Xanthates in mining (update)

Introduction

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)

Hazards

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

Hazards from xanthates include but are not limited to:

• production of toxic / flammable decomposition products (carbon disulphide* (CS2) 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 significant amounts absorbed through the skin
• acute irritation if inhaled or absorbed on skin surface.

For transport and storage purposes, solid xanthate is classified as class 4.2 (spontaneously combustible) under UN 3342 PGII, and for liquid mixtures it is classified as class 8 (corrosive) sub-risk 6.1 (toxic) under UN 2922. Xanthates are also a scheduled material for major hazard facilities.

Xanthates also have the following classification under the UN Globally Harmonized System of Classification and Labelling of Chemicals:

Solid and liquid mixtures of xanthates

• Self-heating substances and mixtures - Category 1
• Acute toxicity (Oral) - Category 4
• Acute toxicity (Dermal) - Category 4
• Skin corrosion / irritation - Category 2
• Serious eye damage / irritation - Category 2A.

Liquid mixtures of xanthates only

• Metal corrosion - Category 1.

In addition, animal studies show xanthates chronically damage the liver and neurological system from long term elevated exposures.

Xanthate stability is affected by:

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

Xanthates readily absorb moisture from the air which can accelerate xanthate decomposition.

One specific cause of decomposition not widely understood is the contact of xanthates with acidic materials. Xanthates break down faster at pH less than 7 generating CS2 as the main decomposition gas product. Typical acidic materials encountered in mining includes copper sulphate solutions (pH 4) and sodium metabisulphite solutions (SMBS - pH4.6).

*Carbon disulphide (CAS 75-15-0)

Carbon disulphide (CS2) is a flammable gas with explosive limits from 0.6% to 60% by volume in air, and an autoigniton temperature of 90oC. It is also classified as Equipment IIC and temperature class T6 for the purposes of hazardous area calculations and classifications.

The Time Weighted Average exposure is 10 ppm with an Immediately Dangerous to Life and Health value of 500 ppm. Anecdotal evidence suggests readings of up to 200 ppm CS2 were measured when simply opening boxes of xanthate.

Considerable literature exists on the effects of CS2. Identified health effects include:

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

Long-term exposures 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

In Queensland, xanthates are one of the most used mining reagents by volume. Unfortunately they have been also 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 of these incidents

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

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

Causes

Incident investigations have highlighted some common causes, including some previously identified in readily available literature. 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.
• incompatible storage—mixtures were stored in the same bunded area as copper sulphate and SMBS.
• 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 no longer required by the ADG code however is recommended that it is still included).
• 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 and unable to maintain risk at an acceptable level.

Controls for hazards associated with xanthates

Follow the hierarchy of controls so risk is as low as reasonably achievable and at an acceptable level. With regard to process safety, appropriate risk assessments such as a HAZOP should also be conducted to identify other preventative and mitigative measures.

Often, elimination or substitution of xanthate with less hazardous reagents isn’t feasible due to the metallurgy of the ore, so consider the following control recommendations when designing for hazards created by xanthates if substitution is not an option.

Substitution:

Where feasible substitute liquid for solid xanthate to eliminate mixing steps in the process and eliminate the need to store and handle the dry chemical.

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.
• Ensure xanthate storage areas are adequately segregated from incompatible materials including SMBS and copper sulphate.
• 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 the AS/NZS 60079 Explosive atmospheres series, and xanthate storage and mixing areas are properly assessed against this standard.
• Ensure vehicles used for clean-up of dangerous goods wastes meet the requirements of the ADG Code.
• Install cooling jackets on any liquid xanthate storage tanks.

As xanthate fumes are flammable, AS1940 The storage and handling of flammable and combustible liquids could assist in identifying possible controls along with AS3780 The storage and handling of corrosive substances for liquid mixtures.

Administration:

Ensure storage, handling and mixing procedures include these activities where appropriate:

• 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
• consider measuring for CS2 in shipping containers when received to ensure none is present.
• continuous static monitoring of CS2 in mixing and storage areas
• clean equipment properly after mixing and use
• label all inner bags from boxed xanthate
• if CS2 is detected in storage use of thermal imaging to identify suspect container(s) / box(es).

Personal protective equipment: 

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

The selection of appropriate respiratory protective equipment depends on the concentration that may be encountered by the wearer. For CS₂ and SO₂, the effective protection may require respiratory protection with ABE filters or supplied air respirators.

References:

1. Geschaftsfiihrer, L, Bearbeiter, G, 1994, Ober Untersuchungen zum Zundverhalten stromender CS2/Luft-Gemische in einer Rohrapparatur (Translated - Top Investigations on the ignition behaviour of streaming CS2 / air mixtures in a crude apparatus), Institut für Sicherheitstechnik

2. ILO, 2000, ICSC card list - 22 Carbon Disulphide at www.ilo.org/dyn/icsc/showcard.listCards2

3. National Industrial Chemicals Notification and Assessment Scheme (NICNAS), 1995, Sodium ethyl xanthate assessment report

4. NIOSH, 2014, https://www.cdc.gov/niosh/idlh/75150.html

5. Safe Work Australia, 2017, Hazardous Substances Information System, http://hsis.safeworkaustralia.gov.au/

6. Shen, Y., Nagaraj, D.R., Farinto, R., Somasundaran, P., 2016, Study of xanthate decomposition in aqueous solutions, Minerals Engineering 93 (2016), Elsevier, pp 10-15

7. Standards Australia AS/NZS 1715:2009 - Selection, use and maintenance of respiratory protective equipment, Sydney

8. Standards Australia AS3780:2008 The storage and handling of corrosive substances, Sydney

9. Standards Australia AS/NZS 60079:2012 series – Explosive atmospheres, Sydney

10. Tipman, N. R., 1962, The reactions of potassium ethyl xanthate in aqueous solution, University of Alberta

Authorised by Luca Rocchi - Chief Inspector of Mineral Mines and Quarries

Contact: Damian Lee, Senior Inspector of Mines, Chemical , +61 7 4747 2157 damian.lee@dnrme.qld.gov.au

Issued by Queensland Department of Natural Resources, Mines and Energy