Introduction

Generally，manufacture process of emulsion explosive(EE for short)mainly includes emulsification and sensitization[1].The emulsification will influence stability and shelf life of EE[2-4].While sensitization will affect the detonation performance，especially for bulk emulsion explosive(BEE for short)[5].Since BEE was introduced into China in 1980s，chemical foaming has always been an optimal way for sensitization and a widely applied，convenient and rapid method to foam the emulsion matrix comparing with physical sensitization[6].However，as a chemical reaction，chemical method being influenced by much more factors was found in the production process[1，7]，such as temperature，concentration，catalyst and so on.It is widely known that the bubble accumulated in emulsion contributes to sensitization and density adjustment.If chemical foaming were not properly controlled，an undesired density or bigger bubbles were more likely to produce，however，adjusting the density could hardly create enough hot spot for better detonation and even result in misfire.Thus it's significant for EE manufacturing to investigate chemical foaming process systemically.

1 Test

BEE was selected as a subject in this study.The formula of emulsion matrix was listed in Table 1.At first，775 g ammonium nitrate(AN)is dissolved into 180 g water，stirring the solution and heating it to 85-95℃；secondly，dissolve 15 g emulsifier into 40 g diesel oil in the 2 L container and it is heated to 30-55℃；set up an agitator onto the bench drill，turn it up to 700-1 000 r/min to stir the mixture；then stop the bench drill1min later and pour the emulsion matrix into enamel cup.Acid is introduced into dispersed phase，and the pH or different strength acid affecting foaming was studied.

Tab.1 Formula of emulsion matrix%

MaterialsH2OANdieselemulsifier mass fraction17.077.54.01.5

5%-10%(mass fraction)NaNO2aqueous solution was the main sensitizer applied and M(NO3)2(M is a alkaliearthmetal)solution with different concentration was used as the accelerant.The main sensitizer mix the accelerant with the matrix sufficiently and homogeneously in the enamel cup，and then pour the mixture into a constant volume container and observe the density change regularly.

2 Analysis

The mechanism of the reaction between andcatalyzed by acid was proved in this application.NH3was generated through thedecomposition reaction；the intermediate，H2NNO，was formed by the reaction between NH3and HNO2/N2O3，and then it decomposed soon and produced N2.The mechanism could be listed as the below Equation(1)-Equation(7).Generally speaking，Equation(1)-Equation(2)could be the first step and Equation(3)-Equation(7)could be the second step[8-9].

Chemical foaming process was divided into three distinct stages found in experiment:

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3)The reactant，after reaction in the kinetic regime，remained a low concentration and thus the foaming rate was low.This stage was called the plateau regime[10].

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1)The amount of NaNO2.5%(mass fraction)NaNO2and 10%(mass fraction)NaNO2solution were applied as sensitizer for foaming.Because of low temperature，the M(NO3)2with the same concentration was used as accelerant.The foaming process was presented in Figure 1.

Several factors affecting the foaming process were investigated here:

Fig.1 Foaming process with different amount of NaNO2

From Figure 1，a higher rate of foaming was found due to a higher concentration of NO2-，in the meanwhile，the temperature and quantity of NaNO2were fixed，thus the NaNO2was significant in foaming process.

1)At the beginning，there was no sufficient amount of NH3and HNO2generated due to a low reaction rate；and N2would be dissolved into internal phase until saturation，so that there were few bubbles and the density scarcely decreased.Thus this stage was called the lag regime.

3)Different strength acid.In the experiment，C acid，B acid and A acid were chosen as the catalyst of sensitization.During the experiment，the same amount of NaNO2and acid were used to reduce variability at the same temperature.In Figure 3，due to the stronger acid，an increased rate of reaction and a decrease of aftereffect were found in the first stage.Generally speaking，C acid was regarded as a medium-strong acid，B acid and A acid were weak acid，and their pKa(only primary dissociation constant considered)were 2.12，3.13 and 4.76 respectively.A larger pKa is along with a larger amount of H＋，moreover，C acid and B acid belonged to polybasic acid，using which would increase the foaming process in first two stages compared with A acid.On the other hand，H＋was consumed too much and thus causing plateau regime coming beforehand.In Figure 3，it showed that when the amount of NaNO2and acid were fixed，the stronger the acid was，the larger the density was，because stronger acid would result in a higher rate of gassing and a large number of bubbles，a bigger density would consequently created due to a higher rate of bubble escaping，otherwise reaction at low concentration couldn't maintain the reaction.

Fig.2 Foaming process under different pH

2)pH.Acid was utilized as a catalyst mostly.In the experiment，one type of acid(organic acid)was used to adjust the pH of the aqueous phase.In Figure 2，it showed that when the amount of sensitization composition was fixed，the foaming rate would grow by the increase of the pH value.When the pH＝4.5，the lag period lasted for 15 min.It suggested that H＋with low concentration couldn't catalyze the intermediate reaction at low temperature.It took a longer time to accumulate reactant to proceed the foaming process.Therefore，reducing the pH appropriately could prompt the rate of sensitization.

2)After the lag period，the foaming process began speeding up with a large number of bubbles generated，which resulting in a large decline of the density of emulsion.This stage was also called the kinetic regime.

Fig.3 Foaming process with different acid

In addition，short starting and kinetic regime would lead to big useless bubbles generating easily；stronger acid or low pH could cause more NOxproduced owing to high amount of side reaction product of Equation(2)-Equation(5)；Since the acid damaged the emulsifier and decreased the stability of matrix under high temperature，a low pH of aqueous solution caused a high probability of demulsification.

4)M2＋.At low temperature，applying NaNO2and acid wouldn't accelerate the foaming process.In the experiment，the M2＋was introduced as accelerant.In Figure 4，when the pH＝4.5，the density of matrix was decreased to 1.15 g/cm3over 60 min after adding 35%(mass fraction)M2＋，in contrast，it decreased slightly without M2＋.It was the reaction mechanism that M2＋made compound reaction with NH3to reduce the reaction difficulty with NH3in the second step and reacting with NO＋(a variant of N2O3)to promote generating N2.So M2＋played a significant role in the foaming process at low temperature.In Figure 5，lag period of two processes wasn't found obvious change in gassing rate.While a lower density in the kinetic regime was found after using a larger amount of M2＋，which proved the inference valid.

Fig.4 Application of M2＋in low temperature foaming process

Fig.5 Foaming process with different mass fraction of M2＋

5)T.T，as a catalyzer，was used in bulk emulsion and packing products for decades[1].Adding a few amount of T，emulsion could create a high foaming rate and the reaction mechanism was showed as Equation(8)-Equation(9)[10].NO＋combining with T reacted much easier with NH3than NO＋.

It's important to note that it's easy to generate useless big bubbles which would be harmful for initiation when，T was applied.Therefore，T should be controlled in a reasonable amount during test or practice.

T was added with 0，0.05%，0.10%and 0.15%(mass fraction)into emulsion at 60℃，and the rate of reaction was showed in Figure 6.When the amount of T was 0.15%，a lot of big useless bubbles generating and sound of collapse of bubble were observed.

Fig.6 Gassing process with different concentration of T

6)Temperature.In the experiment，the new made matrix was cooled to 40℃and 50℃respectively and then 10%(mass fraction)NaNO2solution was introduced to sensitize.The foaming process was showed in Figure 7.In Figure 7，a higher rate of foaming was found when the temperature was 50℃in the lag period and kinetic regime，and a lower top density was tested in the plateau regime.

3 Conclusion

Fig.7 foaming process at different temperature

The amount of NaNO2，pH，different strength acids，catalysts and temperature，which influence the foaming process of emulsion were investigated in this paper.In the industrial production process，many other factors were found，such as device，sensitization mode and emulsification mode.In the test，homogeneous and tiny bubbles were found in lowering the rate of foaming.As a result，we should consider those factors in a comprehensive way and reduce the rate in a proper range to gain a good sensitizing result.Thus，a comprehensive method was set up for better understanding.Besides，through test analysis，trend data could be acquired in directing to solve problems in manufacture process，and further it would improve production control level.

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