DE1567638B2 - PROCESS FOR THE CONTINUOUS PURIFICATION OF AN AQUATIC HYDROGEN HYDROGEN ACID BY DISTILLATION - Google Patents

Description

Hydrofluoric acid, as hydrofluoric acid (H ₂ SiF ₆ ) is often called for the sake of brevity, is the starting material for the production of many other fluorine compounds, such as. B. of sodium silicofluoride, ammonium fluoride, aluminum fluoride or hydrogen fluoride. In most cases, the purity and concentration of the hydrofluoric acid are decisive for the feasibility and economic viability of these processes.

Silicate hydrofluoric acid falls in significant quantities in the digestion of rock phosphates with mineral acids, especially when producing superphosphate or when evaporating dilute phosphoric acid solutions at. The solutions of hydrofluoric acid obtained in the latter case are particularly suitable for further processing too dilute and practically always contaminated, so that they had to be discarded.

The behavior of hydrofluoric acid in distillation is very complicated. From the work of E. B aur in den Ber., 36 (1903), p. 4215, it is known that the vapor of hydrofluoric acid breaks down almost completely into hydrogen fluoride and silicon tetrafluoride. It is also known that, depending on the concentration of the boiling silicofluoric acid, the vapor contains hydrogen fluoride and silicon tetrafluoride in a variable ratio to one another. Hydrofluoric acid with a concentration below 13 percent by weight gives a distillate that is richer in hydrogen fluoride than _{would correspond to the formula H 2} SiF ₆ , and one that is more concentrated than 13 percent by weight gives a distillate that is poorer in hydrogen fluoride. In addition, water and hydrofluoric acid form an azeotrope with a maximum boiling point of about 45 percent by weight H ₂ SiF ₆ , so that rectification of an acid that is more dilute than this results in an accumulation of this acid in the column bottom.

US Pat. No. 2,369,791 has made it known that dilute, phosphate-containing and optionally also silicic acid-containing crude hydrofluoric acid solution can be obtained during the evaporation of dilute hydrofluoric acid and other volatile phosphoric acid containing fluorine or fluorosilicon compounds. Here, phosphoric acid with an initial concentration of about 35 to 45% is evaporated to "about 80% and to avoid silica deposits in the vapors as a result of the reaction of SiF ₄ with the condensation water, the temperatures are kept high enough that the formation of condensation water is prevented. For this purpose, temperatures above 110 ⁰ C, preferably between 116 and 121 ⁰ C, which is achieved by the fact that hot furnace gases hit the surface during evaporation of the phosphoric acid. However, this results in uncontrollable local overheating in the phosphoric acid, which is gradually and inevitably increasing in concentration, and in the above In view of the complicated equilibrium between H ₂ SiF ₆ , H ₂ O, SiF ₄ and HF both in the dilute raw phosphoric acid and in the vapor state, it is by no means certain whether and to what extent hydrofluoric acid or HF and SiF ₄ , the latter in the optimal mo lar ratio of 2: 1, distill off. Apart from that, the condensate is not pure. It contains phosphoric acid and must also be filtered off through a bed of sand. This process is therefore unsuitable for the production of pure hydrofluoric acid.

Surprisingly, it has now been found that the boiling and distillation behavior aqueous Kieselflußsäurelösung changes fundamentally if one does not carry out the distillation of Rohkieselflußsäurelösung of boiling phosphoric acid solution at temperatures below 12O ⁰ C.

The invention thus provides a process for the continuous purification of an aqueous silicofluoric acid solution by distillation at atmospheric pressure, which is characterized in that

ίο boiling, aqueous solution of phosphoric acid wherein aqueous Kieselflußsäure during distillation is continuously added so that the boiling point of the mixture is kept constant at least 120 ⁰ C is presented with a boiling point of at least 120 ° C.

A phosphoric acid concentration of 60 to 65 percent by weight corresponds to a boiling point of 120 ^{° C.} At this minimum boiling point, the hydrofluoric acid distills off congruently and does not form an azeotrope with water. The prerequisite for the feasibility of the process is compliance with the specified minimum concentration of the phosphoric acid solution presented, so that the process according to the invention can practically neither be coupled with the production of concentrated phosphoric acid solution from the dilute one according to the aforementioned USA patent, nor is it anticipated by its findings.

example

21 kg of phosphoric acid each of different densities with the following boiling points were Filled into a polypropylene flask with a diameter of 300 mm and a height of 550 mm and with a Silver heating coil through which steam flows up to the respective boiling point of the water-containing phosphoric acid heated. As soon as the boiling point of the initially charged phosphoric acid was reached, aqueous silicofluoric acid became continuously added dropwise so that it remained constant. The amount added per experiment was Amount of hydrofluoric acid amount about 13 kg per hour. About 2 hours after the start of the experiment, the amount of condensate per unit of time was stant. The product was then collected and analyzed, as was the contents of the evaporator. The results are as follows:

H ₂ SiF ₆
additive
Conc
tration in% boiling point
the H ₃ PO ₁ -
Distillation
temperature
( ⁰ C) H ₃ PO ₁
Conc
tration in% F content
in the
H ₃ PO ₁
in% D.
kg / 1 11th
11th
11th
11th 110
115
120
125 35.5
"49.9
64.2
70.2 9.15
6.35
3.35
1.25 1.36
1.44
1.50
1.54

At the distillation temperatures of 120 and

The condensate was clear at 125 ° C. Even at 115 ° C. slight cloudiness appeared. At 110 ^° C., large amounts of silica separated out, and the contents of the evaporator increased continuously.

The hydrofluoric acid thus obtained contains about 50 ppm P ₂ O ₅ and is completely free of precipitated silica.

In the dilute silicofluoric acid contained, non-evaporable impurities such. B. phosphoric acid, Gypsum, etc., accumulate in the evaporator and must be removed from it continuously or periodically will.

If the vapor obtained by distillation from the initially introduced phosphoric acid is passed into a rectification column for the purpose of concentrating the silicofluoric acid, the hydrogen fluoride is first absorbed in the latter and the silicon tetrafluoride is only absorbed in the upper part, which forms silica under hydrolysis and clogs the column. This can be prevented in an obvious way by sending the more fluorine-rich, concentrated condensate from the bottom of the column towards the vapor coming out of the evaporator, which balances the F content of the entire column contents and splits off silica is prevented at the top of the column. The excess of concentrated silicofluoric acid solution resulting from the further condensation is continuously drawn off at the bottom of the column, while part of the water vapor is drawn off at the top of the rectification column. _{If this contains small amounts of silicon tetrafluoride, the SiF 4} -containing vapor escaping at the top of the column is washed with aqueous fluoride solutions in order to avoid hydrolysis of this compound with precipitation of silica in the post-washer.

An apparatus composed of conventional elements suitable for carrying out the process according to the invention can consist of an evaporator 1 with a downstream packed column 2 serving to rectify the vapor with a device 3 for circulating the condensate, a reflux condenser 4 and a reflux condenser 4 attached to or downstream of the column 2 if necessary, there is an additional column 5 arranged between these two, in which the SiF ₄ -containing water vapor is washed with fluoride solutions (cf. FIGS. 1 and 2).

Claims (1)

Claim :. Process for the continuous purification of an aqueous fluosilicic acid by distillation at atmospheric pressure, characterized in that boiling, aqueous solution of phosphoric acid is presented with a boiling point of at least 120 ⁰ C, with aqueous during distillation fluosilicic acid is continuously added so that the boiling point of the mixture to at least 12O ⁰ C is kept constant. 1 sheet of drawings