WET STICKY ORES

High Clay Ores-A Mineral Processing Nightmare?

Introduction

Clay minerals can cause nightmares with Mineral Processing because of the inherent difficulties of processing high clay ores. These clay minerals typically form in near surface geological environments due to weathering, particularly above the natural water table, sedimentation and diagenesis and are common in hydrothermally altered rocks. Clay minerals associated with alteration is referred to as Argillic alteration. The extremely small particle size, usually <2 microns (sometimes coarser or finer) and with the very high surface area these clay minerals are highly reactive and respond to changes in the processing environment. Some of the main clay groupings encountered are listed below:

The Kaolin Sepentine Group

kaolinite, dickite, nacrite and halloysite

Serpentite Group

chrysotile, lizardite,berthierine

Talc Pyrophyllite Group

pyrophyllite

Mica Group

muscovite, paragonite, roscoelite, caladonite, illite, phengite, serecite, biotite, phlogopite

Smectite Vermiculite Group

montmorillonite, beidellite, nontronite, saponite, stevensite, vermiculite

Chlorite Group

chlorite, clinichlore, chamosite, nimite, pennantite

Mixed layer minerals

Sepiolite-Palygorskite/Attapulgite

Allophane and Imogolite

Clays are dominated by their surfaces. They can be flakes, needles or lathes. Charged ions are attracted to the surface of clays i.e. they are adsorbed onto the surface. This changes the physical characteristics causing them to swell or shrink.

Cation exchange is important with clays. Because of this a large number of clay minerals have a composition determined by the environment in which they form.

Identification

X Ray Diffraction has historically been used to identify clay minerals but other techniques such as infrared spectroscopy and electron microscopy, scanning electron microscopy (SEM) and Qemscan.

Weathering Profile

Clay minerals exist in the areas above the water table but can in some instances exist below it. For example the reaction of feldspars and water to form kaolinite involves the exchange of cations and produces a clay mineral.

In addition the depth of weathering is not uniform but influenced by faulting and fractures within the ore body. This makes it difficult to clearly define the zone of oxidation and therefore selectively mine these zones. Even mining fresh ore there are occasional patches of weathered clay ore within the sulphide zone. This complicates processing where the characteristics of the fresh ore and weathered ore are so different.

Argillic Alteration

Hydrothermal alteration results from the interaction of hot waters with rocks to form clay minerals. This is not uncommon in Epithermal deposits and results in clays persisting at depths which would not be expected. The phenomena of hard quartz rock with interspersed clay (5 to 15% clay) as a result of hydrothermal fluids is quite deceptive with regards to how the rock will process. It looks like a hard rock but the contained clay will cause processing problems such as poor settling and filtering.

Typical Mineral Processing Problems

The Mineral Processing problems are not unique they cover gold, copper, uranium, lead zinc, iron ores and nickel projects. Clays have a major impact on unit operations such as thickening and filtration to such an extent that these unit operations cannot be used on some ores.

Materials handling

The clays can stick to conveyors, idlers and screens. Open stockpiles are not possible (rat hole) with high clay wet sticky ores. The use of bins is highly problematic even with air canons and vibrators. The only option is direct feeding of the ore.

Crushing & Screening

The crushing capacity of high clay ores is very poor. In some instances crushing is not possible. The ore simply won’t go through cone crushers and bridges the opening. A better option is tom SAG mill the ore. Screening results in screen cloths blinding over a period of hours and closing the screen. This requires regular cleaning to be functional.

 Heap leaching

Clays restrict percolation and therefore recovery. While agglomeration mitigates the effect for very heavy clay ores the ore may not be heap leachable.

Milling & Classification

At the Kura gold project the ore was so viscous it would not flow out of the mill even with lower densities. Also cyclone efficiency is negatively affected resulting in a less sharp separation. Clays affect the grinding efficiency necessitating operating the mill with a lower density to flush the clay out of the mill.

Leaching

Clay ores can result in preg robbing and tanks overflowing because of insufficient fall on the tanks and high viscosities. At some early Goldfields CIP projects caustic soda had to be used for pH control because lime exacerbated the thixotropic nature of these pulps. This added to the processing cost but unless caustic was used the thick pulps could not be pumped or transported though leach tanks..

Pumping

Pump capacity is significantly de rated due the presence of clays due to the high viscosity encountered with high clay ores. Centrifugal pumps under thickeners simply won’t pump high clay ores necessitating the change to Bredel pumps.

 Flotation

High clay ores result in higher reagent consumption and poorer selectivity and impeded flotation kinetics. Clays inhibit the flotation of fresh sulphides.

Thickening

High clay ores have very poor settling rates and dirty overflows result which can affect downstream processes such as clarification or solvent extraction. This requires much larger thickeners and higher doses of flocculants.

Filtering

The presence of clays results in very low filtration rates i.e. 50 to 100 kg/m2/day. In some instances filtering has to be ruled out as an option due to the high CAPEX and difficulties involved in filtering, spillage, cloth blinding etc. The filter cake can’t be washed and usually has a high moisture content.

 Tailings

High clay ores result in tailings dams with very low water recovery rates and a high risk of wall failures and leakage from the dam. Clays seal the dam and prevent effective drainage and can sit there as pulp for many months. This prevents sub aerial cyclic deposition.

Project Examples

Tennant Creek could not process high grade gold ores from the Peko ore because of poor settling and filtration characteristics in the Merrill Crowe circuit. Similarly the near surface oxide/clay ores around Kalgoorlie could not be processed until the advent of carbon in pulp (CIP) or carbon in leach (CIL). Bougainville Copper experienced poor flotation with high clay ores in the early commissioning days. Harbour Lights gold project experienced poor crushing rates, hang ups in the fine ore bin and tank overflows due to high clay ores. Boddington 1 CIP plant had similar problems as well as Kelian, Kurara, Mt McLure, Mt Muro, Mina Sertao, and Lefa Corridor. In the case of Mt Muro a specially designed clay washing circuit was inoperable and had to be scrapped because it did not work. The Mt McLure crushing circuit was designed for heavy clays and still proved difficult to operate. The Nifty copper heap leach experienced poor recoveries from the high clay ores which were not anticipated. Phu Bia suffered from unexpected high clay ores causing flotation problems in the early days. Simberi CIP gold project suffered lower throughput and materials handling issues because the high clay ores were not adequately addressed in the design. There are numerous additional examples that could be referred to with similar characteristic problems attributable to high clay ores.

Conclusion

Understanding the Geology, Mineralogy and correctly selecting representative samples of the ore to be processed is critical in getting the Mineral, Processing right. Clays have the potential if not recognised to destroy project economics.

The presence of clays in hard rock comes as a surprise if not fully understood. Similarly waste dilution from country rock containing clay can adversely affect the process.

The presence of clays has a major influence on the process selection and equipment used in the final flowsheet. The industry has paid a heavy price on a number of projects where the clay was not recognised or the process plant was based on a hard rock design which later proved to be totally unsuitable. Trying to mitigate the impact of clay on the process if the percentage of clay in the feed cannot be controlled is a Mineral Processing nightmare.