An Alternative Means of Disposing of Fertilizer Industry's Fluoride Effluent
The following is a presentation given at the International Fertilizer Industry Association's 2000 Technical Conference in New Orleans
Recovery of fluosilicic acid and fluoride bearing waters for the production of a mixture of silica and precipitated calcium fluoride usable for the production of cement
Michele Lavanga*, Sergio Di Lena*, Shane Sullivan** and Alan Ingram** * Fluorid S.p.A, Italy, ** Svedala Ltd., Sweden
(Read full paper at: http://www.fertilizer.org/ifa/publicat/pdf/2000_biblio_125.pdf)
Introduction
Fluorsid is a chemical company, based in Italy, which produces fluorine derivatives, namely aluminium fluoride and synthetic cryolite, for the aluminium industry.
Svedala is a Swedish multinational manufacturer of process equipment for industry.
The reason why Fluorsid, which is not involved in the fertilizer industry, is taking part in this Technical Conference by presenting a paper, is because it shares with many companies of this sector one identical problem: the elimination of waste waters containing fluosilicic acid and hydrofluoric acid. Fluorsid solved this problem with a plant built in cooperation with Svedala.
As fertilizer producers well know, the disposal of fluosilicic acid is a major environmental problem of phosphoric acid and phosphate fertilizer manufacture. Fluorides, such as silicon tetrafluoride and hydrogen fluoride, are among the main polluting streams resulting from phosphoric acid production. Phosphate rocks contain 2 to 4 percent fluorine. Part of the fluorine from the rock is precipitated with the gypsum, another part is leached out with the phosphoric acid product, and the remaining portion is vaporized in the reactor or evaporator. The relative distribution of fluorine among these three forms depends on the type of the rock, the plant configuration and the operating conditions.
From the reactor and the evaporator, fluorine is liberated initially in the form of hydrogen fluoride, but in the presence of silica it readily reacts to form silicon tetrafluoride :
CaF2 + 2H + ---> 2HF + Ca ++
4HF + SiO2 ---> SiF4 + 2H2O
It is common practice to absorb this vapour by water scrubbing, to form fluosilicic acid:
3SiF4 + 2H2O ---> 2H2SiF6 + SiO2
The quantity of fluosilicic acid obtained as a by-product in phosphoric acid production is normally in the range 20 to 40 Kg (as H2SiF6 100%) per ton of P2O5 produced.
At present, only a small portion of this acid is used. In most cases, the effluent is simply discharged to sea or to fresh water rivers.
It is, therefore, evident that the disposal of these effluents is an ever more serious environmental problem for phosphoric acid manufacturers. According to Italian law, the concentration of F - ions in sea or fresh water discharges must be lower than 6 mg/liter.
Fluorsid's experience
Fluorsid has produced, since the early 1970's, hydrogen fluoride, aluminium fluoride and synthetic cryolite (sodium hexafluoroaluminate). These last two products are used by the aluminium industry. Fluorsid is one of the main producers in the world of aluminium fluoride and the largest producer of synthetic cryolite.
For the production of 60,000 tons per year of cryolite and aluminium fluoride, Fluorsid consumes about 90,000 tpy of acid grade fluorspar, which contains on average 1% SiO2. During the first phase of the process, hydrofluoric acid is produced through the attack of fluorspar using sulphuric acid which takes place in rotary kilns at a temperature of 200*C. Silicon tetrafluoride, coming from the reaction between HF and SiO2, then develops together with HF. The SiF4 constitutes a pollutant in the gaseous stream and is separated downstream of the production process through absorption in water. Fluosilicic acid is thus obtained. Annually a total quantity of about 2,200 tons (basis H2SiF6 100%) is recovered:
4HF + SiO --> SiF4 + 2H2O
3SiF4 + 2H2O --> 2H2SiF6 + SiO2
To this, we add hydrofluoric acid contained in the mother liquor of the cryolite production process as well as in fluorinated waters coming from wet scrubbers of other production processes. The total stream is then conveyed to the water treatment plant where Fluorsid overall treats about 600,000 cubic meters per year of waste water containing about 3,000 tpy of fluorine (as F).
In the course of Fluorsid's activity of more than thirty years, to which the experience acquired by its technicians at other companies since the 1950s has to be added, various solutions have been put into operation aimed at the recovery of hydrofluoric acid in order to obtain fluorinated products. Also the literature is full of patents, studies, publications in this sense: Paul A. Smith, of Societe Chimique Prayon-Rupel, has indicated in its presentation at the IFA meeting of Novgorod, in September 1999, nearly 400 bibliographical quotations on the recovery of fluorine from the effluent waters.
The various solutions undertaken by Fluorsid (or by its technicians prior to Fluorsid's existence) have been the following:
1. Production and sale of sodium fluosilicate. This was abandoned because the market is too limited.
2. Production and sale of sodium fluoride. Abandoned for the same reason as above.
3. Production and sale of aluminium fluoride. Abandoned because the resulting product has qualitative characteristics no longer suitable to be employed with the modern technologies for aluminium production (it has in fact a bulk density of 0.6 Kg/dm 3 and cannot be fed into the electrolysis cells with the point feeder system).
4. Production and sale of synthetic cryolite. Abandoned for quality and cost problems.
5. Disposal of fluorinated muds coming from water treatment plant, after neutralization with calcium carbonate and calcium hydroxide.
This last solution was the most economic. However since the early 1990s this method of disposal has become impracticable due to new more restrictive rules and extremely high (60 US$/wet ton) disposal costs. Consequently, Fluorsid has been compelled to look for new solutions.
A careful market survey uncovered in the cement industry an interesting consumer of low concentration calcium fluoride. The cement plants, in fact, add fluorspar during the preparation of the raw cement mix both for obtaining a white cement and to adjust the mineralization of the available raw materials. The CaF2 contents acceptable for this application are in the order of 40-50%.
Fluorsid's attention has been focused on developing a product of this kind. Through laboratory and a pilot plant research, Fluorsid has been able to optimize its own water treatment plant (containing hydrofluoric acid and fluosilicic acid) in order to obtain a synthetic compound whose chemical composition would be acceptable to the cement plants.
The Fluorsid product has a residual humidity at 110*C of about 30%; the chemical composition is found within the following range:
CaF2 50-65%
SiO2 9-10%
CaSO4 8-10%
CaCO3 3-6%
Al2O3 2-3%
Fe2O3 1-2%
MgCO3 1-2%
L.O.I. (450 *C) 10% max.
Mg, Fe and certain heavy metals (Pb, Zn, Cu, Sn), are present at concentrations of a few parts per million, they come from the limestone or from the lime which is used for the neutralization of waste waters. The calcium sulphate originates from the neutralization of acid waste waters containing H2SO4.
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from page 7-8
Applicability of the process to the fertilizer industry
In order to verify the applicability of the process to the fertilizer industry, filtration tests were carried out, both in a laboratory and on industrial plants, of slurry originating from the neutralization of only fluosilicic acid. In a laboratory slurry was filtered coming from the neutralization of H2SiF6 at 15%. Similarly in the industrial plant slurry was filtered coming from the neutralization of about 1% of H2SiF6, after suitable treatment with flocculant, clarification and successive thickening. In both cases, the filtration at 100 bar yielded a good filtercake which presented itself as a hard crust, compact and manageable having the following composition (referred to a dry basis contained humidity 33%):
CaF2 70%
SiO2 17%
CaCO3 6%
Al2O3 2%
Fe2O3 1%
L.O.I. (450 *C) 4%
As one can observe, such a product is of a quality superior to that which is presently obtained by Fluorsid in that it does not contain sulphates and has a higher level of CaF2. With these characteristics, the product is suitable for usage not only in the cement industry but also in the iron and steel industry.
Based on the laboratory and industrial tests mentioned above, Fluorsid believes its process can be successfully applied to the fluorinated muds resulting from the processing of phosphate rock. The fertilizer industry can expect to have a similar result as regards process and product as the Fluorsid's plant, which was set up 9 years ago with a limited investment and that has operated since with a discreet economic margin.
Furthermore, we also think that this type of process is particularly suitable to be used by the fertilizer industry. In fact, the type of process (neutralizing, filtration, etc.) is part of the basic inorganic chemical culture normally present in the fertilizer sector. Moreover, the process does not foresee drying operations and does not give rise to gaseous emissions. The product does not create problems for storage and can be easily handled by a front-end loader. The market is well defined and consists of a few large customers who donÕt require a particularly difficult marketing effort.
Estimate of the Necessary Investment
With respect to the Fluorsid plant, the flowsheet of which is given in Figure 1, a plant for the treatment of fluosilicic acid deriving from the production of phosphoric acid results much more simplified. The foreseen plant requires only a single neutralization with lime which can be carried out at a higher slurry temperature which also favours filterability. Moreover, settling and thickening are not required since the slurry is sufficiently dense to obtain the best results from the filters. A layout of the simplified plant is given in Figure 4.
For a plant of 4,000 tons per month of solids tel quel, corresponding to 1,200 tons per month of 100% H2SiF6 (obtainable from a plant of 400-500,000 tpy of P2O5), the investment can be estimated to be in the order of 2 million US$, broken down as follows:
US$ x 1,000
Tube Press Filters with valves (8 filters) 750
Equipment and Apparatus (tanks, pumps, belts, etc.) 300
Carpentry, Piping and Mechanical Erection 200
Electrical Plant and Instrumentation 200
Civil Works (including warehouse for 5,000 T) 550
Total 2,000
Naturally, the number of filters depends on the filtering capacity desired. If one considers an indicative selling price of 50 US$/t delivered to the customer, the time to recover the investment can be estimated in 2-3 years.
Conclusions
For about 9 years Fluorsid has operated a plant for the neutralization of fluoridated water and, thanks to a filter plant supplied by Svedala, it has transformed the muds obtained from the water treatment plant into a commercial product which has been successfully sold to the cement industry.
Tests effected both in the laboratory as well as on industrial scale have shown that a solution of this nature can also be applied to resolve the problem of fluosilicic acid generated by plants for the production of phosphoric acid.
The investment necessary is very limited as is the cost of operation. The management of the plant and sale of the product obtained do not require a particular effort by the fertilizer producers nor oblige them to enter into new, different sectors of the chemical industry only to resolve a problem which is environmental in nature. The sale of the product covers entirely operating costs and depreciation. Depending on the geographical location of the customers with respect to the producer, a small profit can also result.
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