Measuring up mica

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Published: Friday, 20 February 2009

Following the acquisition of North America’s leading mica deposits in 2008, Julian Danvers et al weigh up Imerys’ new purchases, how they will complement its present portfolio and for which markets they are suited

 
Imerys’ purchases of North American mica plants, Kings Mountain and Suzorite, has added to its operations in France and Feldspar Corp. in the USA. Courtesy Imerys


The world’s largest industrial minerals group continued its acquisition trail at the end of 2008 by purchasing two of the world’s leading mica sources in North America, Kings Mountain Minerals Inc in the USA, and Suzorite Mining Inc. in Canada from Zemex Minerals Group (see IM November ’08, p.6).

Through the acquisitions from Zemex Minerals Group Inc., Imerys purchased two of the most significant mica sources in North America and some of the largest operations outside of its own portfolio.

Kings Mountain and Suzorite were added to mica operations, The Feldspar Corp in the USA, and the 15,000 tpa Kaolin de Bretagne and Feldspath du Morvan in France.

Imerys’ has not hidden its strategy to encourage organic growth through leveraging process technology, fundamental research and deep market intimacy, combined with strategic acquisitions focused on growth in developing regions, in addition to targeting the acquisition of new minerals resources in developed countries.

These specific North American acquisitions gave Imerys the full portfolio of mica grades which add to its present industrial and specialty minerals such as ground and precipitated calcium carbonate, hydrous or calcined kaolin, perlite and diatomite.

Although mica is not nearly the most plentiful mineral used in industrial applications it offers some unique and beneficial functionality with excellent value propositions.

Mica’s position

Mica is a name for a family of complex hydrous aluminium silicates that have a sheet structure and exhibit almost perfect basal cleavage. The two most common and commercially available micas in use are muscovite and phlogopite micas.

There are other members of the mica family that are very rarely encountered named Illites (hydrated mica), Sericites (fine-grained white mica, an ill-defined material that falls structurally between muscovite and illite) and have notably been mentioned for use in body ceramics predominantly in Asia.

Nearly all economical deposits are mined for an array of minerals and in most cases mica is a minor component by volume, but is of key value, Imerys Suzorite phlogopite mica being an exception to this.

Certain deposits are reported to be mined for mica only and in these cases mica is encountered in lodes from low concentration (India), to rather high concentration (China). These deposits can encounter some drawbacks that make them difficult to become economically viable and consistent over time.

The main complexities associated with these deposits is their reduced size making it difficult to ensure a true reliable mining plan, the need to use explosives and the mandatory hand selection to cope with colour variation generally seen inside the lodes, as well as the need to move to underground mining in many cases.

Currently Imerys has a total of five deposits (see map) of both muscovite and phlogopite. As part of Imerys strategy those deposits are close to their customer base and located in stable countries. Having such proximity to its end users offers economies in freight and enables customers to operate a lean ‘just-in-time’ supply chain.

Location of Imerys’ mica operations
 



















Processing mica

From a process perspective Imerys uses mainly three routes to separate and concentrate mica from ore. For altered rocks the primary process routes used are floatation and wet concentration (spiral) while hard rock use dry concentration(crushing and magnetic separation).

To describe the main processes that lead to commercial grades, the flow sheet provides a summary of various steps to produce from the coarsest to the finest particle sizes and their respective outlets on the market.

Deposits based on altered rocks, the run of mine is firstly blunged, screened to remove the bigger stones and the subsequent slurry obtained is further cycloned to remove the fines primarily made of kaolinite or clays.

The overflow of the cycloning process feeds the wet concentration, a low energy demanding step since it uses gravimetric forces and is free from any chemicals. This spiral step allows the concentration of the largest and purest mica as the feed to floatation made of an assemblage of mica, quartz and feldspars which ranges typically 200µm to 1mm.

Depending on the mica size liberated from the ore, the spiral step may not be used and the overflow from cycloning directly floated.

At this stage the mica obtained is concentrated from pure muscovite to a grit level of 10-15% depending on the requests from customers. Some grades named “flake or coarse mica” are directly dried and sieved to ensure perfect top cuts. These are used mainly for sound dampening, decorative products and oil well drilling.

When final applications ask for a finer mica grade then the mica concentrate will follow various milling, sieving and aspect ratio transformations in order to match specifically the characteristics expected by each market.

This size & shape transformation is either done in wet (batch and continuous) or dry conditions depending on which final properties we want to emphasize. Dry and wet ground micas serve the bulk of the applications as described on the panel, Mica’s markets.

Some applications looking for very fine minerals (typically d50 = 5µm) are served with micronised mica obtained through jet air mill.

Last but not least, for some applications such as the welding rods industry that are very sensitive to water/vapour emission (either during their manufacturing step or in the final end use) a further calcination step can be operated to remove the natural 4-5% of loss on ignition contained in most mica crystals.

Wet v dry

This wet route, used for altered rocks, is currently being used at 3 of Imerys’ 5 deposits, covering all muscovite types.

The dry route is less encountered worldwide at least when dealing with industrialized assets. Imerys processes this way both the muscovite from Burgundy and the Suzorite phlogopite.

The ore rock is crushed in jaw crushers and after having being dried is further concentrated to obtain blocky mica (non delaminated mica) and finely delaminated mica. This dry route generates several different types of mica grades differentiating themselves principally by their particle size distribution and their aspect ratio.

Imerys produces one of the lowest bulk density (<200 g/l) commercial grade available worldwide by utilising this dry process with selected Imerys deposits, this can also generate particularly high aspect ratio products also.

Imerys provides the suitable surface treatment needed to enhance compatibility with certain polymer families. However the trend is to see this processed being captured by compounders as part of the know-how and their way to differentiate from each other in their respective markets.

Imerys’ big five

The mineral specificity of each of Imerys’ five deposits combined with a careful selection of the most appropriate and “state-of-the-art” processing equipments allows Imerys to cover any mica demand worldwide.

Imerys’ global research and process development teams are collaborate to leverage and develop new technologies and applications

Experience gained from recent product innovations, such as Barrisurf, a high aspect clay for improved barrier properties in water based coating, allows the group to expand the mineral product portfolio and performance around key functional requirements.

With mica, perlite and vermiculite Imerys has a suite of minerals for niche applications including fire protection and insulation applications.

Imerys’ Suzorite operation in Boucherville, Quebec which produces phlogopite mica, by a process of continued investment is described by the company as “one of the most modern and efficient mineral processing plants in the world.”

As displayed in the panel, the applications where mica finds an interest are numerous and thanks to the broad particle size range and its unique delamination behaviour mica has few cost effective alternatives.

Different mica processing for different markets 
 


Selecting mica for markets

Any particular markets or applications using mica in their formulations to impart one or more of the properties, will select one grade rather than another on the best cost performance ratio of the expected technical benefit at the best value (See Selection criteria panel).

Amongst criteria that contribute to usage value, all applications have in common a high demand with regards to particle size distribution: sharp top cut, limited to no fines and steep distribution (tend as much as possible to monodisperse). This not mentioning the utmost cleanliness and consistency from one batch to the other.

Mica’s 2009 global market split 
 
However, against what is most often perceived, poor brightness is not a definite barrier to any mica use. The vast majority of paints & coatings applications - which is the largest outlet for mica not considering the well-known US specificity of mica into joint compound - is claiming for highest brightness mica such as some very unique muscovite.

Marine and anticorrosive paints, however, are not so sensitive and many of the others applications will not take into account this characteristic but will be very demanding on the bulk density (sound dampening, fire retardant) or the chemistry (welding rods) or the aspect ratio (plastics), amongst others.

The bulk of plastic for automotive are dark to black colour hence fully satisfied with phlogopite that complement ideally fiber glass in a compounded formulation, not only by reducing significantly the price per kg but also since its platy structure reduces the presence of fibre at the plastic surface as a source of possible defect once a paint layer is sprayed onto the plastic piece.

Mica is not an easy mineral to sell due to the numerous final applications and it requires a strong customer technical support to match market needs.

Delivering a cost effective, high performance mica requires close cross functional cooperation ranging from geologists, process engineers, research and development scientists and marketers.

This additional support is where Imerys’ believes it has the edge in the mica market and after adding Kings Mountain and Suzorite is now the world’s leading producer.

Contributors: Julian Danvers, Doug Smith and Richard Enos of Imerys SA.

As an overview, below is a listing of Imerys’ main operations, the mica type mined and processing routes commonly used to concentrate and mill micas.

Imerys’ mica locations

Muscovite

 Location  Operation
 Acid igneous rock : granite,  
 pegmatites, alaskite, aplite.  
 Appalachian Mountains (US)  Imerys Spruce Pine
 Burgundy (France) Imerys Feldspath du Morvan  
 Liberated during the kaolinization  
 of granite. Brittany (France)   Imerys Kaolins de Bretagne
 Kings Mountain (US)  Imerys Kings Mountain
 Georgia (US)   Imerys North America
 Phlogopite  
 (Alkaline ultrabasic rocks)  
 Location  Operation
 Suzor (Canada)  Imerys Boucherville


Muscovite v phlogopite mica

When comparing the two main grades of mica, muscovite and phlogopite, the primary differences are:

  • Muscovite is considerably more white and flexible than phlogopite and as a consequence typically is used for more colour sensitive applications
  • Phlogopite is darker in shade and offers better heat resistance than muscovite. It lends itself to high temperature less colour sensitive applications. Of particular interest is the unique very low bulk density and larger PSD spectrum of Imerys Suzorite mica than any other phlogopite. 

This being said, muscovite mica in particular can exhibit a variety of different properties:

  • Particle sizes are available across a very broad range within a deposit, this determines what process is needed to match the mineral purity and which particle sizes are achievable in commercial products
  • Lattice substitutions often vary from deposit to deposit and as such can impart different behaviours in delamination, flake flexibility, colour, etc
  • Interlayer cations (primarily K) that are not evenly bonded to both cleavage plan generate different levels of cohesive energy
  • Associated minerals (kaolin, feldspar, quartz) within a deposit contribute to the final properties of mica

The various substituting atoms have effects on the physical properties of mica however Imerys’ technical expertise can bring support to customer’s formulators when attempting to substitute one mica for another, to ensure that functionality is maintained or even improved in the application.

Selection criteria 

MAIN PROPERTIES

  • Flexible & toughness
  • lsotropic reinforcement, anti-cracking
  • Thermal stabilty and LCTE
  • Chemical inertness
  • Barrier to gas, electrolytes and UV
  • Vibration damping
  • Electrical & Fire insulation
  • Rheology, lubricity

USAGE VALUE

  • Whiteness, Brightness
  • Aspect ratio & bulk density
  • Purity
  • Particle size distribution
  • Clear top cut
  • Refractories
  • pH
  • Consistency