Mitigation of post blast fume generation in soft ground and wet conditions

Lee Julian, Anand Musunuri, Ewan Sellers, Sarma Kanchibotla

What is blast fume?

The gases generated from an explosive reaction; particularly NO, NO2 and CO.

Is it important for industry to manage?

Value: • Fines for infringement • Lost production from late re-entry • Mines closed • Projects stopped due to potential

for fume

ACARP C23016 Approach

- Known issues – soft ground, wet/deep holes - Review - Develop measurement techniques - Lab experimentation - Field observations - Risk management framework Challenges - Declining coal price - Withdrawal of partner sites

Measurement techniques

mobile laboratory • Water content in ammonium nitrate emulsion

• ANE viscosity

• ANE ammonium nitrate proportion

• Water content in ammonium nitrate prill

• Ammonium nitrate prill purity

• Ammonium nitrate prill size distribution

• Water content of blends

• Prill to emulsion ratio of blends.

Water Ingress tester Index test - Beakers

Detonation of mining explosive

• AN, fuel & Sensitiser • Pressure • Self propagating shock wave • Rapid, exothermic reaction • Expansion of confinement • Post detonation reactions

ideal detonation when explosive reacts instantaneously and completely. In a non ideal detonation not all the chemicals react fully. Deflagration is when the chemical burns

Rock Diameter Product

Good &

Large &

Good

Poor or

Small or

Poor

component

Fume and “Detonation” Oxygen balanced:

3𝑁𝐻4𝑁𝑂3 + 𝐶𝐻2→3𝑁2 + 7𝐻2𝑂 + 𝐶𝑂2 (1) Fuel lean:

5𝑁𝐻4𝑁𝑂3 + 𝐶𝐻2→4𝑁2 + 2𝑁𝑂 + 11𝐻2𝑂 + 𝐶𝑂2 (2) Fuel rich:

2𝑁𝐻4𝑁𝑂3 + 𝐶𝐻2→2𝑁2 + 5𝐻2𝑂 + 𝐶𝑂 (3) Deflagration:

3𝑁𝐻4𝑁𝑂3 + 𝑁𝐻4𝑁𝑂3→2𝑁𝑂2 + 3𝑁2 + 8𝐻2𝑂 (4) Post detonation nitrogen reactions:

2𝑁𝑂 + 𝑂2→2𝑁𝑂2 (5)

2𝑁𝑂2⇔𝑁2𝑂4 (6)

No fume Invisible fume No fume Visible fume Visible fume

Wet holes

What is the source of the water? How long must the water be in contact with the explosives? Was there a single rain event, or continuously flowing ground water? The Queensland Fume Steering Group provided their fume database of over 5000 blasts for review; 1.46 million tonnes of explosives blasted at 42 mine sites reporting 48 categories per blast for 3.25 years.

Wet holes: The data

52% 51% 41%

74% 74%

44%

19% 15% 21%

9% 11%

25%

22%

13% 16%

6% 5%

13%

4%

9% 11% 6% 6%

13%

4% 10% 8%

6% 5% 0%

0% 2% 3% 0% 0% 6%

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

30% EP 40% EP 50% EP 30% EP 40% EP 50% EP

Wet Dry

Fume frequency, Wet vs Dry: HANFO

5

4

3

2

1

0

Wet Dry

Wet holes

Queensland database: Was the blast loaded to design?

Yes No N/A 4876 116 43 97% 2% 1%

Correlation: Low % 70% blend correlates to fume Causality Rain causes ad hoc changes on bench and the products already charged are damaged Proactive Design for rain. Expand patterns

higher PF

Field observations

Wet holes

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0

20

40

60

80

100

120

140

160

180

200

Janu

ary

2011

Febr

uary

201

1

Mar

ch 2

011

April

201

1

May

201

1

June

201

1

July

201

1

Augu

st 2

011

Sept

embe

r 201

1

Oct

ober

201

1

Nov

embe

r 201

1

Dece

mbe

r 201

1

Janu

ary

2012

Febr

uary

201

2

Mar

ch 2

012

April

201

2

May

201

2

June

201

2

July

201

2

Augu

st 2

012

Sept

embe

r 201

2

Oct

ober

201

2

Nov

embe

r 201

2

Dece

mbe

r 201

2

Janu

ary

2013

Febr

uary

201

3

Mar

ch 2

013

April

201

3

May

201

3

June

201

3

July

201

3

Augu

st 2

013

Sept

embe

r 201

3

Oct

ober

201

3

Nov

embe

r 201

3

Dece

mbe

r 201

3

Janu

ary

2014

Febr

uary

201

4

Mar

ch 2

014

April

201

4

May

June

Fum

e ra

ting

Mon

thly

rain

fall

(mm

)

Rainfall and fume

Wet holes Evidence of groundwater could be observed readily through the pit

Borehole camera observed dynamic water down hole

Wet

Water flows along fault. Weak ground Design change

Wet holes:

1 2 3 4 5 6

ANFO

Blend

Typical coal blast design

Stem ANFO BLEND

Issues solution

Water ingress tester

Testing issues and solutions overnight

Separation and Deflagration:

3𝑁𝐻4𝑁𝑂3 + 𝑁𝐻4𝑁𝑂3→2𝑁𝑂2 + 3𝑁2 + 8𝐻2𝑂

Wet holes: Detonation performance

Dry ANFO

Dry or wet ANFO

Wet holes: Detonation performance ANFO rinsed with 4 litres ANFO rinsed with 6 litres

70 000 fps Critical amount of water creates fume

Wet holes: Detonation performance ANFO rinsed with 4 litres ANFO rinsed with 6 litres

3168 2905

Similar Max VoD

Wet holes: Detonation performance

From left to right: ANFO rinsed with 2 litres of water, ANFO rinsed with 6 litres of water, ANFO rinsed with 2 litres of acid, ANFO rinsed with 5 litres of acid, ANFO saturated with water

VOD not strong indicator of fume potential

Soft ground: Modelling the detonation

Two explosives were modelled, ANFO and a 40% ANE HANFO using Vixen2009 code.

Blend

ANFO

Hard rock

weak rock

hot

cool

Sellers; Furtney et al. FRAGBLAST10

Soft ground: Modelling NOx

The rate at which NO converts to NO2 was identified to be a very strong function of temperature. Conversion happens at low temperature.

Soft ground: Modelling NOx in soft ground Bringing the modelled relationships from detonation and NO2 synthesis:

strong

weak ANFO Blend

weak

ANFO Blend

weak

Soft ground: Detonation testing

• Wedge tests to simulate cavity • below critical diamater • Non ideal detonation • Cools more rapidly • DRY ANFO = fume

Methodology to minimise fume

1. Characterise long term influences well ahead of blast design seasonal rain variation, ground water flow, rock mass strength and structure as well as the presence of catalysts , such as carbon, acid or organics

2. Regular explosive testing to track quality variations Quality vital. Affected by small components eg surfactants. Mobile laboratory for constituent tests, Beaker tests, Viscosity

3. Design blasts ahead of time for expected conditions possible rain and long term ground water seepage

4. Implement to design Systems in place to manage necessary variations to the plan. Minimise last minute changes on a blast pattern to add water resistant product during rain by planning ahead and charging blasts with suitable products proactively.

5. Measure during blast Pressure, VoD, temperature.

6. Reactive systems as back up Risk assessment, wind direction simulation, gas detectors and emergency evacuation plans.

Conclusions

• Water flow routes obvious, but unexpected on site. • Post detonation reactions cause visible fume. Many

small factors contribute. • Product selection, downhole quality, testing vital. • In soft ground rapid expansion and cooling leads to

conditions for NO2 • Fume mitigation requires proactive approach and

testing, which will reduce fume, improve efficiency on the mine and also offers cost advantages.

http://hfacs.com/hfacs-framework.html

Acknowledgements

• Australian Coal Industry’s Research Program (ACARP) C23016.

• Steve Simmons, Vishwa Bhushan, Lindsay Ford and Keith Smith are thanked for their support and assistance.

• Gary Cavanough and Miguel Araos for testing. • Site teams

Questions