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Powder Technology1082000160–163
https://www.doczj.com/doc/1a4782618.html, r locate r powtec
The effects on baghouse filter bags during the incineration of
high-fluoride wastes
Zhaoguang Yang
Aptus,Rollins En?ironmental Ser?ices,P.O.Box27448,Salt Lake City,UT84127,USA
Accepted20September1999
Abstract
Fluorides and fluorosilicates have been widely used in many industrial plants.Hence,there are many hazardous wastes which contain high fluoride.Fluoride gases were reacted with fiberglass at high temperature and the reaction studied as a function of percentage of moisture.Keeping the quench tower,in front of baghouse,in a good working condition can efficiently reduce the attack to the filter bags from the fluoride gases.Aptus,wholly owned by Rollins Environmental Services,currently operates several environmental service facilities in the US.One of six high-temperature incinerators in Rollins is located in Aragonite,UT,which burns hazardous and industrial wastes and features a state-of-the-art air pollution control system.q2000Elsevier Science S.A.All rights reserved.
Keywords:Baghouse filter bags;Incineration;High-fluoride wastes
1.Introduction
There is potential substantial discharge of fluorides from various industry plants.Aptus is an environmental service company which burns hazardous wastes,including high-concentration fluoride wastes.The gases through the baghouse contained fluoride stream.Fluoride gases will attack silica oxide which is a major component of the fiberglass bag.Depending on the level of fluorine gases, moisture and temperature,the degradation to the fiberglass can be slow and worse over time.
The primary objective of this study is to investigate the mechanisms and effects of interaction of fluoride gases with fiberglass baghouse filter media.
2.Materials and methods
?.
The fiberglass bags GL65-Tri-Loft were purchased from BHA Group,HF solution was obtained from Baker. All containers,pipets,volumetric flasks,beakers etc.were made of Teflon to avoid fluoride–glass interactions.Sev-eral groups of experiments were designed and performed.
1.Deterioration test of fiberglass bag samples continu-
ously soaked in varying concentrations of fluoride solu-tion.
2.Deterioration test of fiberglass bag samples HF mois-
ture conditions.3.Deterioration test of fiberglass bag samples HF dry
conditions.
For Experiment1,the bags were prepared for experi-mentation by cutting a bag into100-mm square pieces. Five of the sample pieces were then continuously soaked in varying concentrations of HF solution.Fluoride solu-tions used in the experimentation contained initially7000, 5000,3000,1000,and500mg r l HF.The solution volume was100ml for each sample.Another five samples were placed in HF solution concentrations as stated above.The samples were then removed from the solutions,and placed in125-ml containers.The moisture condition was main-tained.Experiment3entailed immersing five more sample pieces in the above stated solution concentrations and then placing the samples in an oven until dry.
The baghouse dust samples were sent to the University of Utah for the X-ray microprobe and X-ray diffraction test.
3.Experimental results
Table1shows the test results of filter bag samples in HF solution.In7000mg r l HF solution,the bag sample started to show slight deterioration after2days.After7 days,it was seriously damaged.In5000mg r l HF solu-tion,the sample exhibited slight deterioration,the same as
0032-5910r00r$-see front matter q2000Elsevier Science S.A.All rights reserved.
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PII:S0032-59109900215-6
()
Z.Yang r Powder Technology1082000160–163161
Table1
Test results of filter bag samples in HF solutions
?.?.
Test period day Bag Condition in various HF concentration solution mg r l
5001000300050007000
1normal normal normal normal normal
2normal normal normal slight deterioration slight deterioration
3normal normal normal slight deterioration deterioration
7normal normal very slight deterioration deterioration deterioration
10normal normal very slight deterioration deterioration serious deterioration
18normal normal very slight deterioration deterioration test ended
28normal normal very slight deterioration deterioration
39normal normal very slight deterioration deterioration
43normal normal very slight deterioration deterioration
in7000mg r l HF solution after2days.From the7th day until the44th day,there was still no serious damage because the pH value increased.The sample in3000mg r l HF solution showed slight deterioration after the7th day until the44th day.Both samples in1000and500mg r l HF solution were in normal condition until the test was fin-ished.Table2indicates the pH changes for the test.Note that following time change the pH value greatly increased. That means if the fresh HF solutions were changed every-day,the deterioration rate of bag samples would be faster.
Table3shows the results of filter bag samples in HF moisture condition.The bag sample in7000mg r l HF solution showed very slight deterioration after9days.The bag sample in5000mg r l HF began to show very slight deterioration after43days.The rest of the samples re-mained in normal condition until the test was completed. The all bag samples in Experiment3stayed in normal condition after41days of testing.
The elements found in baghouse dust samples were: iron,chlorine,calcium,silicon,phosphorous,sodium,alu-minum,magnesium,and fluorine.
X-ray diffraction analysis was performed on the sam-ples.Calcium fluoride and magnesium fluoride were found. The fluoride–silicon compounds cannot be found because of the minimal amount.
4.Discussion
The dissociation constant of HF in aqueous solution has
w x
been measured by many investigators1,2.The data indi-cate that the dissociation constant of HF lies between2.82
Table2
Change in pH of time during the experimentation of Table1
?.?. Test period day pH value in various HF concentrations mg r l
5001000300050007000
1 1.7 1.5 1.00.90.8
39 6.0 5.9 5.0 3.2 3.0
43 6.8 6.7 6.4 3.8 3.0and3.78.The value determined depends on ionic strength,
temperature and simply the means employed of measuring the constant.The value of p k from the most recent work
a
w x w x
3is2.97.Lower pH,more HF occurs,which attacks
w y x
quartz more seriously than F.
Hydrofluoric acid is a weak acid,and,therefore,at low pH levels most of it is present as undissociated HF which attacks conventional laboratory apparatus,especially glass-ware.The filter bags in Aptus are made with fiberglass.It is possible for the low pH HF gases to react with silicone on the filter bags.This reaction will damage the bags.
?
The reactions of dilute HF and NaF a completely
.
dissociated fluoride solution aqueous solutions with quartz were studied as functions of time at various constant w x
temperatures4.With both HF and NaF solutions the
product of the reactions is probably SiF2y.In the case of
6
HF solutions,the reaction obeys a second order rate law with respect to fluoride concentration in solution.The activation energy for the reactions is12840cal r mol and
w x
the Eyring activation enthalpy is10603cal r mol4.It is
?
believed that the following reaction assume moisture HF
.
gases as a solution:
SiF gas q2HF solution°2H q q SiF2y1?.?.?.
46
is rate-controlling in this system.
In the case of dilute NaF solutions the reaction rates are much slower and almost independent of temperature.The reactions follow an apparent first order rate law.It is believed that in this case a diffusion process is controlling the rate of the reaction.
An HF solution readily attacks and dissolves quartz. The equation often cited pertaining to the first step in HF reaction with quartz is:
SiO solid q4HF aq°SiF gas q2H O2?.?.?.?.
242
or if amorphous silicic acid is present on a disturbed quartz,silica or silicate surface the equation should be:
Si OH solid or surface q4HF aq°SiF gas
?.?.?.?.
44
q4H O.3
?.
2
()Z.Yang r Powder Technology 1082000160–163
162Table 3
Test results of bag samples in HF moisture condition ?.?.Test period Days Bag concentration in various HF concentration solution mg r l 500100030005000
7000
1normal normal normal normal normal 8normal normal normal normal normal
9normal normal normal normal very slight deterioration 15normal normal normal normal very slight deterioration 28normal normal normal normal
slight deterioration 43
normal
normal
normal
very slight deterioration
slight deterioration
It is also possible that the amorphous surface on these minerals partially dissolves in water.In this case,the predominant aqueous silicate species at pH values more ?.?.w x acidic than pH 9will be Si OH aq 5.In this case,the 4following reaction should occur:
Si OH aq q 4HF aq °SiF gas q 4H O.4?.?.?.?.?.442?.The SiF gas thus formed reacts with HF:4SiF gas q 2HF aq °H SiF aq 5?.?.?.?.
426or
SiF gas q 2HF aq °2H q q SiF 2y .
?.?.46The stability constant for this reaction is uncertain.w x Mellor 6claims:
H SiF ?.
264k s s 2.5=10.6?.
2
w x SiF gas HF ?.
?.4w x Ryss 7claims:SiF 2y °SiF gas q 2F y 7?.?.64k s 6.5=10y 7,
SiF gas q 2H O °SiO q 4HF aq 8?.?.?.422k s 1.04=10y 8,
SiF 2y q 2H O °SiO q 4H q q 6F y 9?.
22k s 5.4=10y 27
w x and according to Rees and Huddleston 8:w x y SiF gas F ?.?.2
4y 6k s
s 1=10at 208C.10?.
2y
SiF
w x However,Cooke and Minski 9concluded that the ??..‘‘constant’’for reaction Eq.10is not truly constant but varies with fluorosilicate concentration in addition to tem-perature.These authors found that at 258C,k was 1.3=10y 6for 10y 3kmol r m 3fluorosilicate ion but dropped rapidly with concentration to 3=10y 7for 1=10y 2kmol r m 3fluorosilicate ion.
In the present baghouse,the reactions involve quartz ?.fiberglass filter ,a solid,and an aqueous phase containing ?.a fluoride HF,NaF,KF,etc.,most of HF an overall heterogeneous reaction is involved.In the present system,
the overall reaction must involve at least the following ?.steps assume moisture gases as solutions .
?.q y 1.Transport of HF aq or H and F in solution to the quartz–solution interface.
2.Adsorption of these aqueous constituents onto the quartz surface.
3.Reaction between the adsorbed species and quartz.
4.Desorption of soluble products of the reaction from the quartz surface.
5.Diffusion of the soluble products into the bulk of the solution.
6.Further reaction of the products among themselves or with constituents of the solution.
Should the quartz dissolve unevenly,there may also be diffusion of reactants and products through a porous sur-face layer.
Had the reaction rate been controlled through diffusion processes,it would be expected that the reaction would follow an apparent first order rate law.Since it is not the ?.case,it is suggested that reaction 5is the rate-controlling mechanism when an HF aqueous stream reacts with quartz.However,regardless of the reaction mechanism,the prod-uct must be the fluorosilicate ion.It is of interest to note that fluorosilicic acid is a rather strong acid roughly com-w x parable to sulfuric acid in strength 10.
When NaF was substituted for HF,the reaction with quartz was much slower although similar amounts of ?y .fluoride HF q F were ultimately converted to another ?2y .form presumably SiF .Since the reaction follows an 6apparent first order rate law,the rate-controlling mecha-nism must be different in the case of the dilute NaF solution.In such a system,NaF will attack fiberglass filter bag very slowly.
In either case,the reactions are quite complicated and must be slow.Thus,diffusion probably controls the reac-tion rate.Considering these reactions,the accompanying rise in pH is not surprising.
Thus,how effective quartz is in reacting fluoride from a fluoride aqueous effluent stream will depend on pH of the ?.stream.In low pH ;3,HF with quartz is a homoge-neous chemical rate control,it is a temperature-dependent ??.?..reaction,too.Note that all reactions from Eqs.1–10happened in the solutions or water.Tables 1and 3and
()
Z.Yang r Powder Technology1082000160–163163
Experiment3also confirm the reaction of fluoride with quartz is very slow without solution and water.If the
?.
quench tower spray dryer works well and reduces the moisture from dust gases to the baghouse,that can mini-mize the damage of filter bags.The Aptus baghouse operation indicates that moisture of baghouse dust will determine the life of the filter bag.If the moisture of baghouse dust was less than1%,there will be almost no chemical attack for the bags.Moisture higher than5%will degrade the life of the filter bag.The cold winter weather probably causes the condensation of moisture gases for the system,specially during shut down and starting.
5.Conclusions and suggestions
The following conclusions have been made.
?.1Considering quartz data obtained also suggest that if an HF solution is reacted with quartz the reaction follows an apparent second order rate law with respect to fluoride ?y.
concentration HF q F and temperature.
?.?.
2At a higher pH value;pH6the reaction rate is much slower,first order and apparently controlled by diffusion.
?.3If a dilute NaF or KF gases r stream are reacted with quartz the reaction is much slower and follows a pseudo first order rate law with respect to fluoride concentration. Unlike the reaction in the HF system,the reaction in the NaF or KF system is very little temperature-dependent.
?.4Fluoride gases can react with solid or liquid calcium and magnesium,but no evidence exists to show that they can rapidly react with solid quartz in high-temperature ranges.Fluoride can rapidly react with SiO,Ca,and Mg
2
in the fluoride solutions at ambient temperature.Hence, keeping the quench tower in excellent working conditions can minimize the damages to the baghouse.
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