WO1999007635A1

WO1999007635A1 - Process for aluminum fluoride production

Info

Publication number: WO1999007635A1
Application number: PCT/US1998/016208
Authority: WO
Inventors: Klaas J. Hutter; Robert J. Danos
Original assignee: Raytheon Engineers and Constructors Inc
Current assignee: Morrison Knudsen Corp
Priority date: 1997-08-06
Filing date: 1998-08-05
Publication date: 1999-02-18
Anticipated expiration: 2000-02-06
Also published as: AU8768198A

Abstract

A process is disclosed for drying hydrated aluminum fluoride, produced from the reaction of hydrated alumina with hydrofluosilicic acid, utilizing a turbo-tray drying system so as to reduce the production of particle fines and recycle thermal energy from upstream operations, resulting in an improved and more economical anhydrous aluminum fluoride product.

Description

PROCESS FOR ALUMINUM FLUORIDE PRODUCTION

The present invention relates generally to improvements in the production of aluminum fluoride for use in various applications, particularly as the flux for electrolytic baths in connection with manufacturing aluminum from various ores. The process of this invention results in surprising and advantageous production synergies, energy efficiency, and an aluminum fluoride product that minimizes troublesome fine particulates which can be difficult to handle.

BACKGROUND OF THE INVENTION

Aluminum is the most widely used nonferrous metal in industrial applications.

Major geographic producers include the United States, the former Soviet Union, the People's Republic of China, and Australia.

Aluminum is produced from two sources: in primary aluminum production the mineral bauxite is the starting material, while in secondary aluminum production, scrap is the starting material. In primary aluminum production, metallurgical-grade alumina is extracted from bauxite and purified through electrolysis in what is known as the Hall- Heroult process to produce aluminum. In the smelting cell, the alumina is dissolved in a molten bath of cryolite. Aluminum fluoride (A1F₃) is then typically added to the electrolytic cell solution to replace the fluorine absorbed into the cell linings and lost through volatilization.

Primary aluminum production is highly energy intensive, making the availability of cheap electricity a high priority in choosing a smelter plant site. Improvements in the Hall-Heroult process have been made to improve both the energy efficiency and decrease the environmentally unacceptable fluorine emissions. The quantity of aluminum fluoride required for this process is approximately 20-25 kg per metric ton of aluminum ingot produced. Thus, a cost-effective, high-quality supply of aluminum fluoride is indispensable to commercial aluminum manufacture. In the past, various processes have been used to produce aluminum fluoride. As reported in a 1964 technical article (F. Weinrotter, "Now Commercial: A1F₃ Synthesis From Superphosphate Byproduct," Chemical Engineering. April 24, 1964, at 132-134), which is incorporated herein by reference, the conventional production of aluminum fluoride from Al(OH)₃ and HF became commercially impractical as conventional sources of HF dwindled, and the costs of working with HF increased. In its place, a newer and more economical process developed at Linz, Austria, Osterreichische Stickstoffwerke A.G. utilized hydrofluosilicic acid (H₂SiF₆) and alumina to make A1F₃. The hydrofluosilicic acid (hereafter FSA), a byproduct in the manufacture of superphosphate, was much cheaper than HF.

In a typical process for producing A1F₃ from FSA, hydrated alumina from a feed bin is weighted into a set of batch reaction tanks. The fluosilicic acid (FSA) solution is pumped from storage tanks through a heat exchanger to the batch reaction tanks. In the reaction tanks, fluosilicic acid reacts with hydrated alumina to form a metastable, soluble form of aluminum fluoride trihydrate (alpha-form) and silica according to the following reaction (wherein "s" denotes a solid and "aq" an aqueous solution):

(1) Al₂O₃ (s) + H₂SiF₆ (aq) + 5 H₂O → 2 AlF₃*3H₂O (aq) + SiO₂ (s)

Control of process conditions in this reaction is critical to obtaining high reaction efficiencies and high product quality. When the reaction is substantially complete, slurry from the reaction tanks is pumped to a silica filter. Silica removed by the silica filter, which discharges from the filter, is slurried in a tank and then pumped to the neutralization system. The filtrate from the filter, containing the aluminum fluoride in the hydrated, soluble alpha form, is sent to a green liquor holding tank from which it can be pumped to a set of precipitation tanks.

In the precipitation tanks, the concentrated aluminum fluoride trihydrate (AlF₃*3H₂O) is heated with steam and agitated with air to irreversibly convert the soluble alpha form of the aluminum fluoride trihydrate to the insoluble beta form according to the following sequence: (2) AlF₃*3H₂O (aq) — ^ → AlF₃*3H₂O (s) The resulting slurry formed in the precipitator tanks is cooled before sending it to a filter feed tank. The cooled slurry is pumped to a vacuum filter which separates the aluminum fluoride trihydrate (beta form) crystals from the so-called spent liquor.

In conventional processes, the moist aluminum fluoride trihydrate filter cake is then fed to a flash dryer which removes free moisture and water of hydration resulting in converting at least a portion of the aluminum fluoride trihydrate to aluminum fluoride hemihydrate according to the following sequence (3):

(3) AlF₃*3 H₂O (s) — ^to → A1F₃*0.5 H₂O (s)

(4) A1F₃*0.5H₂O (s) — ^fe → A1F₃ (s)

The flash dryer operation may, in some applications, utilize waste heat from the calcining operation (sequence (2) above). The flash dryer product is then conveyed to a calciner and cooler where the mixture of aluminum fluoride tri- further dehydrated by calcination to anhydrous aluminum fluoride per sequence (4) above, and then cooled. The cooled A1F₃ product is then conveyed to an A1F₃ storage bin. From here it is loaded out by gravity to rail cars or trucks.

In summary, in the foregoing commercial process, aluminum fluoride is produced by the reaction of FSA and hydrated alumina; aluminum fluoride trihydrate is crystallized from the solution; and, following separation from spent liquor, aluminum fluoride trihydrate crystals are dried and calcined to yield anhydrous aluminum fluoride. It has been found in practice, however, that the aluminum fluoride trihydrate crystals have a tendency to breakdown during drying, calcination and handling. The final aluminum fluoride product thus contains a relatively high concentration of dust and fines of less than 325 mesh material that is considered an expensive nuisance during handling and processing of the material. Conventional processes employing a flash dryer typically generate as much as 3-6 % of these undesired fines. Accordingly, it would be highly desirable in this art to find an efficient and economical technique for carrying out aluminum fluoride production in a way that reduces the generation of particle fines (less than 325 mesh) to a level below 1 % , preferably below 0.5 %, by weight of final product. These and other problems with and limitations of the prior art are overcome in whole or in part with the improved aluminum fluoride process of this invention.

OBJECTIVES OF THE INVENTION

Accordingly, a general object of this invention is to provide an improved process for commercial production of aluminum fluoride from FSA and alumina.

A specific object of this invention is to provide a process for producing aluminum fluoride having a reduced content of fine particles.

A more specific object of this invention is to provide a process for producing aluminum fluoride wherein the content of fine particles below a size of about 325 mesh is less than about 1 %, preferable less than about 0.5% , by weight.

Another object of this invention is to provide an aluminum fluoride process that utilizes an alternative drying apparatus and processes to maximize A1F₃ crystal size and minimize breakdown of the crystals. Yet, another object of this invention is to provide an aluminum fluoride process that utilizes certain process synergies to improve energy efficiency in connection with an alternative drying apparatus and process.

Specifically, it is an object of this invention to provide an improved aluminum fluoride process which efficiently utilizes a turbo-tray type drying system and results in a surprisingly reduced production of fines at relatively low capital cost, while utilizing waste heat from other process steps, and with no detectable physical or chemical degradation of the resulting aluminum fluoride product.

Other objects and advantages of the present invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the process, involving the various components and the several steps, and the relation and order of one or more of such components or steps with respect to each of the others, as exemplified in the following detailed description and as illustrated by the drawing.

SUMMARY OF THE INVENTION

The improved aluminum fluoride production process of this invention comprises the coordination of an FSA-hydrated alumina process with a turbo-tray type drying system in order to improve energy efficiency, maximize crystal size, and minimize crystal breakdown. In particular, waste heat from one or more of the calcining steps in the FSA-hydrated alumina process is utilized to operate a turbo- tray drying system to complete the drying of aluminum fluoride product. It has been found that this energy-efficient process also surprisingly results in an extremely low level of fines, reducing by as much as 90% or more the level of fines produced with more conventional flash drying technology.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a process flowchart illustrating an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 schematically illustrates an aluminum fluoride process utilizing a turbo-tray drying system in accordance with this invention. Hydrated alumina from feed bin 10 is added to a reaction chamber 12 through feed line 14, where it is mixed with a suitable quantity of hydrofluosilicic acid fed to reactor 12 via feed line 16 from an upstream source, for example a superphosphate operation as described in the 1964 Weinrotter article. Reaction chamber 12 may be provided with mixing means, such as stirrer 18, and vent means. Flow stream 20 leaving reaction chamber 12 contains an aqueous solution of aluminum fluoride trihydrate in the alpha form and silica particles. Flow stream 20 is directed into a solid-liquid separator 22, such as a centrifuge or filter tank. Wash water may be added as needed to separator 22 via feed line 24, and hydrated silicic acid is removed through exit line 26 while the liquor containing the dissolved aluminum fluoride trihydrate is removed through exit line 28. This filtrate/liquor may either be sent to a holding tank or else, as shown in Fig. 1, directly to a set of one or more crystallizers or precipitator tanks 30, 32, and 34.

The crystallizers or precipitator tanks 30, 32 and 34 are provided with heating means, such as steam heating jackets (not shown), to heat the contents of these units, and with agitation means, such as stirrers 31, 33 and 35 respectively. By means of the applied heat and agitation, the soluble alpha form of the aluminum fluoride trihydrate is converted to a slurry of the insoluble beta form. Exiting the crystallizers or precipitator tanks via stream 36, the slurry is cooled and directed to a second solid-liquid separator 40, such as a filter tank or centrifuge. In separator 40, aluminum fluoride trihydrate crystals are separated, and removed via stream 42, from the spent liquor, which is removed via stream 44. Wash water may be added as necessary to separator 40 through feed stream 46.

Stream 42 carrying the wet, hydrated aluminum fluoride is then directed to the top of a turbo-shelf dryer unit 50. Turbo-tray or turbo-shelf dryers are generally well known in the drying art, although such technology has not previously been applied to an aluminum fluoride process, nor have the surprising benefits from so doing been recognized. Various types of turbo-tray dryers are taught, for example, in U.S. Patent Nos. 3,728,797; 3,777,409; and 3,681,855; which patents are incorporated herein by reference.

In general, and as the term is used in this application, a turbo-tray dryer, such as a dryer unit 50, is a continuous dryer consisting of a stack of rotating annular shelves in the center of which turbo-type fans revolve to circulate the air over the shelves. Wet material, in this case wet, hydrated aluminum fluoride, enters through the roof, falling onto the top shelf as it rotates beneath the feed opening. After completing one revolution, the material is wiped by a stationary wiper through radials slots onto the shelf below, where it is spread into a uniform pile by a stationary leveler. The action is repeated on each shelf, with transfers occurring once in each revolution. From the last shelf, material is discharged through the bottom of the dryer. A steel-frame housing may consist of removable insulated panels for access to the interior. All bearings and lubricated parts are exterior to the unit with the drives located under the housing. Parts in contact with the product may be of steel or special alloy. The trays can be of any sheet material, such as enameled steel, asbestos-cement composition board, or plastic-glass laminates.

The rate at which each fan circulates air can be varied by changing the pitch of the fan blades. In final drying stages, in which diffusion controls or the product is light and powdery, the circulation rate is considerably lower than in the initial stage, in which high evaporation rates prevail. In the majority of applications, air flows through the dryer upward in counterflow to the material. In special cases, required drying conditions dictate that air flow be concurrent or both countercurrent and concurrent with the exhaust leaving at some level between solids inlet and discharge. A separate cold-air supply fan may be provided if the product is to be cooled before being discharged.

By virtue of its vertical construction, the turbo-type tray dryer has a stack effect, the resulting draft being frequently sufficient to operate the dryer with natural draft. Pressure at all points within the dryer is typically maintained close to atmospheric. Most of the roof area is used as a breeching, lowering the exhaust velocity to settle dust back into the dryer. Heating means, such as a row of steam pipes, can be located in the space between the trays and the dryer housing, where they are not in direct contact with the product. High thermal efficiencies can be obtained by reheating the air within the dryer. Waste steam from the upstream calcining operation may advantageously be used as the heating medium. The high cost of heating electrically generally restricts its use to relatively small equipment. For materials which have a tendency to foul internal heating surfaces, an external heating system is employed.

The turbo-tray dryer can handle materials from thick slurries to fine powders. It is not suitable for fibrous materials which mat or for doughy or tacky materials. Thin slurries can often be handled by recycle of dry product. Filter- press cakes are granulated before feeding. Thixotropic materials are fed directly from a rotary filter by scoring the cake as it leaves the drum. Pastes can be extruded onto the top shelf and subjected to a hot blast of air to make them firm and free-flowing after one revolution.

In accordance with the present invention, hydrated aluminum fluoride stream 42 is fed to the turbo-tray dryer inlet 52. The temperature of feed 42 to dryer inlet 52 may typically range from about 70°F to about 125 °F. The temperature at the top and bottom of the dryer may typically range from about 300 °F to about 550 °F or hotter. The pressure drop through the dryer is typically on the order of about 1 to 2 inches H₂0, with the top of the dryer operated at about 0.25 to 1 inches H₂0. The dryer tray speed and the dryer turbo fan speed may each be adjusted as necessary. Hot air (or other gas), typically at a temperature of about 400 °F to about 700 °F or hotter, is circulated by natural draft through the center or stack portion 54 of dryer 50. Anhydrous aluminum fluoride is recovered from the dryer outlet 56 as final product stream 60. As previously discussed, it has been discovered that this final aluminum fluoride product stream has a surprising and highly desirable low level of fines.

The following examples further illustrate the benefits and advantages of this invention. The tests performed in connection with the following examples compared the performance of a conventional flash dryer, operated in the normal manner, with a Wyssmont Turbo-Tray Dryer, Model K-20, obtained for purposes of these tests. The Wyssmont K-20 dryer unit is rated at 150 lbs. of feed/hour, producing 106 lbs ./hour of dried product and running up to 44 lbs ./hour of volatiles (H₂0). The air flow rating is 122 ACFM at 575 °F, with temperature in the dryer nominally at 425 °F at the top and 450 °F at the bottom. Nominal heat consumption is approximately 390,000 BTU/hour.

The tests included monitoring and varying the following parameters for operation of the Wyssmont K-20 dryer: date/time feed rate, #/hr (or rate setting on the dryer) feed temperature, °F dryer bottom temperature, °F dryer top temperature, °F dryer ΔP, inches H₂0 dryer top pressure, inches H₂0 dryer tray speed setting, number dryer turbo fan setting, number tray cake thickness, inches inlet gas temperature, °F

Tests were run for several hours. For each run, samples were taken from the feed common to the flash dryer unit and the Wyssmont K-20 unit. The data and test results presented in the following examples were drawn from the test log sheets.

Example 1

An aluminum fluoride process in accordance with the present invention was operated under the following turbo-tray dryer conditions:

Residence Time Dryer Temperature first 15 mins. 400°F next 55 mins. 450 °F

In this example, it was found that a total of 70 minutes of dryer residence time was required to produce a substantially anhydrous A1F₃ product equivalent in dryness to a current flash-dryer produced A1F₃ product. But, whereas the flash- dryer product was found to contain 3-6% by weight fine particulates of less than about 325 mesh size, the A1F₃ produced according to Example 1 contained only about 0.6% by weight of such fines.

Example 2

In a second test, the turbo-tray dryer was operated as follows:

Residence Time Dryer Temperature first 15 mins. 450°F next 45-55 mins. 500°F

In this example, a total dryer residence time of about 60-70 minutes was required to produce a substantially anhydrous A1F₃ product of comparable dryness to a flash-dryer produced product. In Example 2, however, the A1F₃ product was found to contain only about 0.2% by weight of fines of less than about 325 mesh. This is a dramatic improvement of a better than 90 % reduction in the presence of such fines compared with flash-dryer A1F₃ product.

Example 3 This example compared the A1F₃ particle size distribution (using screens of varying mesh size) for a first typical hydrated aluminum fluoride feed with the dried product resulting from processing that feed in two ways: (a) in a conventional flash dryer; and, (b) in a turbo-tray dryer in accordance with the present invention. The test data is presented in Table 1 below:

Table 1

Screen Size Percent (wt. %) Held on Screen + or Passed (-)

Mesh

Dried Feed Flash Drver Turbo-Trav Drver

+ 100 1.2 1.0 7.0

+ 150 32.5 31.8 38.0

+200 85.4 82.4 89.0

+325 98.8 96.4 99.6

- 325 1.2 3.6 0.4 Thus, Table 1 shows that, for the dried feed used in Example 3, a 100 mesh screen retained only 1.2% by weight of the particles in the feed, whereas a 325 mesh screen retained 98.8% by weight of the particles in the feed and passed 1.2% of the particles in the feed. The portion of the particles passed through a 325 mesh screen is what would be regarded as undesirable "fines."

When the feed of Example 3 was dried in a conventional flash dryer, the result was the production of smaller, fractured particles completely across the size distribution range, including a greater production of undesirable fines. Thus, Table 1 shows that at 100, 150, 200 and 325 mesh screen sizes, a smaller percentage of the particles were retained on the screen as compared with the dried feed.

Correspondingly, the percentage of fines produced in the flash dryer product approximately tripled to 3.6% , compared with 1.2% for the dried feed.

By contrast, when the feed of Example 3 was dried in a turbo-tray dryer in accordance with this invention, the result was the production of larger particles completely across the size distribution range, including a correspondingly smaller production of undesirable fines. Thus, Table 1 shows that at 100, 150, 200 and 325 mesh screen sizes, a larger percentage of the particles were retained on the screen as compared with either the dried feed or the flash dryer product. In particular, the percentage of particle fines passed by the 325 mesh screen was only one-third the corresponding percentage for the dried feed, and a surprising one-ninth the corresponding percentage for the flash dryer product.

Example 4 This example was comparable to Example 3 but the screen tests were performed on a second, different hydrated aluminum fluoride feed to try to confirm the surprising comparative data of Example 3. The data is presented in Table 2 below: Table 2

Screen Size Percent (wt. %) Held on Screen (+) or Passed (-) Mesh

Dried Feed Flash Drver Turbo-Trav Drver

+ 100 4.6 2.6 5.8 + 150 36.6 31.5 37.2 +200 87.8 82.8 88.8

+325 99.0 96.8 99.6 - 325 1.0 3.2 0.4

Similar to Table 1 , Table 2 shows that, across the entire size distribution, the flash dryer product resulted in smaller particle sizes and a correspondingly higher percentage of particle fines as compared with the dried feed (3.2% versus 1.0%). By contrast, again across the entire particle size distribution, the turbo-tray dryer product in accordance with the present invention resulted in larger particle sizes and a correspondingly smaller percentage of particle fines as compared with the dired feed (0.4% versus 1.0%) and, even more strikingly, as compared with the flash dryer product (0.4% versus 3.2%).

The experimental results clearly showed that the Wyssmont K-20 turbo-tray dryer performed satisfactorily as a dryer for hydrated aluminum fluoride, performing as well as, or better than, a conventional flash dryer. Test data (based on loss-on-ignition or LOI data) showed that both dryer types as operated resulted in about a 37-39% weight reduction in the product, correlating well with the theoretical water content (39.13 % by weight) of hydrated aluminum fluoride (A1F₃ ^• 3H_O). Because of the effectiveness of the turbo-tray dryer of this invention, it should be possible to operate with a relatively smaller calciner thereby resulting in additional capital and operational savings. Although the manufacturer's rating for the Wyssmont K-20 turbo-tray dryer utilized in the foregoing tests was for feed at a rate of 150 lbs ./hour, when used in the aluminum fluoride process of this invention, it was found that the dryer demonstrated operational stability and achieved satisfactory drying (measured by LOI data) over a wide range of feed rates from a low of 74 lbs ./hour up to 234 lbs ./hour. This makes the turbo-tray dryers of this invention adaptable to a broader range of hydrated aluminum fluoride feed rates than conventional flash dryers.

Finally, the test data demonstrated that the turbo-tray dryer of this invention clearly outperformed the conventional flash dryer in minimizing the production of product fines (i.e., particles passed by a 325 mesh screen). While the flash drying process actually resulted in increasing the weight percent of fines (relative to dried feed), the turbo-tray drying process of this invention resulted in a surprisingly large reduction in fines relative to both the flash dryer product and the dried feed.

Since certain changes may be made in the above-described apparatus and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted in an illustrative and not in a limiting sense. Thus, those skilled in the art will recognize, or be able to ascertain, using no more than routine experimentation, numerous equivalents to the specific substances and procedures described herein. Such equivalents are considered to be within the scope of this invention.

Having described the invention, what we claim is:

Claims

1. A process for aluminum fluoride production comprising the steps of: (a) reacting alumina and hydrofluosilicic acid to form a hydrated aluminum fluoride solution;

(b) heating and agitating said aluminum fluoride solution to convert substantially all of said hydrated aluminum fluoride to its insoluble form, and thereafter separating the solid hydrated aluminum fluoride from the liquor; and (c) treating said solid hydrated aluminum fluoride in a turbo-tray drying system for a sufficient time and at a sufficient temperature to yield substantially anhydrous aluminum fluoride.

2. A process according to claim 1 further wherein the heat for operating said turbo-tray drying system in step (c) is at least partly supplied by waste heat from step (b).

3. A process according to claim 2 wherein the heat from step (b) is supplied by steam, which is thereafter directed to said turbo-tray drying system.

4. A process according to claim 3 further wherein said steam from step (b) is passed through a row of steam pipes of said turbo-tray drying system countercurrently to the direction of movement of said solid hydrated aluminum fluoride.

5. A process according to claim 1 wherein said solid hydrated aluminum fluoride is fed to said turbo-tray drying system at a temperature of about 70°F to about 125°F.

6. A process according to claim 1 further comprising the steps of: (a) feeding said hydrated aluminum fluoride after separation from the liquor to the top of a turbo-shelf dryer comprising a continuous vertical stack of annular, slotted shelves rotating about a centrally-located vertical channel; (b) circulating heated gas vertically through said channel and radially from said channel over said shelves; (c) after the partially-dried aluminum fluoride has substantially completed one revolution on a shelf, sweeping said aluminum fluoride through the shelf slots so that it falls onto the next lower shelf; and (d) recovering substantially anhydrous aluminum fluoride from the bottom of said dryer.

7. A process according to claim 6 wherein said heated gas is air.

8. A process according to claim 7 wherein said air is heated to a temperature of about 400°F to about 700°F.

9. A process according to claim 6 wherein the pressure within the dryer is maintained close to atmospheric pressure.

10. A process according to claim 6 wherein said dryer further comprises a plurality of turbo-type fan means, including fan blades, for circulating said heated gas over said shelves.

11. A process according to claim 10 further comprising varying the rate at which different fan means along said channel circulate heated gas by varying the pitch of the fan blades.

12. A process according to claim 1 wherein said substantially anhydrous aluminum fluoride produced includes less than 1% by weight of particles passed by a 325 mesh screen.

13. A process according to claim 1 wherein said substantially anhydrous aluminum fluoride produced includes less than 0.5% by weight of particles passed by a 325 mesh screen.

14. A process according to claim 6 wherein said substantially anhydrous aluminum fluoride includes less than 1% by weight of particles passed by a 325 mesh screen.

15. A process according to claim 6 wherein said substantially anhydrous aluminum fluoride includes less than 0.5% by weight of particles passed by a 325 mesh screen.