Xanthates in mining

Xanthates are a group of chemicals typically used in sulphide flotation in mining applications.

Common xanthate products are sodium ethyl xanthate (SEX), sodium isopropyl xanthate (SIPX), sodium isobutyl xanthate (SIBX) and potassium amyl xanthate (PAX).

Where is the risk?

Xanthates are classified as liable to spontaneous combustion in the Australian Dangerous Goods (ADG) Code. They pose a number of hazards due to their nature, the vast quantities used in industry and the climate conditions at most Queensland mines using them.

Hazards from xanthates include but are not limited to:

• production of toxic/flammable decomposition products (carbon disulphide (see below) and potentially, alcohol vapours)
• spontaneous combustion that creates toxic combustion products (sulphur dioxide, carbon monoxide and carbon dioxide)
• low order explosions from ignition of decomposition products
• acute harm if ingested or absorbed in significant amounts through skin
• acute irritation if inhaled or absorbed through skin.

In addition, animal studies indicate xanthates are linked to chronic damage to the liver and neurological system after long-term elevated exposure.
(see the National Industrial Chemicals Notification and Assessment Scheme (NICNAS) Sodium ethyl xanthate assessment report, May 1995, for further information)

Their stability is affected by:

• long storage periods at high temperature
• moisture content (in the manufactured product and moisture absorbed during storage)
• length of storage and
• the pH of any mixtures.

Xanthates are hygroscopic so readily absorb moisture from the air which, at high enough levels, can accelerate decomposition (NICNAS Sodium ethyl xanthate, May 1995).

Carbon disulphide

When xanthates decompose, they produce carbon disulphide (CS₂). This is a flammable gas with explosive limits from 1% to 50% by volume in air, and an autoigniton temperature of 90°C. (see the ILO ICSC card list 22 Carbon Disulphide)

The Time Weighted Average exposure is 10ppm (see the Safe Work Australia Hazardous Substances Information System) with an Immediately Dangerous to Life and Health value of 500ppm (see the CDC–NIOSH website). Anecdotal evidence suggests readings of up to 200ppm CS2 were measured when simply opening boxes of xanthate.

Considerable literature exists on the effects of CS₂ with studies conducted in various countries. Identified health effects (NICNAS Sodium Ethyl Xanthate, May 1995) include:

• irritation to eyes, skin and respiratory tract
• acute poisoning effects including tremor, prostration, dyspnea, cyanosis and vascular collapse
• psychosis or narcosis may result from acute levels of up 500–1000ppm

Long-term exposure at high levels are responsible for:

• nervous system effects including symptoms of fatigue, insomnia, headaches and irritability
• increased susceptibility to heart disease including heart attack, high blood pressure and angina
• links to eye damage, reproductive effects and hearing loss.

Incidents and outcomes

In Queensland, xanthates are one of the most used mining reagents, by volume; they have been involved in many chemical incidents in recent years, including:

• Boxes of xanthate spontaneously combusting in storage areas
• Xanthate breaking down and leaking out of their boxes
• Fire in the air space of a xanthate storage tank ignited by welding sparks
• Accidental mixing of xanthate with sodium metabisulphite (SMBS) created a flammable atmosphere that was ignited by a spark from a tool on steel
• Solid xanthate rapidly decomposing when added to water, creating a large toxic plume
• A vacuum pump motor caught fire and caused a low order explosion in an industrial vacuum truck cleaning up xanthate waste from a sump.

Consequences

Safety and health consequences of these incidents included inhalation symptoms (nausea and vomiting), dizziness, burns, the evacuation of surrounding areas, and an explosion that ejected material up to 30m.

Potential consequences could have included serious inhalation symptoms, on-going health concerns and possible fatality.

Causes

Incident investigations have highlighted common causes, including some previously identified in readily available literature (e.g the NICNAS report). Some of the common causes include:

• Moisture. Boxes of solid xanthate accumulated moisture when left open for long periods or in humid conditions, leading to spontaneous combustion. Also, shortening of the drying process during manufacture led to xanthate with a high moisture content being supplied.
• High temperature. Xanthate was stored in high ambient temperatures for long periods which led to increased decomposition.
• Length of storage. Stock was not rotated properly, allowing old stock to 'age' for long periods.
• Inappropriate mixing. No labelling on the inside bag of boxed xanthate led to confusion over the contents which were then inadvertently added to SMBS. (This labelling is required by the ADG code and has been communicated to known suppliers in Queensland.)
• Poor training and procedures. The hazards of xanthates and their waste products were not identified by workers when inadequate procedures and training on chemicals and associated hazards left them unaware of the hazards or how to maintain risk at an acceptable level.

Controls for hazards

Follow the hierarchy of controls so risk is as low as reasonably achievable and at an acceptable level. When elimination or substitution of xanthates with less hazardous reagents isn't feasible due to the metallurgy of the ore, consider the following control recommendations when designing for hazards created by xanthates -

Substitution

Where feasible substitute liquid for solid xanthate to eliminate mixing steps in the process.

Engineering

• Store xanthate boxes to allow sufficient ventilation to disperse any fumes and ensure storage areas prevent build up of humidity.
• Use extraction hoods with scrubbers to eliminate xanthate dust from mixing.
• Remove people from involvement in the mixing process by using cranes and other equipment.
• Ensure electrical equipment in xanthate mixing, storage areas and clean up equipment complies with AS/NZS 2381 Electrical equipment for explosive atmospheres-Selection, installation and maintenance.
• Ensure vehicles used for cleanup of dangerous goods wastes meet the requirements of the ADG Code. (Section 6.9 might require advice on dangerous goods, in Queensland, from the Department of Transport and Main Roads.)
• Install cooling jackets on any liquid xanthate storage tanks.

As xanthate fumes and liquids are flammable, AS 1940 The storage and handling of flammable and combustible liquids could assist in identifying possible controls

Administration

Ensure storage, handling and mixing procedures include these activities:

• Train workers (including relevant contractors) to identify hazards associated with xanthates, including xanthate waste.
• Rotate stock properly (old stock used first) and keep minimal stock at the mine.
• Prepare xanthate bulky bags only immediately before use.
• Monitor CS₂ levels in areas known to have high occupational exposure levels.
• Clean equipment properly after mixing and use.
• Label all inner bags from boxed xanthate as required under the ADG code.

Personal protective equipment

Include appropriate respirator or fresh air supply mask, eye protection including goggles, and protective clothing including appropriately rated coveralls, gloves and boots where necessary.

Authorised by Phil Goode - A/Chief Inspector of Mines

Contact: minesafetyandhealth@dnrm.qld.gov.au

Issued by Queensland Department of Natural Resources and Mines