GB2159136A - Process for the manufacture of hydrofluoric acid by reaction of sulphuric acid with fluorospar in a rotating oven - Google Patents

Description

1

SPECIFICATION

GB 2 159 136 A 1 Process for the manufacture of hydrofluoric acid by reaction of sulphuric acid with fluorospar in a rotating oven The present invention relates to the manufacture of hydrofluoric acid by the reaction of sulphuric acid with fluorospar in an externally heated rotating oven.

The reaction in which fluorospar is attacked by sulphuric acid is endothermic. In order to proceed under good conditions on an industrial scale, it requires the application of agitation and a heat input. As it proceeds up to 70-80% conversion, this reaction passes through several stages which are successively 10 and/or simultaneously found to be corrosive and crust-forming.

For this part of the reaction, the plant employed should be capable of overcoming these disadvantages, that is to say, in the one hand, be constructed in or coated with anticorrosive materials and, on the other hand, be equipped with means of agitation such as to prevent or destroy the caking on the walls.

Beyond 70-80% and in any event beyond 80% conversion, the reaction presents no more corrosive and 15 crustforming phenomena.

Several processes for the manufacture of hydrofluoric acid from fluorospar and sulphuric acid have therefore been developed to reduce these disadvantages.

A first category of ovens implements the complete reaction by being subjected to all the phases of the reaction. Also included in this category are ovens equipped with raw- material premixers which, even when the raw materials are preheated, deliver to the oven a reaction mixture which is characterised by a conversion which is limited and less than 25-30%, taking into account the very short residence time of fered by equipment of this type.

A plant of this type must process the various stages of the reaction, which implies:

- either employing scraping devices to ensure heat transfer despite the encrustations, and also protec- 25 tive devices protecting the oven walls against corrosion; - or employing largesize ovens in which it is possible to hold a large volume of reaction mixture with which the raw materials introduced will mix, in order to reduce the corrosive effects and, by avoiding the encrustations, to promote heat transfer.

Because of the possible appearance of caking on the walls and also of the corrosiion which develops 30 thereon, a plant of this type requires a proven technology, which is costly and requires regular stoppages for maintenance.

In a second category of processes, the oven still implements all of the reaction. However, a suitable quantity of the reaction mixture which has reacted more or less partly is recycled at the spar/sulphuric acid composition which is equivalent to a conversion of 80% and in any event greater than 70% is en- 35 sured at the head of the oven.

Consequently, a plant of this type operates as if the reaction had to be implemented only in the part where the conversion varies from 80% to 97-99%, this being by virtue of a recycle flow adapted to the oven regime.

In a plant of this type large-size ovens must be available, in order to provide the reaction mixture with 40 an adequate residence time because the reaction kinetics remain very slow once 80% conversion has been exceeded. In addition, the starting reaction mixture is in the form of a paste leading to the forma tion of blocks which unadvoidably impede the progress of the reaction.

In a third category of processes, the reaction is carried out from 0 to 97-99% by coupling up a prereac tor and an oven. The raw materials, preheated beforehand, are introduced into a specially designed apparatus of a sufficient volume and equiped with arrangements permitting heat input. This enables a conversion of between 40 and 50% to be attained at the exit of the prereactor. This prereactor then deliv ers to the oven a pulverulent reaction mixture which is slightly moist (impregnated with sulphuric acid).

Since the conversion at the exit of the prereactor is restricted to a value well below 70%, the oven must still undergo the second, corrosive and crust-forming phase; it is consequently necessary to employ 50 scraping devices combined with anticorrosive coatings, or to have largesize ovens available, to enable a large volume of reaction mixture to be held.

The present invention greatly reduces the above disadvantages of the prior art and provides a process for the manufacture of hydroflouric acid by reaction of sulphuric acid with fluorospar, comprising:

a) feeding the sulfuric acid and the fluorospar to a prereaction or premixer device operating so as to 55 produce at its exit a pulverulent product the conversion of which is from 40 to 50%; then b) introducing the said pulverulent product at a temperature of from 20 to 12WC into a rotating oven operating at a temperature from approximately 80 to 35WC in which hydrofluoric acid and calcium sul phate are formed, and simultaneously c) recycling the reaction product to the oven such that from 3 to 3.5 moles of calcium sulphate are 60 recycled per mole of calcium fluoride contained in the puiverulent product entering the oven.

2 GB 2 159 136 A The various elements of the device which can be employed in the present process, and their operation, are now described in greater detail below.

2 Prereactor.' This apparatus, which is constructed of materials which resist corrosion in the presence of hydrofluoric acid and sulphuric acid permits intimate mixing of the reactants and ensures continuous working thereof to promote the mass and heat exchanges which are necessary for the reaction to proceed. The design of this apparatus also permits a high heat input by heating all the surfaces in contact with the reaction mixture and also a uniform forward movement of the product which restricts the back-mixing phenom ena.

Such an apparatus, fed with the raw materials which are correctly preheated to 80-120'C, makes it pos sible to produce at the exit of the apparatus a pulverulent product at 80- 100'C the conversion of which is 40-50%, the hydrofluoric acid produced by the reaction being, for the most part, removed as gas. Accord ing to a preferred embodiment, the mean residence time in the prereactor is between 8 and 15 minutes.

Inside the prereactor the temperature must be sufficient and adjusted so that the required conversion can be ensured at the exit of this apparatus.

The means of heating and heat input must be compatible with the behaviour of the materials of con struction. Furthermore, the exothermic reactions of sulphuric acid with the inpurities in the fluorospar produce a part of the required heat input. One of the means of heat input consists in introducing oleum and steam by means of a suitable device and in appropriate quantity and proportion. However, this intro- 20 duction must be carried out by the use of a suitable device to avoid local over-concentrations and the corrosion which these can produce.

Since the kinetics of the reaction of sulphuric acid with fluorospar depend on the temperature region in which this reaction is carried out, the volume of the prereactor must be adjusted in a known manner as a function of the reactant temperatures, the heating of the apparatus itself and the exchange coefficient 25 obtained.

For reactants which are suitably preheated and an intimate mixing of the reaction mixture, an appropri ate value of the useful volume required in the prereactor is approximately 750 litres per tonne of hydro fluoric acid produced per hour overall.

This apparatus should preferably be connected to the oven via an assembly equipped with a sealing 30 device which prevents exchanges with the surrounding atmosphere and ensures the removal of hydro fluoric acid as gas towards the gas-treatment unit.

Premixer When a prereactor is not available, step (a) of the process according to the invention can also be car- 35 ried out in a premixer. This known device does not permit generally a large heat input, but merely pro vides an intimate mixing and working of the reactants which may be preheated or not.

If there is no heat input very little conversion of the sulphuric acid and fluorospar occurs and therefore in order to obtain at the exit of the premixer a mixture whose components correspond to a conversion rate of 40 to 50% and have the pulverulent character according to the invention, calcium sulphate is added to the sulphuric acid and the fluorospar, in a ratio of about 0.7 to 1 mole of calcium sulphate per each mole of calcium fluoride introduced initially.

The residence time in the premixer is generally very short. In the absence of heat input, practically no reaction takes place in it and consequently the production of hydrofluoric acid is very slight. Neverthe less, as in the case of the prereactor, the premixer should preferably be connected to the oven via a sealing and degasing device.

Oven The oven receives the reaction mixture leaving the prereactor and ensures its treatment up to the re quired conversion. The oven in question is typically constructed in ordinary steel which withstands the 50 operating temperatures. The heating may be providedjor example, by virtue of the circulation of hot gas in a thermally lagged jacket.

The operating principle consists in recycling a product which has almost the characteristics of the an hydrite extracted, so as to produce, after mixing with the reaction mixture leaving the prereactor or the premixer, a composition equivalent to a conversion of 80% and in any event greater than 70%.

The recycled product flow rate depends on the efficiency of the first member of the device and on the composition of the recycled product. Nevertheless, it can be considered that, for a conversion of 40 to 50% at the exit of the prereactor or the premixer, the recycle rate is advantageously from 225 to 300 kg/h per tonne of the total hydrofluoric acid produced per day (that is 3 to 3. 5 moles of recycled CaSO, for 1 mole of CaF, entering the oven).

The operating parameters may be readily adjusted as a function of the quality of raw material, the particular dimensions of each apparatus, the heating temperatures of the raw materials and the tempera tures of the heat transfer fluids employed for heating the prereactor and the oven.

3 GB 2 159 136 A 3 The reaction mixture is preferably introduced into the oven at a temperature of 20 to 1000C approxi mately and preferably leaves at a temperature of approximately 300'C. Under these conditions, the pre ferred mean residence time is between 60 and 150 min, the optimum mean residence time being approximately 120 min. The oven is consequently dimensioned for such requirements.

If desired, a small amount of by-products containing hydrofluoric and (diluted HF or fluorosu(phonic 5 acid may be recycled.

The process of the present invention permits continuous manufacture of hydrofluoric acid in excellent yields, of the order of 98-99%.

Since the problems of caking and wall corrosion have been greatly reduced, the frequency of stop- pages due to incidents or maintenance is thus reduced to only routine maintenance operations and to 10 access to the apparatus for mechanical checking. The maintenance costs are consequently lower than those of the processes of the prior art.

Furthermore, the intimate mixing of the reactants in the prereactor or premixer and the continouus recycling of the products to the oven ensure a stability of the yields together with an excellent and con- stant quality of the by-product anhydrite, which facilitates its exploitation.

Furthermore, the process of the invention can also be applied to the modification of existing plants. In effect, by having available a residence volume corresponding to a mean residence time of approximawtely 120 min, the oven can be seen to have its instantaneous production capacity doubled after modification according to this invention (that is addition of a prereactor and recycling). The mean pro- duction capacity becomes above double of that of the prior art plant before modification, because the duration of the stoppages for troubles, breakdowns or maintenance is reduced.

It is the same, though at a lower degree (gain in capacity of about a third), in the case of a plant of the premixer + oven type.

This invention is now illustrated and compared to the prior art in the following Examples, wherein a fluorospar of "Acid Grade" quality and a 99 weight - % sulphuric acid were used.

Example 1

A rotating oven 11.9 m in length and 1.9 m in diameter was operated according to one of the following configurations:

- with a scraping device (configuration A) - with a premixer without recycling (configuration B) - with a prereactor without recycling (configuration C) - with internal recycling of anhydrite (configuration D) - according to the invention with a prereactor and internal recycling of anydrite (configuration E) - according to the invention with a premixer and recycling of anhydrite to the premixer and the oven 35 (configuration F).

4 GB 2 159 136 A Table 1 below shows the operating conditions and the production capacity in each case.

TABLE 1

4 5 Con- Rate of flow of Temperature f Q of Temperature Speed of figurraw materials the raw materials M of the oven ation (kglhour) products rotation entering the (revolu tionsf 10 min) Fluorospar HSO, Fluorospar HSO, A 1466.6 1800 20 20 20 3.5 15 B 1627.5 2001.75 20 80 60 3.5 20 C 2215.2 2724.6 80 100 80 3.5 D 1808.3 2224.2 20 20 20 8 25 E 2893.3 3558.6 80 100 80 8 F 1981.1 2446.5(a) 20 20 20 8 30 (a) In addition, 2300 to 3500 kg/h of anhydrite at 200C were recycled to the premixer.

Continuation of Table 1 35 Con- Oven heating Recycled product Temperatures (C) Production figur- temperatures (anhydrite) of the products capacity ation (' C) leaving the oven (tonnesIday) 40 Flow rate: Temp.

Kg/h 0C Raw Solid gas 45 HF (anhydrite) A 3501400 to 5501600 120-t20 280-t20 16 B 3501400 to 5501600 120 280 20 18 C 4501500 to 550/600 120-1:20 280 20 24.5 50 D 4001450 to 5501600 9000 to 15000 260 20 120 20 280 20 20 E 4501500 to 5501600 7200 to 9600 260 20 120 20 280 20 32 F 450/500 to 5501600 8250 to 13200 260-t20 120 20 280-t20 22 GB 2 159 136 A 5 Example 2

Example 1 is repeated in a rotating oven 18 m in length and 2.5 m in diameter and with the flow rates modified as shown in Table 11 below, the other operating conditions remaining identical.

TABLE 11 5

Configur- Flow rate of raw Flow rate of recycled Production ation materials (kglhour) product (anhydrite) capacity in kglh (tonnesIday) 10 Fluorospar HSO, A 2893.3 3558.6 32 1 B 3255.0 4003.5 36 c 3978.3 4893.2 44 20 D 3707.1 4559.5 18500 to 28700 41 E 5786.6 7117.3 14400 to 19200 64 F 3978.2 4893.0 16500 to 26400 (b).44 25 (b) In addition, 4600 to 7000 kg/h of anhydrite at WC were recycled to the premixer.

Claims (6)

1. A process for the manufacture of hydrofluoric acid by reaction of sulphuric acid with fluorospar, comprising:

a) feeding the sulfuric acid and fluorospar to a prereaction or a premixer device operating so as to produce at its exit a pulverulent product the conversion of which is from 40 to 50%; then b) introducing the said pulverulent product at a temperature of from 20 to 12WC into a rotating oven operating at a temperature of from 80 to 3500C in which hydrofluoric acid and calcium sulphate are formed, and simultaneously c) recycling the reaction product to the oven such that from 3 to 3.5 moles of calcium sulphate are recycled per mole of calcium fluoride contained in the pulverulent product entering the oven.

2. A process according to claim 1, in which step (a) is carried out in a prereactor fed with sulphuric acid and fluorospar which are preheated to a temperature of 80 to 120'C, the means residence time in the prereactor being from 8 to 15 minutes.

3. A process according to Claim 1, in which step (a) is carried out in a premixer fed with sulphuric acid, fluorospar, and calcium sulphate in a ratio of 0.7 to 1 mole of calcium sulphate per mole of calcium 45 fluoride introduced initially.

4. A process according to any of claims 1 to 3, in which the mean residence time of the product in the rotating oven is from 60 to 150 minutes.

5. A process according to claim 1 substantially as described in Example 1 or 2.

6. An apparatus suitable for carrying out the process of any one of claims 1 to 5, comprising a pre- 50 mixer or prereactor whose exit is connected to a rotating oven with means for recycling the product from the oven back into the oven.

Printed in the UK for HMSO, D8818935, 10185, 7102.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.