US3033674A - Smelting of zinc in ceramic retorts - Google Patents

Description

3,fl33,fl74 Patented May 8, 1962 3,033,674 SMELTING OF ZlNC IN CERAMXC RETURTS Kenneth A. Phillips, Collinsville, llh, assignor to American Zinc, Lead and Elmelting Company, St. Louis, Mo., a corporation of Maine No Drawing. Filed Dec. 16, 1959, Ser. No. 859,862 ll tClaim. (Cl. 7586) This invention relates to the smelting of zinc. One process by which zinc metal is commonly produced involves mixing zinc oxide, which may be in the form of Zinc sinter, with carbonaceous reducing material and sodium chloride, charging banks of horizontal retorts with the mixture, heating the retorts externally, and condensing the reduced zinc metal. The retorts are commonly hollow cylinders of ceramic material, oriented transversely of a long furnace, with their inner ends supported on a longitudinal center wall and with their mouths opening through a side wall of the furnace, much like boiler tubes in a header. The retorts are arranged in banks symmetrically about the center wall, so that two chambers full of retorts are provided, one on either side of the center wall, the retorts opening through opposite long side walls of the furnace. The retorts are heated, externally, by hot gases in the space between the center wall and the side wall through which the retorts open. The retorts, which are about ten inches in (outside) diameter, and five feet long, must be substantially 'imper vious to Zinc vapor, resistant to attack by coal ash and the gangue constituents of zinc ore, must have high strength and good resistance to plastic flow at temperatures up to about 2300 F. They must be transferred, at high heat, from the firing kiln or a soaking pit, to the furnace; a retort whose temperature after firing drops below 1600 F. being almost certain to fail quickly. Industry wide, the average life of silica retorts is about 45 days. It can be seen, then, that it is of the utmost importance that the deterioration of the retorts, which is rapid enough as it is, be not accelerated.

Immediately outside the side walls of the furnace, the open ends of the retorts are capped with condensers, which are in the form of truncated cones. The ends of these cones are stuffed with wet anthracite or coke, in such a way that the carbon monoxide produced in the reaction between the carbonaceous material and the Zinc oxide, can escape, and that most of the metal fumes cannot. The condensers are maintained at a heat willciently high to keep the Zinc molten, but low enough to condense the metal vapors. For example, the retorts may be kept at about 2150 F. and the condensers at about 950 F. From time to time, the plug in the condensers is knocked out, and the zinc metal is tapped.

As has been indicated above, sodium chloride, common salt, is used in the charging mixture. Without the use of this salt or other suitable chloride, the recovery of zinc metal is relatively low, because of the formation of what is known as blue powder, fine particles of zinc metal coated with a substantially monomolecular layer of zinc oxide. This metal is not lost in the process, but simply recirculates, lowering the efliciency of the process. When sodium chloride or zinc chloride is used, the condenser surface is coated with a flux, which encourages the wetting of the surface by the zinc metal, and permits the recovery in a single cycle of about 85% of the zinc vapor as molten zinc. In a high-lead zinc ore, the presence of the lead facilitates the condensing of the zinc. When a low-lead zinc is desired, a zinc ore low in lead must be used, and the recovery of the zinc in liquid form is more diflicult, and the use of an efiicient halide flux is doubly important.

It was suggested in an early patent (Moulden et al., No. 1,030,676) that other halides might be used as fluxes in the reduction process, but chlorides are the only specific examples given. It has been known in soldering fluxes, to use a mixture of chlorides and fluorides. It has also been known to use a mixture of chlorides and fluorides in the recovery of zinc metal from dross, cf. Lytle, No. 2,578,977 and Deterding, No. 2,701,194. The addition of fluorides has been known to tend to make the contaminated metal easier to coalesce into a liquid pool. However, in the primary smelting process, the use of even one percent of fluorite by weight of sinter results in a very serious attack on the refractory walls of the retorts. The degree of attack is roughly proportional to the percentage of fluorite added. Although the evidence is somewhat difficult to evaluate, it is believed that the degree of attack is also increased when coarse particles of fluorite are employed.

One of the objects of this invention is to provide a more efficient method of smelting zinc ores in ceramic retorts, which utilizes the fluxing capacity of fluorides without seriously damaging the retorts.

Other objects will become apparent to those skilled in the art in the light of the following description.

In accordance with this invention, generally stated, a process of smelting zinc ores in refractory retorts is provided, in which a relatively small amount of finelyground fluorite is used in conjunction with sodium chloride, in such a way as to unexpectedly increase the recovery of metallic Zinc over the recovery to be expected from the use of either the fluorite or the salt, singly. The process of this invention does not lead to uneconomic deterioration of the retorts.

The fineness to which the fluorite is ground seems to determine its eflectiveness as a fluxing medium, its efliciency varying, within limits, inversely as the particle size. However, merely using finely-ground fluorite alone does not give the desired results. In order to achieve the desired fluxing, it is necessary that the fluorite be admixed with common salt in the charge mix.

An example of the process of this invention, and a comparison of the results obtained with it, and the results obtained by conventional methods, and results obtained with the use of fluorite alone, is as follows:

Example 1 Retorts approximately 9% inches in diameter and 61 inches long, inside dimensions, were filled with thoroughly mixed charges having the compositions shown below.

The charge of retort 1 represents a conventional charge. The charge of retort 2. represents a simple substitution of fluoride for common salt. In retort 2, the fluorite was ground moderately fine, with of it passing a 200 mesh screen. Retort 3 represents the charge used in the process of this invention. The fluorite in retort 3 was ground sufi'iciently fine so that all of it passed a 200 mesh screen.

The temperature ofthe gases surrounding the retorts at the beginning of the forty-eight hour cycle was 1750 F.; the temperature at the conclusion of the cycle was 2220 F. The zinc metal was drawn four times during the'cycle.

and filtered to recover the solid contents thereof. The

following data represent the averages for a number of retorts charged and treated in the same way.

The ladle sk-immings are drawn from the condenser, along with the liquid metal therein. They contain the coal or coke used to stuff the condenser and all zinc particles or zinc compounds which are not collected as liquid metal and cast into slabs. The ladle skimmings and rich residues from the front of the retort are recirculated in the next mixed charge. The lean residues are discarded or are treated by some method other than retort smelting. In commercial operations the condenser fume is not collected; it is wasted to the atmosphere.

The simultaneous use in retort 3 of 1.67% of salt and 0.20% of fluorite, based upon the sinter weight, raised the output of slab zinc by over 3%. In retorts 1 and 2 the use of one halide alone allowed an equivalent amount of zinc to partially oxidize and to enter the blue powder. The latter is recirculated on the following charge, and most of the zinc is recovered in the second pass through the furnace, 'but the retreatment step reduces furnace capacity and increases operating costs.

It should be noted that the large amount of fiuorspar in the charge of retort 2 had a seriously deleterious effect upon the ceramic surface of the retort, whereas the small amount of fine fluorspar of the charge in retort 3 had no discernible deleterious eflect upon the retort.

Example 2 Retorts approximately 9% inches in diameter and 61 inches long, inside dimensions, were filled with thoroughly mixed charges having the compositions shown below. The fluorite in retort 1 was ground very finely, so that all of it would pass a 200 mesh screen. The fluorite in retort 2 was ground only to pass through a 10 mesh screen.

Dry pounds of material charge Retort 1 Retort 2 Sinter, 65% Zn 180 180 Reduction fuel, 86% C 55 55 Salt, NaCl 3 3 Fluorite, GaFz .36 1.08

The temperature of the gases surrounding the retorts at the beginning of the 48-hour cycle was 1750 F.; the

temperature at the conclusion of the cycle was 2220" F. The zinc metal produced was drawn four times during the The table above shows that 0.36 lb. of fine fluoride was as eiiective in promoting the condensation of zinc as was 1.08 lbs. of coarse fluorite. To be eifective, the fluorine in the fluorite must be carried to the condenser, where it will enter a chloride-fluoride flux. The following table shows that substantially equal amounts of fluorine travel to the condenser when a small amount of fine fluorite is used as when a larger amount of coarse fluorite is used.

Assay percent fluorine in products Retort 1 Retort 2 Trial 1 Trial 2 Trial 1 Trial 2 Ladle skimmings 25 28 24 26 Condenser fume:

Before 1st draw 18 13 20 .15

1st to 2nd draw .08' .09 .09 .19

2nd to 3rd dra .11 17 14 .14

3rd to 4th draw 15 22 20 .13 Averages l3 15 16 15 The metallurgical results were therefore alike; the physical results are diiferent. The small amount of fluorite used in the retorts marked 1 did not seriously damage the retorts used; the larger amount of coarse fluorite used attacked the retorts marked 2 seriously enough to render the use of fluorite uneconomic at some smelters.

Example 3 Retorts approximately 9% inches in diameter and 61 inches long, inside. dimensions, were filled with thoroughly mixed charges having the compositions shown below. The fluorite in retort 1 was ground very finely, so that all of it would pass a 200 mesh screen. The fluorite in retort 2 was ground only through a 10 mesh screen.

Dry Pounds of Material Charged Retort 1 Retort 2 Sinterchargedmoo coo-(7::

Sinter, 65% On Reduction fuel, 86% 0-- Salt, NaCl Fluorite, CEtFz and treated in the same way.

Pounds of zinc Dry pounds of contents product Retort 1 Retort 2 Retort 2 Slab zinc drawn Rich residues- Lean residues-.- Condenser fume 5 5 This series of tests showed that the beneficial efiects of fluorite substantially disappeared when the amount 0t fine fluorite used was decreased below 0.1% of the smter weight or when the amount of coarse fluorite used was a 6 pie 1. No significant attack of the fluorite was noted and converted to zinc vapor, and condensing said zinc upon the retorts used in the series of tests covered by vapor. Example 3.

It is therefore believed that the amount of fluorite added should not be less than 0.1% nor more than 0.6% of 5 References (med m the file of this patent the sinter Weight. The best balance between recovery UNITED STATES PATENTS and retort life is achieved when 0.2% to 0.3% of finely- 79,701 Stevens July 7, 1868 ground fluorite is used, with about 0.5 to 2.0% of com-1 713,043 Armstrong Nov. 4, 1902 mon salt, by weight of the sinter. At least 90% -of the 1,999,209 Queneau Apr. 30, 1935 fluorite should pass a 150 mesh screen. 10 2,578,977 Lytle Dec. 18, 1951 Having thus described the invention, what is claimed 2,701,194 Deterding Feb. 1, 1955 and desired to be secured by Letters Patent is: 2,867,037 Lawton J an. 6, 1959 The method of smelting Z1110 ore 1n ceramic retorts I GTHER REFERENCES comprising charging said retorts with a mixture of oxidic zinc ore, reduction fuel, sodium chloride and fluorite, at Hackhs Chemical Dictionary, by Julius Grant, 3rd least of the fluorite being of a fineness at least to Edition, The Blakiston Co., 1944, page 214 relied upon.

pass a 150 mesh screen, said sodium chloride being pres- Chambers Technical Dictionary, by C. F; Tweney 'ent in the charging mixture in an amount between about and L. E. C. Hughes, Revised Edition with Supplement,

0.5 and 2.0 percent by weight of the oxidic zinc ore and 1944, theMacmillan Co., page 177 relied upon. the said fluorite being present in the charging mixture in 20 Chemical Encyclopedia, by C. T. Kingzett, 3rd Edition,

an amount between about 0.1 and 0.6 percent by Weight D. Van Nostrand Co., 1924, page relied upon.

of the oxidic zinc ore, heating said retorts and said mixture Chemical Engineers Handbook, by Perry, 3rd Edition, to a temperature at which, over a period of time, the ma- McGraw-Hi1l Book Co. Inc, 1950, page 1111 relied jor portion of the oxidic zinc ore is reduced to metallic zinc upon.