of the neodymium with cheaper praseodymium could increase corrosion resistance.  Replacing from 1 to 12 percent of the neodymium with dysprosium, erbium or holmium will increase the high temperature operating limits of the magnet.  Dysprosium is the more abundant so it is typically used.  The magnet cost is very high, hence, the use is restricted to uses like jet engines and rockets.  The magnetic effects are a result of the unpaired 4f electrons.  These electrons are embedded in the atom and shielded by the 5s and 5p electrons so they have little impact on the chemistry. 

 

Many of the REE can be used to color glass or ceramics.  However, only praseodymium is abundant and cheap enough to be widely used in this capacity.

 

Gadolinium and Dysprosium have even atomic number abundance and high neutron absorption cross sections and have been used as control rods in nuclear reactors.

 

Gadolinium's seven 4f electrons have parallel spin.  This property is why Gd complexes are used as MRI contrast agents.  With one of the strongest known formation constants 1, 4, 7, 10 tetraazacyclododecane - 1, 4, 7, 10 tetra acetic acid (DOTA) reacts with Gd to form Gd(DOTA) H2O.  DOTA is 8 coordinate, plus one water results in a 9 coordinate Gd complex.  Most REE coordination complexes are 8, 9 or 10 coordinate.  Ligand variations have been developed that make it possible to use MRI to detect things like pH, O2 and temperature on a localized basis in the body.  A similar yttrium complex made with radioactive 90Y3+ is used in cancer therapy Y(DOTA) H2O.

 

A phosphor is a luminescent material that absorbs energy in a portion of the electromagnetic spectrum and emits energy in another portion of the electromagnetic spectrum.  A modern color display screen would use a UV-blue LED of the IniGajAlkN variety where i + j + k = 1 to supply the light.  Thousands of phosphor formulas have been made that can produce just about any color desired.  Major classes of phosphor are the phosphorus oxides and boron oxides.   The metals are either of the Ca, Sr, Ba plus two type or Al, Sc, Y, La plus three type.  Small amounts of REE can be added to adjust the color.  Europium is the only REE that has stable plus two and plus three states this is one of the reasons it is important as a phosphor dopant.  A few phosphor compositions are La0.95Eu0.05PO4, Gd0.95Eu0.05Al3B4O12, La0.985Sm0.01Ce0.005OBr, Y1.85Eu0.15W0.98Mo0.02O6 and Y2.85Ce0.15Al5O12.  The well know europium red phosphor is M2Si5N8 (M + Ca, Sr, Ba) Eu2+ doped for red.  Depending on how much europium is added Sr2P2O7 can be either yellow or blue.

 Specialty Plant:  This facility has several small SX circuits, drying kilns and product blending and bagging facilities. 

 

 Old Cracking Plant and Old SX-1 and SX-2, and Old CeO2 Plant.