Minerals in waste remediation

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Published: Sunday, 26 July 2009

With a shift in US environment policy and a global push for a cleaner environment, George Hawley reviews the potential of minerals in waste disposal

Industrial minerals, with or without modification, play a key role in removing and sequestering harmful waste products.

A number of recent developments including Sweden’s plans for utilising bentonite in nuclear waste disposal have highlighted this growing trend in the industry (see IM July ’09, p.24: Bentonite’s nuclear disposal role).

There are many end uses including the well known flue gas desulphurisation (FGD) to the lesser know application in acid mine drainage and land remediation.



Nuclear disposal techniques utilise minerals
with absorbing properties, particularly bentonite.
Courtesy Steve Morgan/Greenpeace

Flue gas desulphurisation

Power plants emit large amounts of sulphuric oxide gases, derived from the sulphur in the fuel. These gases cause acid rain that damages foliage and leads to eutrophication of lakes and streams.

Sulphur trioxide is also generated as a by-product from Selective Catalyst Reduction (SCR) used in some power plants to control NOx.

Coal has the highest sulphur content, followed by oil, which may be high in sulphur, like Venezuelan crude. Natural gas can be ‘sweet’ with low sulphur or can be high in hydrogen sulphide.

Sulphur compounds are removed from the flue gases by treatment with limestone or lime. The utility power plant category dominates the FGD market at 90%.

In the USA in 2007, Flue Gas Desulphurisation (FGD) consumed 14.5m. tonnes of limestone and 3.73m. tonnes of lime – both products add carbon dioxide to the atmosphere.

Gypsum is formed by the reaction. Where the power plant is near a wallboard plant, it is used as a substitute for mined gypsum.

One such new project is Bulgaria where Balkanstroy Group AD has started construction of a gypsum board and gypsum mortar facility near Stara Zagora. The $55m. plant will consume 360,000 tpa of FGD gypsum (IM 4 June 2009: Bulgarian gypsum project).

In the USA last year, 8m.tonnes of this synthetic gypsum were used out of a total consumption of 18.7m. tonnes, the rest being mined gypsum. Some magnesium hydroxide slurry is also used in FGD. The advantage is that the magnesium sulphate produced is soluble and thus solid waste problems are minimised.

Wollastonite reacts with sulphuric, carbonic and hydrochloric acids, so can substitute for lime and limestone. Diopside, a by-product of some wollastonite sources, should also be useful in FGD.

Acid Mine Drainage

Pollution by toxic heavy metals, including arsenic, cobalt, copper, cadmium, chromium, lead, silver and zinc , and by aluminium and manganese, occurs when rock excavated or exposed in an underground mine, is leached out by water. Leaching is accelerated in the low pH conditions created by Acid Mine Drainage (AMD) from sulphide ores.

Pollution also occurs when chemicals such as cyanide, sulphuric acid and flotation reagents spill, leak, or leach from the mine site into nearby water bodies. These chemicals can be highly toxic.

The US Toxic Release Inventory (TRI) requires mining, among other industries, to report on their release of toxic chemicals. In 2005, although mining represented less than 0.5% of the industrial activity, it produced 27% of all toxic chemicals reported, an amount of 530m. tpa. Of this, 97% was from drainage from mine tailings.

Some 80 facilities are not reporting tailings and waste rock to NPRI so the actual release of toxics could be much higher. Remediation of AMD is performed mainly by adding limestone and lime, but also by caustic soda, and hydrogen sulphide.

In 2007, in the USA, 3.74m. tonnes of limestone and 104,000 tonnes of lime were used in AMD remediation.

The use of both these products results in the release of carbon dioxide into the atmosphere. An environmentally preferred alternative is the use of wollastonite.

Wollastonite has a neutral pH but this rises to an alkaline 10.4 in contact with water due to the hydrolysis to lime and silica.

In acid waters, this process is accelerated and a surface layer of amorphous silica remains on the wollastonite particle. The lime captures the sulphuric acid in AMD as insoluble calcium sulphate, with potential use in gypsum wall board.

The amorphous silica is porous and has a high surface area. For wollastonite milled to 63 microns in size, the reactive surface area increases from 1.19 to 47.6 m²/g after 15 days in mine drainage fluid.

This silica has been shown to adsorb arsenic, copper, zinc, iron and aluminium.

Liners and caps

Swelling bentonite is also used as an admix 3 to 5:1 with native soil in layers 1.5 to 3 cm thick to form an impermeable liner for tailings ponds, in the same way that it is used in landfills. It may also be used to cap waste rock piles.

Sodium-bentonite swells when contacted by water, thus forming an impermeable self-healing barrier.

Around 139,000 tonnes of bentonite were used in waterproofing and sealing applications in 2007.

India and China are he most promising markets for geosynthetic clay liners (GSL). Amcol International Corp. currently has GCL production facilities Australia, China, India (via a j-v with Ashapura), Poland, South Korea, and the UK.

Amcol produces bentonite GCLs for landfill protection from its six plants worldwide, and it has nearly a 50% market share. The company is eyeing China, India and Brazil as the major growth markets.

Nuclear waste disposal

Disposal of nuclear waste and dealing with spills, is an on-going problem, which is likely to increase with the replacement of carbon dioxide generating fossil fuels by nuclear power.

There have been a number of nuclear waste spills – notably in Chernobyl (Ukraine), Windscale (UK), Chalk River (Canada ) ,Savannah River Site (USA) , Maxey Flat (USA), and the Areva plant in Tricastin (France).

Solid nuclear wastes are encapsulated in glass, copper or special cements. Liquid wastes are concentrated by ion-exchange before encapsulation.

Deep well burial is one method to dispose of spent fuel. To prevent damage of containers and release of radioactive nuclides, SKB of Sweden, places the copper capsules in holes backfilled with bentonite. This cushions the capsules against crushing by rock movement, and seals against leakage in or out.

Radioactive elements are released by nuclear accidents and in the course of normal activities.

These elements are long lived and can persist for a lifetime in the body when inhaled or ingested. The result is often cancer.

Radioactive caesium, 137 Cs products of the Chernobyl disaster were carried by air streams and have been detected as far west as Vermont.

It has been shown that radioactive chemicals can be removed from waste streams and sequestered by various minerals. The minerals are mainly clays, micas, zeolites, silicas and modifications thereof.

Bentonite’s role in nuclear waste disposal



Adapted from SKB

Mica

Finnish group Kemira Oy discovered that phlogopite can extract and sequester radioactive elements from contaminated waters by ion exchange.

Sridhar Komarneni and Rustum Roy of Pennsylvania State University discovered that phlogopite acts as a highly selective sieve for capturing radioactive isotopes of ceasium, cobalt and strontium from liquid wastes. These are immobilised in a solid form that can be disposed of as solid waste.

Mica can be used by dispersing it in water or soil and then filtering out the caesium-laden product or by letting caesium-contaminated humans or animals ingest the material, which extracts the caesium from the body and is then excreted.

Phlogopite is 30 times more effective than muscovite and 6 times more effective than biotite, in the sequestration of radioactive cobalt, 60Co.

When the finely ground phlogopite is converted to the sodium form by ion exchange, the 60Co, radioactive cobalt, uptake is doubled. Phlogopite is also 16 times more effective in the sequestration of radioactive strontium, 85 Sr.

However natural muscovite is 13 times more effective than phlogopite in sequestering radioactive cesium, 134Cs.

Clays

The Savannah River Site, Aitken, South Carolina, USA was built by E.I. du Pont de Nemours – now chemicals giant Dupont – for the Atomic Energy Commission in the 1950s as a site to produce weapons grade nuclear materials.

The site is highly contaminated due to dumping and releases of chemicals in the 1950s. Over 500 Curies - of Caesium 137 were released into cooling water. This has contaminated the canals and wetlands to the extent of 3,000 acres.

The ceasium winds up in the sediment but rises to the surface annually. Since ceasium replaces potassium, it is concentrated in the wetlands plants and garden vegetables and thence into foraging animals.

Residents in the area are advised not to eat dear meat and home grown vegetables.

Caesium isotope 137Cs with a half-life of 30 years, is the most prevalent radioactive isotope in people exposed to nuclear radiation. Offsite people in the SRS area have high levels of 137Cs. SRS has the highest 137Cs concentration ratio in the world – 6.8+/- 2.3. The recommended maximum level is 0.1.

The river covers 310 square miles. Remediation is on-going and the major sites are planned to be remediated by 2025.

Illite

A 2002 study was made, on an area of 8 acres with one canal, of the use of various minerals to decontaminate wet lands at the Savannah River. Various clays were evaluated; kaolin gave poor results and illite mica the best.

The conventional method is to dig out the sediment and truck it to a dump. The cost, for an 8 acre site, was estimated to be $109m.

Tests were conducted using illite (fine grained muscovite ) as an on-site additive, in a layer 0.1 inch thick, consuming about 100 tonnes.

The illite absorbs the 137Cs and sinks to the bottom of the wetland and stays there, holding the cesium tenaciously. It is reported not to release it over time or exposure to ammonium ion.

The cost to decontaminate the 8 acres on –site by illite was estimated to be only $19m. The illite price used was $450/tonne delivered, this the illite needed to remediate 3,000 acres would be 37,500 tonnes.

The best of the illite minerals tested was Todd Light ball clay produced by KT Clay, TN, specification 89% < 5 microns and 46% < 0.5 microns.

Mineralite 3X and 4X from MMC also demonstrated good results – these have 95-98% -325 mesh.

Saponite

Saponite is a magnesium-containing clay similar to a non-swelling bentonite. It is produced by IMV Nevada.

Saponite from the Khmelknytsky saponite deposit in the Ukraine, is being used mainly as a detoxicant for animals in the Chernobyl region that have ingested radioactive elements in their feed.

Zeolites

Zeolites, both naturally occurring and synthetic, are used in the decontamination and disposal of radioactive wastes. After the Chernobyl nuclear accident, mordenite zeolite was added to animal feed in Sweden to remove 137Cs from the gastrointestinal tracts of contaminated animals.

Minerals in mercury removal

Mercury is released into the atmosphere from cement kilns, but it is coal burning plants that are the largest source of mercury emissions, about 40 tonnes annually– 40% of the total emissions. The top 50 power plant polluters in the US emit over 18,000kg of mercury annually.

This has resulted in Environmental Protection Agency (EPA) regulations which call for 80% reduction by 2010 and 90% by 2015.

The conventional flue gas treatment is Activated Carbon Injection (ACI), at a cost of about $2,530/tonnes, which can reduce mercury emissions by 90%.

Activated carbon is made by pyrolysis of nutshells, wood, or coal. It has very high surface area of 500 – 1500 sq. m/g. It may contain graphene layers to account for the high surface area. It is often treated with sulphur or iodine to increase efficiency. Activated carbon, iodine treated, can absorb up to 5kg mercury for each tonne

When used, ACI adds carbon to the fly ash produced from the power plants. The carbon in the fly ash can cause problems when used in the manufacture of concrete. Millions of tons of fly ash are used in the USA and Canada for the production of concrete. The presence of too much carbon into the fly ash in some instances makes the fly ash unusable in this application.

From mine...



India’s Ashapura Minechem mine bentonite from Gujarat
state for use in Geosynthetic Clay Liners which it produces
through subsidiary Ashapura Volclay.

...to market



Geosynthetic Clay liners - a lining material filled with
bentonite clay - are one of the largest uses of industrial
minerals in the waste disposal market.

Cement

EPA now estimates mercury emissions in the US from cement clinker production at 10,500 kg/year. Part of it is from the coal fuel used, but it is also present in the limestone and shale feed. EPA legislation is pending to regulate feed or emissions concentration limits to 120 microgramme/ dry standard cubic metre.

The goal is to reduce mercury emissions by 81- 93%.

Other mercury remediation methods

Zeolites are being proposed for the extraction of mercury. Large scale tests, in power plants, are underway, using chabazite zeolite from Canada based Zeox Corporation.

A US patent has been applied for by Millenium Inorganic Chemicals – now part of Cristal Global – for precipitated silica, which has been treated with copper or other transition elements and with sulphur-containing silane which gives better results in removing mercury from gaseous effluents.

Highly porous silica has been developed by the researchers at the Pacific North-west National Laboratory. This, when treated with thiol – a sulphur/hydrogen compound – has the ability to capture mercury ions in wastewater. The thiol attracts mercury ions, but ignores other ions in the wastewater.

The synthetic silica from decomposition of wollastonite and/or diopside has a high porosity and surface area and may also be able to sequester mercury.

Prior patents (US 6,719,828 and 7,048,781) use silicates such as montmorillonite and vermiculite, treated with metal sulfides, for the same purpose.

It is possible that high aspect ratio wollastonite, treated the same way, could also sequester mercury. Also expanded graphite, with its high surface area, and graphene content may behave like activated charcoals and absorb mercury.

Obama minerals bounce

The use of industrial minerals in environmental remediation is likely to grow with the growth of legislation, especially during the presidency of Barack Obama. The new US Fuel Efficiency Policy which could spell an end to “gas guzzling” vehicles, is the most sweeping environmental act since the Clean Air Act of the 1970s which is the basis for today’s pollution regulations.

Passing such an act within the first six months of his presidency, Obama has given a strong environmental statement of intent to the world which is expected to pave the way for the use of many minerals in waste remediation and emissions capture.

The minerals most likely to be affected by such trends will be limestone, lime, magnesium hydrate, micas, clays, zeolites, activated carbon, graphite, wollastonite, diopside and synthetic silicas.

Contributor: George Hawley, of George C. Hawley & Associates, Canada, is an industrial minerals consultant.