MANUFACTURE OF EMULSION EXPLOSIVES
The present invention relates to a method of producing an emulsion explosive. In particular, the present invention relates to the production of an emulsion explosive under relatively low shear conditions.
According to conventional techniques, emulsion explosives are manufactured by feeding components to mixing devices which subject the components to high shear forces thereby forming an emulsion. The apparatus required and installation, maintenance and running costs mean that such techniques tend to be capital intensive. It would be desirable to provide a process for manufacturing an emulsion explosive which is less expensive and which may therefore readily be employed in remote areas where the capital investment for high shear mixing is unwarranted. The present invention seeks to provide an alternative to current capital intensive manufacturing processes.
In accordance with the present invention it has been found that it is possible to produce an emulsion explosive using sparging to provide the shear necessary for emulsification. Accordingly, the present invention provides a method of producing an emulsion explosive, which method comprises sparging with gas a mixture comprising an aqueous solution of an oxygen-releasing salt and a water-immiscible fuel. Sparging with a gas agitates the mixture thereby forming an emulsion which comprises a continuous phase of the water-immiscible fuel and a discontinuous phase of the aqueous solution of the oxygen-releasing salt.
A further advantage associated with the present invention is that it is inherently safe. In part this is because there are no moving parts in contact with the emulsion during its manufacture. Furthermore, sparging allows devices requiring electricity such as pumps or compressors to be far-removed from the emulsion. Sparging also avoids the need to use devices which impart high shear forces, such as static mixers, to form the emulsion. These factors, together with the simplicity and ease of practice of the invention represent significant advantages in this particular field.
Sparging may be by injection of a gas to form an emulsion and this may be achieved by a variety of sparging techniques. Usually, the gas is injected into the mixture at a pressure of from 10 to 100 psi. However, pressures outside this range may be adopted. Any gas may be employed to sparge the mixture provided that such gas does not react with or adversely effect any components of the emulsion. Mixtures of gases may be used. It is preferred however that the gas which is used for sparging is air. The injection of gas results in turbulence and agitation of the mixture such that an emulsion may form.
Typically, the means through which the gas is injected (hereafter the sparger) is an elongate conduit of approximately circular cross-section. Conveniently, the conduit may be a plastic, rubber or metal tube or pipe. Plastic or rubber hosing may for example be used. The cross- sectional diameter of the conduit is typically 10 to 50mm, for example about 25mm.
The sparger is provided with a number of orifices through which the gas is injected. One end of the conduit is connected to a gas supply, for instance a compressor when compressed air is used to sparge the mixture. The other end of the tube is typically sealed, although it may be provided with an orifice or orifices through which gas is injected into the mixture. The orifices may be holes or circumferential cuts in the conduit. The sparger may be made by piercing/drilling or cutting a tube or pipe.
Sparging takes place in any suitable mixing tank or container and it is possible to adapt a storage-type tank for practice of the invention. Typically, the mixing tank is cylindrical with its long axis running horizontally. The dimensions of the tank will depend upon the volume of emulsion to be prepared. As a practical example, mention may be made of using a cylindrical steel tank 4m in length and 1.8m in diameter.
The sparger may comprise a series of interconnected pipes which may be positioned at or adjacent the bottom of the mixing tank. The interconnected pipes are preferably are designed so that the bubble generating orifices are evenly distributed across the base of the mixing tank. Alternatively, it has been found that adequate mixing can be provided by a single perforated tube running along the length of an elongate tank at or adjacent the bottom. For example, a 20
mm diameter tube having equally spaced holes may be used.
The gas bubble size generated by the sparger is generally determined by the size of the orifices in the sparger and by the pressure under which the gas is injected. Usually, the size of the orifices of the sparger is from 2 to 12 mm in diameter, or possibly even larger. The orifices may be in a linear array or staggered array, or some other arrangement. Plural arrays of the same or different type may be used.
The degree of agitation of a sparger may be described in terms of the air rate, i.e. volume of gas/(tank horizontal sectional area.minute). It is preferred that the degree of agitation is in the range of from 1 to 2 ft/min, most preferably about 1.5 ft/min, to provide complete agitation
The period for sparging will be dependent upon the degree of agitation. Typically, with a degree of agitation of about 1.5 ft/min a total batch of about 6 tonnes, the period of agitation will be in the range of from 15 to 180 minutes. With higher degrees of agitation the period may be less, and with lower degrees of agitation the period may be increased.
The period of agitation influences the viscosity of the emulsion. The temperature of the emulsion may also influence viscosity. In practice secondary refinement of the emulsion for instance, using pumps or static mixers, is employed to achieve a desired viscosity. By using secondary refinement in combination with the sparging method it is possible to optimise the processing time required to obtain an emulsion having a desired viscosity. Typically, sparging takes place until the emulsion has a Brookfield viscosity of 7 Brookfield Units at 65-70°C, after which the emulsion is subjected to secondary refinement.
Typically, the mixture being sparged is at a temperature above the fudge point of the aqueous solution component, i.e. above the temperature at which crystallisation in the salt solution occurs. Usually the temperature is from 30-90°C.
By simple experimentation, the sparging variables, for instance bubble size, period and sparger, configuration and positioning may be selected to achieve an emulsion having the desired
characteristics, the latter typically being the viscosity and robustness of the emulsion. Suitable characteristics will depend upon the intended end use of the emulsion explosive. Usually, the viscosity and robustness are influenced by the period of sparge. It is believed that the configuration and size of the holes in the sparger, and the injected volume and velocity of gas are not especially critical to the ability of the method of the invention to form an emulsion.
In a preferred embodiment of the invention, optimum sparging has been found to occur if the gas bubbles are directed against and impinge a surface of the tank /container holding the mixture and results in reduced emulsification times. This is believed to cause increased turbulence in the mixture. Preferably the surface is the bottom of the container. The sparger is typically spaced from the surface and this spacing may be varied in order to control the turbulence achieved. The spacing should be such that for a given set of sparging characteristics (gas flow rate etc) at least a proportion, and preferably the majority and more preferably all, of the gas bubbles impinge against the surface. The spacing of the sparger from the surface is thus at least partly dependent upon the velocity of the gas leaving the sparger.
Emulsion explosives are well known by those skilled in the art, but for the sake of clarity some explanation of the possible compositions is given below.
Suitable oxygen-releasing salts for use in the aqueous phase of the emulsion include the alkali and alkaline earth metal nitrates, chlorates and perchlorates, ammonium nitrate, ammonium chlorate, ammonium perchlorate and mixtures thereof. The preferred oxygen-releasing salts include ammonium nitrate, sodium nitrate and calcium nitrate. More preferably the oxygen- releasing salt comprises ammonium nitrate or a mixture of ammonium nitrate and sodium or calcium nitrates.
Typically the oxygen-releasing salt component may comprise from 45 to 95 % w/w and preferably from 60 to 90 % w/w of the total emulsion composition. In compositions wherein the oxygen-releasing salt comprises a mixture of ammonium nitrate and sodium/calcium nitrate the preferred composition range for such a blend is from 5 to 80 parts of sodium/calcium nitrate for every 100 parts of ammonium nitrate. Therefore, in the preferred composition the oxygen-
releasing salt component comprises from 45 to 90 % w/w (of the total emulsion composition), ammonium nitrate or mixtures of from 0 to 40 % w/w sodium/calcium nitrate and from 50 to 90 % w/w ammonium nitrate.
Typically the amount of water employed in the emulsion is in the range of from 0 to 30 % w/w of the total emulsion composition. Preferably the amount employed is from 4 to 25 % w/w and more preferably from 6 to 20 % w/w.
Suitable organic fuels for the continuous "oil" phase of the emulsion include aliphatic, alicyclic and aromatic compounds and mixtures thereof which are in the liquid state at the formulation temperature. Suitable organic fuels may be chosen from fuel oil, diesel oil, distillate, furnace oil, kerosene, naphtha, waxes such as microcrystalline wax, paraffin wax and slack wax, paraffin oils, benzene, toluene, xylenes, asphaltic materials, polymeric oils such as the low molecular weight polymers of olefines, animal oils, vegetable oils, fish oils and other mineral, hydrocarbon or fatty oils and mixtures thereof. Preferred organic fuels are liquid hydrocarbons generally referred to as petroleum distillates such as gasoline, kerosene, fuel oils and paraffin oils.
Typically the organic fuel or continuous phase of the emulsion comprises from 2 to 15 % w/w and preferably 3 to 10 % w/w of the total composition.
The emulsion may include one or more emulsifiers chosen from the wide range of emulsifiers known in the art. The emulsifier may be one of the well known emulsifiers based on the reaction products of poly[alk(en)yl] succinic anhydrides and alkylamines, including the polyisobutylene succinic anhydride (PiBSA) derivatives of alkanolamines. Other suitable emulsifiers include alcohol alkoxylates, phenol alkoxylates, poly(olyalkylene)glycols, poly(oxyalkylene)fatty acid esters, amine alkoxylates, fatty acid esters of sorbitol and glycerol, fatty acid salts, sorbitan esters, poly(oxyalkylene) sorbitan esters, fatty amine alkoxylates, poly(oxyalkylene)glycol esters, fatty acid amines, fatty acid amide alkoxylates, fatty amines, quaternary amines, alkyloxazolines, alkenyloxazolines, imidazolines, alkylsulphonates, alkylarylsulphonates, alkylsulphosuccinates, alkylphosphates, alkenylphosphates, phosphate esters, lecithin,
copolymers of poly(oxyalkylene)glycols and poly( 12-hydroxystearic)acid and mixtures thereof. Low molecular weight emulsifiers may be used, particularly sorbitan monooleate. Preferably, the emulsifier is a polyisobutenyl succinic anhydride.
Typically the emulsifier of the emulsion comprises up to 5 % w/w of the emulsion. Higher proportions of the emulsifying agent may be used and may serve as supplemental fuel for the composition but in general it is not necessary to add more than 5 % w/w of emulsifying agent to achieve the desired effect.
Generally emulsions themselves are not detonable and in order to form an explosive composition, the emulsion must be mixed with sensitising agents such as a self explosive (e.g. trinitrotoluene or nitroglycerine) or a discontinuous phase of void agents. Suitable void agents include glass microballoons, plastic microballoons, expanded polystyrene beads and gas bubbles, including bubbles of nitrogen generated in-situ by chemical gassing agents and entrained air. These are typically added after the emulsion has been formed. Investigation of potential to entrain air and hence create sensitising voidage through the process of air sparging has demonstrated that the air bubbles are too large to result in sensitisation.
If desired, other optional fuel materials, commonly referred to as secondary fuels, may be incorporated into the emulsion in addition to the water-immiscible organic fuel phase. Examples of such secondary fuels include finely divided solids and water-miscible organic liquids which can be used to partially replace water as a solvent for the oxygen-releasing salts or to extend the aqueous solvent for the oxygen-releasing salts. Examples of finely divided materials such as sulphur, aluminium, urea and carbonaceous materials such as gilsonite, comminuted coke or charcoal, carbon black, resin acids such as abietic acid, sugars such as glucose or dextrose and vegetable products such as starch, nut meal, grain meal and wood pulp. Examples of water- miscible organic liquids include alcohols such as methanol, glycols such as ethylene glycol, amides such as formamide and urea and amines such as methylamine.
When present, the optional secondary fuel component of an explosive emulsion typically comprises up to 30 % w/w of the total composition.
There may also be incoφorated into the emulsion other substances or mixtures of substances which are oxygen-releasing salts or which are themselves suitable as explosive materials. For example the emulsion may be mixed with prilled or particulate ammonium nitrate or ammonium nitrate/fuel oil mixtures.
Other optional additives may be added to the emulsion explosive compositions hereinbefore described including non-associative thickening agents or chemical thickening agents such as zinc chromate or a dichromate either as a separate entity or as a component of a conventional redox system such as for example, a mixture of potassium dichromate and potassium antimony tartrate.
The mixture of the aqueous oxidiser salt solution and water-immiscible fuel is preferably formed by the addition of separate preblended phases. The aqueous oxidiser salt solution is preferably formed in a separate mixing tank and heated to a temperature above the fudge point of the solution. In a preferred embodiment the oxidiser salt solution is itself sparged so as to agitate and mix the solution.
The water-immiscible fuel may be blended with an emulsifier in a separate mixing tank prior to incorporation into the mixture for manufacturing the emulsion. In a preferred embodiment the blend of fuel and emulsifier is sparged so as to agitate and preblend the water immiscible organic fuel.
The oxygen-releasing salt solution and the water immiscible fuel are preferably simultaneously incorporated into the mixing tank and sparged as has been described above. The formed emulsion may then be pumped from the mixing tank, preferably via a number of static mixers which may increase the refinement of the emulsion, for storage and/or transportation to the blast site.
In an embodiment of the present invention, the sparging technique described herein may be used to recycle (reform) a broken emulsion. In this embodiment the broken emulsion is sparged with a gas as described herein, preferably using compressed air, to reform said emulsion. In
recycling a broken emulsion it may be preferable to add additional emulsifier to facilitate reformation of the emulsion. It is preferred to add a low molecular weight emulsifier such as sorbitan monooleate.
The method of the invention may also be employed to change the characteristics of an existing emulsion. For instance, the method may be used to introduce into the emulsion additional aqeuous solution or water-immiscible fuel. In this way the method may be used to convert one kind of emulsion into another having different compositional characteristics. This embodiment is particularly useful where the characteristics of an initial emulsion are unsuitable for a particular purpose. For instance, an initial emulsion may have a high viscosity and thus may be too viscous to pump, and the method of the invention may be employed to reduce emulsion viscosity by inclusion of additional fuel component.
Further, according to the present invention there is provided an emulsion explosive when prepared by the method described herein.
The present invention will now be described by way of example only with reference to Figure 1. Figure 1 is a schematic representation of one embodiment of the process of the present invention. Figure 1 is in no way limiting with regard to the scope of the invention described herein.
Figure 1 shows a mixing tank (1) into which the components of the emulsion may be introduced. A source of compressed air (2) is fed into the mixing tank (1) along conduit (3). Conduit (3) leads to the sparger (4). Sparger (4) has holes (4a) spaced equidistantly along its length. After the contents of mixing tank (1) have been sparged sufficiently to form an emulsion the contents are then removed from mixing tank (1). Mixing tank (1) is connected by a conduit (5) to a diaphragm or progressive cavity pump (7) such that the contents of mixing tank (1) may be withdrawn. Additional emulsifier may be introduced from storage tank (6) through conduit (10) or directly into the tank (6), prior to the formed emulsion passing through the pump (7). Additional emulsifier introduced in this manner will facilitate the refinement of the emulsion by static mixers (8) after exiting the pump (7). The refined emulsion may then be fed through
conduit (9) into a storage facility (not shown).
The present invention will now be described with reference to the following non-limiting examples.
EXAMPLE 1
4200 kg of ammonium nitrate was dissolved in 1050 kg of water which was heated to 65°C, above the fudge point of the solution (approximately 60°C). The solution was sparged whilst the ammonium nitrate was being dissolved in the water in order to provide improved mixing.
A fuel blend was prepared by mixing 320 kg of distillate with 80 kg of a polyisobutylene succinic anhydride-ethanoloamine derivative and 15 kg of sorbitan monooleate. The distillate was dyed to indicate the efficiency of mixing.
The solution was transferred to the sparge tank. The sparge tank was a 6000 1 stainless steel cylindrical tank, 4 m in length and 1.8 m in diameter disposed horizontally.
The sparge was a 20 mm diameter tube extending in a straight line along the bottom of the tank and spaced 75 mm from the bottom with two linear arrays of 21 x 5 mm holes equally spaced along its entire length. The holes in each array are staggered relative to the other array and their axes are inclined at about 30° on respective sides of the vertical, opening downwardly.
The sparger was connected to a 250 cubic feet per minute (cfm) compressor. The fuel blend was added to the ammonium nitrate solution and air was injected through the sparger at 45 psi. Emulsification was observed after 4 minutes. Samples were taken after 10 and 30 minutes of sparging and the emulsion viscosity and droplet size measured. The emulsion viscosity (in Brookfield Units, BU) is measured using a Brookfield viscometer with no. 7 spindle @ 50 rpm and temperature 65-70°C. The droplet size is assessed by microscope photography. The results are shown in Table 1 below:
TABLE I
The samples taken after 10 and 30 minutes were both stable after 24 hours storage. Stability is assessed by putting a sample of the emulsion in a freezer in an attempt to drive crystallisation.
If the emulsion is unstable, droplets coalesce and crystallisation occurs. This is somewhat of a crude assessment but nevertheless is a good indicator as to emulsion quality.
After 30 minutes further samples were pumped from the sparging tank using a 2 inch diaphragm pump through three 3/4 inch SMX static mixers (40 cycles/minute). The results are shown in Table II below.
TABLE II
EXAMPLE 2
Example 1 was repeated except that the sparger had two linear arrays of 15 x 5 mm holes, a 150 cfm compressor was used and the air pressure was 90 psi.
The fuel blend was added to the ammonium nitrate solution and air was applied through the sparger at a pressure of 90 psi. Emulsification was observed after 4 minutes. Samples were taken after 15 minutes and the results are shown in Table III below:
TABLE III
EXAMPLE 3
Example 2 was repeated except that the sparger had two linear arrays of 4 x 3 mm holes. The air back pressure on the compressor was 100 psi at 100 cfm.
The fuel blend was added to the ammonium nitrate solution and air was applied through the sparge. Emulsification was observed after 4 minutes. Samples were taken after 5 and 13 minutes and the results are shown in Table IV below:
TABLE IV
After 13 minutes further samples were pumped from the sparging tank using a 2 inch diaphragm pump (400 kPa, 84 cycles/minute) through 3/4 inch SMX static mixers. The results are shown in Table V below.
TABLE V
In a further example, Example 2 was repeated but using a single array of 20 x 2 mm holes pointing vertical down to the bottom of the tank, and this resulted in successful emulsification of the mixture in even less time than Example 2. Sparging with the holes pointing upwards into the tank was found to be successful, but emulsification took very much longer.
Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within its spirit and scope. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.