All of this sounds very similar to the prodcution of superphosphate and the flouride byproducts
below is an edited extract from a literature review of fluoride managment during SSP production
"The exact chemistry of the reaction between sulphuric acid and phosphate rock is not precisely understood due to the unknown composition of phosphate
rock. The simplified reaction of fluorapatite and sulphuric acid is commonly used as a good representation of this process. In this process sulphuric
acid reacts with the fluorapatite to form phosphoric acid, calcium sulphate and a fluoride based compound. The phosphoric acid goes on to react with
another fluorapatite molecule to form monocalcium phosphate and an additional fluoride compound. This process can be seen below;
2{3[Ca3(PO4)].CaF2} + 18H2SO4 → 18CaSO4 + 12H3PO4 + 2CaF2
3[Ca3(PO4)].CaF2 + 12H3PO4 → 9Ca(H2PO4)2 + CaF2
3CaF2 + 3H2SO4 → 3CaSO4 + 6HF
9Ca(H2PO4)2 + 9H2O → 3Ca(H2PO4)2.H2O
21CaSO4 + 42H2O → 6CaSO4.2H2O
3[Ca3(PO4)].CaF2 + 7H2SO4 + 17H2O → 3Ca(H2PO4)2.H2O + 7CaSO4.2H2O + 2HF
The current mechanism of fluoride evolution is the production of hydrogen fluoride from the reaction of fluorapatite and sulphuric acid. It has then
been suggested that the hydrogen fluoride gas is initially converted into hydrofluoric acid. A reaction then occurs with the silicates in the
phosphate rock to form silicon tetrafluoride. This reaction can be seen in the 2 equaitons below where M2SiO3 represents the silica based compounds.
6HF → 3H2F2
M2SiO3 + 3H2F2 → SiF4 + M2F2 + 3H2O
there is only partial volatilisation of fluoride and it is predominantly in the form of silicon tetrafluoride. This is assuming there is a high rate
of conversion from hydrogen fluoride to hydrofluoric acid
The established set of reactions with hydrogen fluoride as an intermediate product has been put under dispute. It was found the reaction between
concentrated hydrofluoric acid and silicates are usually very slow; therefore, there should be more hydrofluoric acid present in the gas evolved. It
was even found that hydrofluoric acid reacts with the fluorapatite to form available phosphate rather than with the silica to enhance the
volatilisation of fluorine as seen below. These results also coincide with experiments completed where quartz flour and other silicates were added to
low silica phosphate rocks where it was found that there was no increase in the fluorine volatilisation. Also with experiments where 12% hydrogen
fluoride was directly added to the process and there was no increase in fluoride evolution observed. It has been said it is more likely that silicon
tetrafluoride is directly produced but there are currently no reaction pathways which detail this process
Ca10(PO4)6F2 + 10H2F2 → 6H3PO4 + 10CaF2 + 2HF
An experiment was completed which showed that the addition of calcium fluosilicate increases the evolution of the total fluoride evolution. The
percentage of total fluoride evolution remained very consistent between 35.4-35.9%. This indicates that the level of fluoride evolution is directly
related to the presence of calcium flurosilicate compounds. It was concluded that fluorine evolution behaves like hexafluorosilicic acid and not like
hydrofluoric acid.
Fluoride evolution is the main by-product from the production of superphospahte. The majority of the fluoride is evolved from the initial reaction of
phosphate rock with sulphuric acid in the form of silicon tetrafluoride. The silicon tetrafluoride vapours are then run through a wet scrubbing system
in the presence of excess water. It is generally accepted that the silicon tetrafluoride is hydrolysed to form HFA and silicon dioxide. This reaction
has even been shown to take place within the reaction chamber as there is an excess of water vapour present with the silicon tetrafluoride. There has
been a wide variety of different reactions which have been shown to form the HFA and silica based products as seen in below. The first three reactions
agree on the formation of HFA but differ in the type of silica molecule that will be formed. Three different types of silica have been hypothesised to
form they are silicon dioxide (SiO2), hydrated silicon dioxide (SiO2.2H2O) and silicon hydroxide (Si(OH)4) below. Most workers have cited at least one
of these reactions but no experimental evidence has been produced.
3SiF4 + 2H2O → 2H2SiF6 + SiO2
3SiF4 + 4H2O → 2H2SiF6 + SiO2.2H2O
3SiF4 + 4H2O → 2H2SiF6 + Si(OH)4
It has also been noted that when silicon levels are high fluodisilicic acid (H2SiF6.SiF4) is formed in equilibrium with HFA in solution. In this case
the silicon tetrafluoride is thought to be hydrolysed the reaction expressed below.
(3+2n)SiF4 + 2H2O → 2H2SiF6.nSiF4 + SiO2
Any of these reactions are possible. There has been insufficient experimental work completed; therefore, it can not be said which ones take place. It
has even been hypothesised that each of the reactions will occur depending on the reaction conditions, such as, temperature, pressure, pH and many
others. A significant amount of work has been completed on the gaseous phase reaction between silicon tetrafluoride and water which is often used in
the production of high purity amorphous silicon and silicon dioxide fibres . It was stated that other chemicals besides HFA are formed during the
hydrolysis of SiF4. These include hexafluorodisiloxane (SiOF6) and perfluorotrisiloxane (Si3O2F8). Mass spectrometer readings showed that the compound
SiF3OSiF2OSiF3 was present. The Majority of this research is not applicable to the wet scrubbing system used in the production of SSP. This is because
reactions are completed at temperatures often above 200°C and are completed with an excess of silicon tetrafluoride and extremely low concentration
of oxygen and hydrogen. There was a study completed that examined the gaseous phase reaction between silicon tetrafluoride and water at atmospheric
conditions. This was completed because of the potential for the formation of hydrogen fluoride which poses a safety risk. This work may relate to the
conditions found above the reaction of phosphate rock and sulphuric acid. The reactions put forward are shown below. The results from this work
concluded that hydrogen fluoride is not produced during the exposure of silicon tetrafluoride to humid air. An unidentified hydrolysed compound was
produced.
4SiF4 + 2H2O → 2(F3Si)2O + 4HF
2SiF4 + 2H2O → H2SiF6 + SiO2 + 2HF
SiF4 + 2H2O → SiO2 + 4HF
A solution is produced from the hydrolysation of silicon tetrafluoride during the production of Superphosphate this is mainly comprised of HFA
(H2SiF6) and silicon dioxide (SiO2) in solution with water. It has been shown by several workers to be an inaccurate simplification. This is because
the scrubber liquor has been found to have a fluoride to silica ratio (F:Si) of 4.7-5.35. The ratio is significantly lower than the F:Si ratio of 6 of
pure HFA. Several workers have stated that the low ratio is due to the formation fluodisilicic acid (H2SiF6.SiF4) which is present in a state of
equilibrium with the HFA compound. The equilibrium position is mainly influenced by the concentration of fluoride ions. This hypothesis does hold as
the properties and composition of the solution act as a mixture of HFA and hydrated SiF4.2H2O. It has also been proposed that the species SiF5- is in
equilibrium with SiF4 and SiO2. It has also been stated that the HFA may be present in either mono-, di-, and tetra-, forms of hydration. A
significant amount of work has been completed relating to the vapour pressure and vapour phase composition of scrubber liquor solution and pure HFA.
Several experiments were completed to examine the volatility of HFA. It was found that when HFA was heated a distillate of higher concentration could
be produced. It was identified that this could be due to the volatilisation of HFA or the decomposition by means of the reaction shown below.
Additional experiments were completed where the HFA was again heated, but the vapour produced were passed through glass fibre which would adsorb the
hydrogen fluoride, if it was present. In this experiment no distillate was produced regardless of the condenser temperature. A similar experiment was
then completed where the humid air was added to the receiver vessel and a distillate containing HFA was again formed. It was therefore concluded from
these experiments that HFA is a non-volatile acid similar to sulphuric acid, and cannot exist under ordinary conditions in the vapour state. This
means pure HFA cannot be isolated as it decomposes to silicon tetrafluoride and hydrogen fluoride.
H2SiF6 → SiF4 + 2HF
It was also noted in the experiments that vessels which contained HFA became etched. This was most pronounced at the surface line of the liquid but
there was consistent etching below the liquid level. The etching faded about an inch above the surface of the solution. There was little to no etching
in further vessels which were connected up. It was hypothesised that this process occurred due to the decomposition of HFA to hydrofluoric acid and
silicon tetrafluoride. The hydrofluoric acid rapidly went on to react with the silicon in the glass to form silicon tetrafluoride and water. This
result coincides with the observation that when scrubber liquors contain high levels of silica the vapour phase will be rich in silicon tetrafluoride.
This is compared to the solutions used inother experiments, which were low in silica, so formed elevated levels of hydrofluoric acid that went on to
react with the silica in the glass. These results are significant to the process because this explains the relatively low levels hydrogen fluoride
found within the fluoride vapour evolved from the phosphate rock and sulphuric acid. This is due to the reaction of hydrogen fluoride with the silica
which is of great abundance within the scrubber liquor.
The affect of variations in the concentration of HFA were also examined. It was found that the maximum practical concentration of HFA within an
industrial scrubbing system was between 22-25%. Above 25-26 wt% it was found that there was a sudden marked change in the character of the suspended
silica. It was found to act as a stiff foam causing rapid expansion in volume. This significantly reduces the efficiency of fluoride scrubbing to a
point where there can be overflow between vessels and a significant reduction in fluoride absorption capacity occurs. Foaming was only found to happen
in a scrubbing system, as it was not found when HFA was concentrated 60.92% at room temperature (20-22°C). It was found that HFA was stable in this
form for long periods of time.
The reaction of HFA and sulphuric acid has been of significant interest since recycling of scrubber liquor was developed. This was developed as a
fluoride management process that reincorporated fluoride, which was initially present within the phosphate rock back into the finally product. It was
found that this resulted in significant re-evolution of fluoride within the process. This is directly attributed to the reaction between sulphuric
acid and HFA due to a clear relationship between several sulphuric acid based factors. Influencing factors included the concentration of sulphuric
acid and proximity of sulphuric acid injection to HFA injection.
There were two main reaction pathways which have been attributed to the model where sulphuric acid is the driving factor for the re-evolution of
fluoride from HFA. This can be seen in the Equations below.
H2SiF6 + 6H2SO4 → 6HSO3F + SiO2 + 4H2O
6HSO3F + 6H2O → 6H2SO4 + 6HF
4HF + SiO2 → SiF4 + 2H2O
H2SiF6 + 2H2SO4 → 2HSO3F + SiF4 + 2H2O
4HSO3F + SiO2 + 2H2O → SiF4 + 4H2SO4
The overall generalised process of sulphuric acid catalysis of HFA decomposition can be seen in the Equation below.
H2SiF6 + 6H2SO4 → 6H2SO4 + 2HF + SiF4
It should be noted that these reactions are purely theoretical and can be rearranged to give the same final product. In all of these reactions
sulphuric acid only acts as a catalyst in the formation of hydrogen fluoride and silicon tetrafluoride from HFA.
Several experiments have been completed on this process with varying results. It has been found that the level of fluoride evolution can vary between
75 and 100% as the fluoride to silica ratio is decreased from 6:1 to 4:1. This relates back to what is seen in the reactions above where they can both
proceed to completion in the presence of excess silica. It is more likely to follow the reactions seen in Equations below
H2SiF6 + 2H2SO4 → 2HSO3F + SiF4 + 2H2O (23)
4HSO3F + SiO2 + 2H2O → SiF4 + 4H2SO4
as the evolution of hydrogen fluoride in the absence of silica is not observed. There is also the alternative possibility that the main reason for
high fluoride re-evolution is due to the heat which is released with the mixing of the HFA and sulphuric acid. It should also be noted that none of
the reactions explain the repeated observation of the formation of voluminous white solid. The solid flows like a gel at the mixing tee of the
sulphuric acid and HFA which suggests a silica or calcium sulphate compound. Considering the amount of research that has been completed on the
recycling of HFA, there is relativity little known about the reactions that govern this process.
It is common practice to recycled HFA back into the reaction of phosphate rock and sulphuric acid. This reaction acts as a means of locking the
fluoride into a solid state which is of negligible environmental effect. It also supplements the use of sulphuric acid as an active hydrogen donating
molecule. Two different reactions between phosphate rock and HFA have been hypothesised. It has been suggested by two separate workers that calcium
hexafluorosilicate, phosphoric acid and hydrogen fluoride are the produced from the reaction between phosphate rock and HFA as seen in the Equation
below. An alternative theory suggests that calcium fluoride, phosphoric acid and silicon dioxide are formed from this reaction as seen in the other
Equation below.
3[Ca3(PO4)2].CaF2 + 10H2SiF6 → 10CaSiF6 + 6H3PO4 + 2HF
3[Ca3(PO4)2].CaF2 + 3H2SiF6 + 6H2O → 10CaF2 + 6H3PO4 + 3SiO2
Both of the above reactions produce phosphoric acid and a compound that traps the fluoride within the solid state. This coincides with the
observations made that the addition of HFA acts to supplement the action of the sulphuric acid as an active hydrogen donating compound to aid in the
acidulation of the phosphate rock. It has also been found that approximately 25% of the recycled fluoride is contained within the final SSP as a solid
compound.
Several workers have found that the solid fluoride based product was in the form of calcium fluoride. It was also found by another worker that when
phosphate rock was reacted with the scrubbing liquor, only 85% of the stoichiometric portion of the rock could be added before an extremely voluminous
precipitation of silica was formed. It was found that there is a clear reduction in the vapour pressure of fluoride above scrubber liquor with the
increased proportion of phosphate rock. All of these observations indicate that the reaction shown in the Equation below is the most likely to occur.
It was stated that further analysis is required to be completed on the reaction between phosphate rock and HFA.
3[Ca3(PO4)2].CaF2 + 3H2SiF6 + 6H2O → 10CaF2 + 6H3PO4 + 3SiO2"
hopefully some of this information may be usefull for you current or future experiments
[Edited on 28-11-2012 by feacetech] |
)
It would seem that
due to the hydrolysis to HF, if there is a thin coating of water enough is produced in high enough concentrations in those areas to etch the glass. Do
not do this with glass you like... This was a brand new beaker. 