US2139196A - Process of making zinc oxide - Google Patents

Description

1938 w. T. MAIDENS PROCESS OF MAKING ZINC 0XI'DE Original Filed Dec. 14, 1933 grwc/wbo'n I amaze-MW 1m;

Patented Dec. 6, 1938 PATENT OFFICE PROCESS OF MAKING ZINC OXIDE William T. Maidens, Columbus, Ohio, assignor to American Zinc, Lead & Smelting Company, St. Louis, Mo., a corporation of Maine Application December 14, 1933, Serial No. 702,374 Renewed August 17, 1937 7 Claims.

' This invention relates to improvements in the manufacture of zinc oxide and more particularly to methods for the manufacture of round particles of zinc oxide and controlling the size of such particles. According to one well known method for making zinc oxide, a mixture of zinc ore and coal is spread upon a fiat grate through a series of charging doors of a suitable furnace A and at a high temperature (approximately 1400" C.). The zinc is reduced to metallic zinc and volatilizedas a gas. As zinc vapor, diluted with the products of combustion, passes over the charges of coal and zinc ore it becomes oxidized through the action of air admitted to the furnace and, as it alternately meets fresh amounts of zinc vapor and air, particles of zinc oxide are formed. The relatively coarse pigment particles of zinc oxide follow an irregular path through the flue and are finally delivered by suction to a trail where they are cooled so that they can be safely filtered in a bag room.

In the operation of such a plant charges of coal and zinc ore are delivered at periodic intervals to the grate, so that by the time the last charge is inserted through the last door in the series, the ash or clinker is ready to be withdrawn from the point where the first charge is inserted and the new charge is introduced. Under these circumstances the characteristics of the product formed are continually changing, and further, weather conditions and other factors beyond the control of the furnace operation make it impossible to obtain a uniform product of the desired quality.

According to the present invention, the furnace is operated under more uniform conditions. The charges of coal and zinc ore are introduced into the furnace as heretofore, but the draft of air to support combustion is limited and so regulated that the products instead of being a mixture of zinc vapor and zinc oxide with C0, C02 and nitrogen, consist of no zinc oxide and a ratio of carbon monoxide to carbon dioxide sufficient to maintain zinc in the elemental state at the furnace temperature which is preferably maintained at approximately 1200 C. This regulation of the draft of air results in an atmosphere practically completely nonoxidizing to zinc vapor above the charge in the furnace. The gases and zinc vapor under a slight positive pressure are then withdrawn from the furnace. In order to maintain the uniform flow of gas, it is desirable to control the suction in the trail.

The temperature and composition of the gases can be held very uniform and this plays an important part in the uniformity of the zinc oxide made by this process.

In the reduced state the gases from the furnace are passed to a suitable tower or chamber Where combustion takes place due to the introduction of air into the current of gases on the way from the furnace to said chamber. Preferably the passage of the gas from the furnace to the combustion chamber is controlled to the end that the relative proportion of air and gas entering the combustion chamber may be so maintained that an intense flame results. The air is introduced at this point by suction or pressure at high velocity so that there will be considerable turbulence in the combustion chamber. When the furnace is operated under these conditions, relatively large round particles of zinc oxide rather than needle shaped particles are formed. By round particles is meant that no one dimension predominates, rather than that the particles are spherical. It is known in the art that when zinc oxide in the form of needles is used in the manufacture of rubber, the result is a stiff rubber, whereas the round particles make a soft rubber. When the furnace is operated under the conditions of the present invention, relatively large, non-acicular particles are developed due to the growth of the small particles. The zinc oxide of this character although desirable for certain round particlezinc oxide of fine particle size, and

prevent particle growth, water sprays are introduced into a tower or combustion chamber and no other change is made; the air for oxidation is admitted in slightly greater quantities than is theoretically necessary to oxidize the zinc vapors as when coarse zinc oxide is being made. By reason of the exothermic reaction when air is admitted there would be a rise in temperature above that at which the gases leave the furnace but for the excess of air admitted above that necessary to oxidize the zinc vapors. This excess tends to neutralize the rise in temperature due to oxidation but is insufficient to lower the temperature of the combined gas and zinc oxide particles below that at which the gases leave the furnace, to wit, approximately 1200 C. In the tower, however, the water sprays at first somewhat lower the temperature (to approximately 1100 C.) and subsequently the water sprays serve to still further reduce the temperature to approximately 850 C. From the tower the gases are passed into an adjoining chamber in which the temperature falls to approximately 750 C., and the round particle size zinc oxide, together with associated gases, pass from the second chamber into the trail on the way to the bag room. Shortly after entering the trial the temperature is decreased to approximately 650 C. These temperatures may vary somewhat above or below those given, but are approximately as stated.

As pointed out above, this treatment results in small, fine, round particles of zinc oxide. Apparently, the water vapor even at the high temperature indicated, is appreciably adsorbed by the zinc oxide when the water vapor is present in large amounts, producing a film on the zinc oxide particles that inhibits particle growth. Whatever may be the reason, the result, as indicated above, is particles of fine, round zinc oxide. The amount of air added in making the fine particles is little, if any, more than that employed in making coarser particles, and it is, therefore, clear that the uniform fineness cannot be due to the action of the air introduced.

Fine particle sized zinc oxide made by this process may carry a greater acidity than is desirable, and a second phase of the present invention lies in the discovery that this method can be so modified as to make zinc oxide with unusual and desirable properties. It has been universally believed heretofore that the refining of zinc oxide by reheating at 600 C. to 700 C. would unduly coarsen the particles, but when the fine zinc oxide particles of the present invention are thus reheated, there is obtained a product of medium acidity that is free of unduly fine particles and also free of unduly coarse particles, resulting in remarkably uniformly sized zinc oxide particles.

Accordingly, the present invention further consists in refining the round particles of zinc oxide of the present invention by reheating, preferably in the presence of organic material whereby a product of medium acidity free from unduly fine particles and free from unduly coarse particles is obtained.

For the purpose of enabling the invention to be understood, reference is made to the accompanying drawing, which is a diagrammatic view of a plant for making round particles of zinc oxides.

In the drawing, in which like reference numerals indicate like parts throughout, I indicates a furnace with any suitably perforated grate, either stationary or travelling, as may be desired, and 2 indicates charging openings for introducing coal and zinc ore into the furnace. The draft of air passing up through the grate for supporting combustion is regulated and controlled by any suitable means well known in the art. As here shown, the draft of air is regulated or controlled by means of a fan 3 delivering air under low pressure to a conduit 4, and the passage of air from said conduit to the several sections of the furnace is controlled by suitable dampers 5. Care is taken to so regulate the supply of air that the atmosphere above the grate shall be at all times a practically non-oxidizing atmosphere, consisting of the products of combustion and the vaporized zinc. The temperature in this furnace may vary somewhat above or somewhat below 1200 C. At 6 is indicated the usual or any suitable tower into which the fumes from the furnace pass through an opening I, which is suitably controlled by baflies 8 to maintain a degree of back pressure in the furnace. The mixed products of combus tion and zinc vapors pass from the tower 6 through a channel 9 into a secondary chamber I from which chamber they are drawn out through the trail I I by means of a suction fan I2 and pass into the bag room I3. The rate of movement of the vapors from the furnace and of the zinc oxide particles and gases on the way to the bag room may be controlled by the speed of the fan I2, and if desired, by a suitable valve I4 introduced in the trail.

As the products of combustion and zinc vapor pass from the furnace I into the tower 6, air is introduced under pressure at I5 under such conditions as to produce considerable turbulence, the air admitted being in somewhat greater quantities than is theoretically necessary to oxidize the zinc vapor but insufficient to lower the temperature below that at which the vapors leave the furnace.

This results in an intense flame. While in the tower 6 the gas is subjected to the action of water spray I6, and this water spray serves to somewhat lower the temperature of the combined gas and zinc oxide particles so that they pass from the tower 6 to the channel 9 at a temperature of approximately 850 C, and thence into the chamber II], which chamber they leave at a temperature of about 750 C. and enter the trail II where the temperature is still further lowered, so that they enter the bag room I3 at a proper filtering temperature. The combustion of the zinc vapor is initiated in the tower 6 and the small round particles of zinc oxide are subjected therein to the action of the water sprays and then pass onward to the chamber I0 where the temperature is somewhat lower, and thence to the trail and to the bag room.

It will be observed that the temperature of the combined gas and zinc oxide particles is progressively reduced on the way from the furnace to the trail, and the zinc oxide particles are subjected to the action of water spray to arrest the increase in size of the particles, with the result that the small, although not unduly fine particles of zinc oxide are secured.

As indicated above, the zinc oxide particles thus obtained may require treatment to control the acidity thereof, and for this purpose the medium size round particles of zinc oxide are refined by reheating, preferably in the presence of organic material, and by such refining process there is obtained a product of medium acidity free from unduly fine particles and free from unduly coarse particles, thus producing a remarkably uniform sized fine particle zinc oxide of the desired acidity.

It will be understood by those skilled in the art that various forms of apparatus may be employed in the practice of the process, and that the invention is not limited to the particular apparatus herein shown and described. Any means for producing zinc vapor in a non-oxidizing atmosphere and carrying the same with a current of inert gas into a combustion chamber where the temperature is somewhat lowered and the particles of oxide formed are subjected to water vapor, after which the temperature of the oxide particles and the accompanying gases is slowly lowered on the way to the bag room, may be employed, the limits of the invention being defined in the accompanying claims.

Having thus described the invention, what is claimed is:

1. The process of making zinc oxide of small non-acicular form which consists in vaporizing zinc in a reducing atmosphere, then rapidly mixing the zinc vapor with an oxidizing agent sufficient to oxidize but not appreciably cool the vapor, and then subjecting the mixed vapor and oxidizing agent to the action of water spray.

2. The process of making fine particles size zinc oxide of medium acidity, which consists in vaporizing zinc in a reducing atmosphere, then mixing the zinc vapor with sufficient air under turbulence to oxidize without appreciably cooling the vapor, then lowering the temperature of the mixture with water spray, and then refining the product by heating. I

3. The process of making zinc oxide of small non-acicular form which consists in subjecting zinc ore to the action of burning coal in a furnace, so controlling the air draft to the furnace that the resulting zinc vapor is in a reducing atmosphere inthe furnace, then passing the vapor and reducing gases from the furnace and mixing them with suificient air under turbulence to oxidize without appreciably cooling the vapor and then spraying the mixture with water.

i. In a process for making zinc oxide in small particle form, the steps of burning zinc vapor with air insufficient to materially cool the vapor and simultaneously spraying the burning vapor with water.

5. The process of making uniform size oxide of non-acicular form and medium acidity which consists in vaporizing zinc in a reducing atmosphere at a temperature approximately 1200 C., passing the zinc vapor and associated neutral gases into a combustion chamber, mixing sufficient air with said vapor and gases on their way to said combustion chamber to oxidize the vapor without materially cooling the same and then spraying water into said combustion chamber, whereby the temperature of the vapor and gases is reduced to approximately 1100 C. and the increase in the size of the zinc oxide particles is arrested, then passing the gases and oxide particles into a second chamber, maintaining the temperature in said second chamber at approximately 850 C., then passing the zinc oxide and associated gases into a bag room while gradually lowering the temperature, and then refining the zinc oxide by reheating.

6. In a process of making zinc oxide of small non-acicular form, the steps of burning zinc vapor with air without thereby materially reducing the temperature, simultaneously spraying the burning vapor with water, and then passing the resulting zinc oxide to a bag room at a slowly and progressively decreasing temperature.

7. The process of making zinc oxide, of nonacicular form which consists in vaporizing zinc in a reducing atmosphere at a high temperature, introducing air under turbulent conditions and in an amount insufficient to lower the temperature, whereby the gases are held at a high temperature for an appreciable period, and then spraying the gases with water.

WILLIAM T. MAIDENS.

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