WO2002032808A1

WO2002032808A1 - Recovering hf acid from calcium fluoride

Info

Publication number: WO2002032808A1
Application number: PCT/US2001/031054
Authority: WO
Inventors: Samuel F. Sweat; Claude E. Breed; Kenneth E. Mcgill
Original assignee: Hf Extraction, Llc
Priority date: 2000-10-18
Filing date: 2001-10-04
Publication date: 2002-04-25
Also published as: AU2002211418A1

Abstract

Calcium fluoride is converted to hydrofluoric acid and a calcium silicate aggregate by reaction with silica and partially dissociated steam at elevated temperatures.

Description

RECOVERING HF ACID FROM CALCIUM FLUORIDE

Background

Engelson (USP 2,631,083) produces purified calcium fluoride and silica, and reuses any HF produced. Hennig (USP 3,421,853) did not appreciate and does not disclose the conditions necessary to obtain the desired results. Hayford (USP 3,719,747) proposes the same reaction as Hennig, but at much higher temperatures, which forms a melt; moreover, the proposed process is not economical.

Summary of the Invention

When aqueous hydrofluoric acid is reacted with phosphate in particles of calcium- bearing phosphate rock, a slurry containing filter-grade phosphoric acid and a solid component, comprised primarily of calcium fluoride, are obtained. By filtering the slurry and washing the thus-obtained filter cake, a source of calcium fluoride suitable for a starting material is thus obtained. This is merely exemplary; any other source of calcium fluoride is equally suitable for the subject process. Calcium fluoride (CaF₂) is reacted with silica (SiO₂) and steam (H₂0) at a temperature high enough to dissociate a portion of the steam into nascent hydrogen (H°) and nascent oxygen (0°), which react with the silica to release hydrogen fluoride (HF) gas and calcium silicate (CaSiO_x- wherein x is 3 or 4) solids. It is significant to have the temperature low enough to avoid producing the calcium silicate in melt form. An object of this invention is to produce hydrofluoric acid with simple available equipment and available raw material. A further object is to provide such a process which is compatible with production of phosphoric acid. A further object is to devise a method which employs less energy than existing technology. Another object is to produce a highly desirable and recyclable product, as well as salable clinker. A still further object is to accomplish the foregoing in a profitable and environmentally safe manner.

Brief Description of the Drawing Figure 1 is a process flow diagram.

Details The temperature at which a portion of steam dissociates is readily achievable, e.g., in a kiln. Although not so limited, the operating temperature is suitably in the range of from about 2500°F (1371°C) to about 3000°F (2968°C), preferably about 2800°F (1538°C).

With reference to Figure 1 and Table 1, calcium fluoride (line #1) is fed to a mixing chamber, in which it is thoroughly mixed with a stream of silica (Si0₂) (line #2). The mixture is granulated or pelletized into a particle size that makes an efficient feed to a heated kiln. To help bind the materials together and form pellets, a binder is optionally used (line #3).

The pellets are then fed to the front end of a kiln (line #4); the kiln is fired from the discharge end (counter current flow) using gas-, oil-, or coal. A stream of steam or water (line #5) is injected into the discharge end of the kiln. About 2 to 5% of the water vapor disassociates in the kiln and supplies the nascent hydrogen and nascent oxygen necessary for the resulting reaction to go to completion. Because only about 2 to 5% of the steam disassociates readily, the amount of steam required is up to 20 times the stoichiometric requirement.

The retention time in the kiln is usually about 2 to 3 hours. The operating temperature for the kiln is from about 2500° to about 3000° or preferably about 2800°F.

Reaction takes place within the kiln, and HF gas is liberated. A solid (not a melt) calcium silicate is formed. A rotary kiln is desirable, but not necessary, to allow the calcium silicate to form into well rounded pellets to facilitate their end use. Table 1 Description of process lines - CaF₂ Regeneration

An HF rich gas stream (line #6) exits the kiln with the exhaust gases and is recovered in the HF absorption train. A high temperature electrostatic precipitator and a heat recovery unit are installed prior to the HF absorption train to clean up the gas steam and to recover the excess heat from the kiln exhaust gases for use in the evaporation loops. The HF recovery train, which is proven and commercially available technology, consists of a series of countercurrent absorbers. HF acid having a concentration of about 5 to 10% is discharged from the recovery train (line #7) and is then further concentrated in a commercially available HF evaporator to 25% to 37% HF. A portion of the HF is then optionally recycled back to the phosphoric acid reactor, and the remainder (surplus from the rock) is available for sale or concentrated to 70 to 100% HF. When the HF absorbers are operated at concentrations less than

5% HF, the evaporation costs are prohibitive; above 10% HF, the efficiency of the scrubbers drops off drastically. Therefore the recovery system is normally operated at a maximum HF concentration between 6 and 7%.

A final tail gas scrubber utilizes lime slurry to achieve essentially 100%o HF recovery. Exhaust from the absorption train is vented to the atmosphere (line #9). The calcium fluoride formed in the scrubber is fed back to the mixing chamber (line #10) and is recycled through the kiln.

The calcium silicate formed during the reaction is optionally rolled inside a rotary kiln, thus forming regular sized/shaped pellets (aggregate or clinker), and is discharged from the end of the kiln (line #11). This clinker is cooled with water for heat recovery.

The calcium silicate contains essentially all impurities that were in the original phosphate rock. However, the calcium silicate takes on a glassy form and tends to encapsulate the impurities within an essentially inert shell. This inert material (aggregate) is safely usable for construction fill, cement clinker, roadbeds, etc. In the disclosure and claims, wherever "about" appears, a ± 5% variation is contemplated. Wlierever "substantially the same as" appears, the contemplated product varies in essential properties no more than 5% from that which is expressly described.

The term "nascent" is a term used to describe the abnormally active condition of an element. In the case of a diatomic element (such as H₂ or 0₂), it can exist in a monatomic state and be highly reactive, for example H° or 0°. The high temperature in the kiln creates the conditions for this phenomenon to occur which then results in the reaction mechanisms described.

The invention and its advantages are readily understood from the preceding description. Various changes may be made in the process and product without departing from the spirit and scope of the invention or sacrificing its material advantages. The process and product hereinbefore described are merely illustrative of preferred embodiments of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method of processing calcium fluoride which comprises the following steps: a) admixing calcium fluoride with silica (natural or synthetic), b) reacting the resulting admixture with steam at a temperature high enough to dissociate at least part of the steam, thus obtaining hydrogen fluoride and a calcium silicate aggregate, c) recovering a major part of the hydrogen fluoride and converting the remainder of it to calcium fluoride, and d) recycling the calcium fluoride from step (c) to step (a).

2. A method of claim 1 wherein binder is introduced into step (a) to form pellets.

3. A method of claim 2 wherein the binder is calcium lignosulfonate.

4. A method of claim 1 wherein step (b) is conducted in a kiln.

5. A method of claim 4 wherein the kiln is a rotary kiln.

6. A method of claim 4 wherein the kiln is a fluidized bed calciner.

7. A method of claim 1 wherein the temperature is within a range of from about 2500° to about 3000°F.

8. A method of claim 7 wherein the temperature is about 2800°F.

9. A method of claim 1 wherein the recovered part of the hydrogen fluoride from step (c) is concentrated to from 70% to 100% HF.

10. A method of claim 1 wherein the remainder of the hydrogen fluoride converted in step (c) is converted in a lime scrubber.

11. A method of claim 1 wherein the calcium fluoride of step (a) is in the form of washed filter cake obtained from filtering a slurry reaction product of aqueous hydrogen fluoride and phosphate in particles of calcium-bearing phosphate rock.

12. A method of claim 11 wherein binder is introduced into step (a) to form pellets.

13. A method of claim 12 wherein step (b) is conducted in a rotary kiln at a temperature in the range of from about 2500° to about 3000°F.

14. A calcium silicate aggregate which is substantially the same as that obtained from step (b) of claim 13.

15. A method of claim 12 wherein step (b) is conducted in a fluidized bed calciner at a temperature in the range of from about 2500° to about 3000°F.

16. A calcium silicate aggregate which is substantially the same as that obtained from step (b) of claim 15.

17. A calcium silicate aggregate derived from calcium-bearing phosphate rock and containing essentially all impurities that were associated with the phosphate in said rock, said aggregate having a glassy form which encapsulates the impurities within an essentially inert shell.

18. A method of claim 1 which is conducted in a kiln in which introduced water and/or steam is partially dissociated into nascent hydrogen and nascent oxygen.