Cost effective new formulation of ANFO by using Biomass Briquette as Additive
By: Partha Das Sharma
Endeavour is to promote cost effective composition of ANFO, which do not compromise with the energy output (VOD) very much.
ANFO (or Ammonium Nitrate/Fuel Oil) is a widely used bulk industrial explosive mixture in mines and quarry operation. It consists of 94 % porous Ammonium Nitrate (NH4NO3), (AN) that acts as the oxidizing agent and absorbent for the fuel – 6 % number 2 fuel oil (FO), popularly known as High Speed Diesel (HSD). This forms a reasonably powerful commercial explosive. ANFO is non cap-sensitive explosives and requires a large shockwave to set it off.
ANFO has found wide use in coal mining, quarrying, metal mining, and civil construction in undemanding applications where the advantages of ANFO’s low cost and ease of use matter more than the benefits offered by conventional industrial explosives, such as water resistance, high detonation velocity, and performance in small diameters. It is estimated, use of this product accounts for an estimated 80% of explosives used annually, globally. To keep the cost down, bulk ANFO is used, i.e., mixed at the mines / quarry near the borehole by a bulk truck or stationary mixer.
2. Industrial Ammonium Nitrate and its use:
Ammonium nitrate is widely used as a fertilizer in the agricultural industry. In many countries its purchase and use is restricted to buyers who have obtained the proper licence. This restriction is primarily because it is an attractive and simple component used in the production of fertiliser bombs.
In the mining industry, the term ANFO specifically describes a mixture of solid ammonium nitrate prills and heating oil. In this form, it has a bulk density of approximately 840 kg/m3. The density of individual prills is about 1300 kg/m3, while the density of pure crystalline ammonium nitrate is 1700 kg/m3. AN prills used for explosive applications are physically different from fertiliser prills; the former contain approximately 20% air. These versions of ANFO which use prills are generally called explosives grade, low density, or industrial grade ammonium nitrate. These voids are necessary to sensitize ANFO: they create so-called “hot spots”. Finely powdered aluminum can be added to ANFO to increase both sensitivity and energy; however, this has fallen out of favour due to cost. Other additions include perlite, chemical gassing agents, or glass air bubbles to create these voids.
AN is highly hygroscopic, readily absorbing water from air. It is dangerous when stored in humid environments, as any absorbed water interferes with its explosive function. AN is also water soluble. When used in wet mining conditions, considerable effort must be taken to dewater boreholes.
Other explosives based on the ANFO chemistry exist; the most commonly used are Emulsions. They differ from ANFO in the physical form the reactants take. The most notable properties of emulsions are water resistance and higher bulk density.
AN is a substance with relatively weak explosive properties and is not considered an explosive in many classifications. Adding some combustible material, such as nitro compounds, liquid hydrocarbons, solid Hydrocarbons or metal powders to AN increases significantly its explosibility and Energy. Uniform mixing of oil and ammonium nitrate is essential to development of full explosive energy. High explosives boosters are sometime spaced along the borehole to assure propagation throughout the column.
The popularity of ANFO is largely attributable to its low cost and high stability. In most jurisdictions, ammonium nitrate need not be classified as an explosive for transport purposes; it is merely an oxidizer. Many mines prepare ANFO on-site using the same diesel fuel that powers their vehicles, although heating oil, which is nearly identical, may cost less than diesel fuel due to lower fuel tax. Many fuels can theoretically be used; the low volatility and cost of fuel oil makes it ideal.
ANFO has a wide variety of applications in dry hole blasting conditions. It is one of the most cost effective blasting agents available for use in large hole diameter mining through to small hole diameter quarrying. Pneumatically loaded, ANFO is also effective in underground development and tunneling applications.
Features and benefits of ANFO are, (a) it is a dry and free flowing product, allowing delivery by loose pour or pneumatic loading, (b) its low bulk density provides excellent charge distribution in the blasthole, (c) it provides excellent heave energy.
3. Effort in reduction of Cost of ANFO by using Biomass briquette or White Coal as additive:
With recent escalations in Ammonium Nitrate and fuel prices, there have several attempts to reduce the cost of ANFO, without sacrificing the blast performance.
In order to reduce the cost of ANFO, green fuel – Biomass Briquette or White Coal – additive is used.
Nowadays white coal is made from Groundnut shells, Cotton hulls and salks, Castor seed shells, Forest leaves; wood chips and shavings, Sugarcane bagasse, Rice husk and paddy straw, Mustard waste, Coir dust, Coffee husk, Sunflower waste, Maize stalks, Bajra cobs, Sesame seeds oil cake, Wheat straw etc.
Benefits of white coal are (a) White coal is cheaper than coal and fire wood, (b) There is no sulphur in the white coal, therefore no toxic gases, (c) Moisture content is nil, (d) Biomass briquettes have a higher practical thermal value, (e) Briquettes have consistent quality, have high burning efficiency, and are ideally sized for complete combustion, (f) Combustion is more uniform compared to coal due to higher quantity of volatile matter in briquettes, (g) Low ash contents and (h) The calorific value of the finished briquettes is approximately 4000 to 5000 kcal/kg.
Environmentally, the use of biomass briquettes produces much fewer greenhouse gases, specifically, 13.8% to 41.7% CO2 and NOX.
In this system it has been tried to apply combination of low-cost liquid Hydro-carbon fuel (used Diesel) and Biomass Briquette or White Coal i.e., Solid green Fuel (crushed) with porous Ammonium Nitrate, in order to make the blend cost effective, at the same time, without compromising the Energy output of such blend during the blast.
4. Development of the new Formulation:
- As uniform mixing of fuel and ammonium nitrate is essential to obtain maximum explosive energy, several methods of mixing in the field and ingredients used can be employed to achieve optimum result.
- It has been experimented, Ammonium Nitrate blended with solid green fuel, in the form of crushed Biomass Briquette or White Coal and liquid Hydro-carbon fuel (in the form of Diesel oil and used oil) in various proportions to enhance blasting efficiencies.
- The hydrocarbon required for combustion in ANFO is provided by Fuel Oil / Used oil to the extent of 4%.
Therefore, in effect, the blend consists of following four components: (a) Free-flowing Ammonium Nitrate, (b) Liquid fuel oil / Used Oil, (c) Solid green crushed Biomass Briquette or White Coal.
|ANFO / Biomass Briq Cost Calculation:|
|Saving in cost||of ANFO / Briq||3915.00|
|%age saving||of ANFO / Briq||11.01|
Cost Effectiveness – Reduces overall cost by about 11%
- Maximum possible size reduction of Biomass Briquette should be done before use.
- Replacing some portion of Diesel with used oil may reduce further cost.
- First Ammonium Nitrate is to be mixed with required percentage of Diesel or oil (as mentioned), then crushed / grind Biomass Briquette is to be mixed with above ANFO.
5. Blast Performance:
- The new blends comprising of ANFO-waste oil-Solid green fuel (Biomass Briquette) blend has been tested.
- Besides lower the blasting costs, the formulation is comparative in terms of rock fragmentation to standard ANFO (94% AN with 6 % FO).
- The blast output results into low flyrock and consequently effective casting of the blasted rock.
- This helps in safer and secured mucking & loading of the blasted material as well as lower mucking cost since the entire blast was well-fragmented.
- This results in optimum energy utilization of the blast energy and that too for fragmentation and prevents loss of energy resulting from following unsafe and wasteful means: (a) Fly Rock, (b) Vibrations, (c) Noise (d) better heave and (e) preventing of unwanted spreading of blasted material.
- VOD of the new formulation found to be 3600 ± 500 m/sec.
Effectiveness is the most advantageous factor – Reduces overall cost keeping blast performance at par / better than conventional ANFO.
Lessened tendency to have fly rock, lessened amount of back break outside drill pattern was observed. Moreover, less “orange smoke” observed during side-by-side comparisons of ANFO and ANFO-Briquette blasts, suggesting less NOx (orange smoke) formation.