Magnesium desulfurization agent

Details Magnesium desulfurization agent Magnesium desulfurization agent

2000-04-10

2002-03-05

Andrews, Melvyn (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Compositions

Solid treating composition for liquid metal or charge

C075S313000, C075S315000, C075S316000, C075S560000, C075S566000, C075S568000, C420S023000

Reexamination Certificate

active

06352570

ABSTRACT:

The present invention relates to a method of desulfurization of molten iron and more particularly to a desulfurization agent used to desulfurize molten pig iron.
BACKGROUND OF THE INVENTION
Specifications for the sulfur content of finished steel are decreasing to extremely low levels to make high strength low alloy steel, and steels resistant to hydrogen induced cracking, among other applications requiring low sulfur contents. In combination with the economic benefits of blast furnace operations producing molten pig iron with increased sulfur contents, the desulfurization of molten pig iron external to the blast furnace before the molten pig iron enters the steel making furnace has become a practical necessity. Over the years, a wide variety of materials and mixtures have been used to desulfurize pig-iron. It has long been known that various calcium compounds are good desulfurization agents. It has also been known that magnesium, alone or in combination with various alkaline metal oxides, is also a good desulfurization agent. There have been several patents which disclose the use of calcium oxide and magnesium as the primary desulfurization agents. (See Skach U.S. Pat. No. 4,765,830; Skach U.S. Pat. No. 4,708,737; Green U.S. Pat. No. 4,705,561; Kandler U.S. Pat. No. 4,139,369; Kawakami U.S. Pat. No. 4,137,072; Koros U.S. Pat. No. 3,998,625). Furthermore, desulfurization agents disclosing the use of calcium carbide as the primary desulfurization agent have also been known and well documented. (See Freissmuth U.S. Pat. No. 3,598,573; Todd U.S. Pat. No. 3,929,464; Braun U.S. Pat. No. 4,395,282).
The use of a desulfurization agent that includes magnesium and iron carbide or high carbon ferromanganese is disclosed in Luxemburg Patent No. 88,252 dated Jan. 3, 1999 and invented by Axel Thomas. The desulfurization agent disclosed in Thomas '252 includes a majority of iron carbide or high carbon ferromanganese. The desulfurization agent also includes magnesium, and one or more additives to improve the formed slag. The particles of iron carbide or high carbon ferromanganese are selected to be the same or slightly greater in size than the particles of magnesium. The particle sizes of the iron carbide or high carbon ferromanganese and magnesium range from 0.5 to 1 mm. As a result, the particles of iron carbide or high carbon ferromanganese do not coat the particles of magnesium, or vice versa. The iron carbide or high carbon ferromanganese and magnesium can be coated with titanium oxide to improve the fluidity of the particles and to slow the melting rate of the particles. The iron carbide or high carbon ferromanganese and magnesium can be mixed together prior to injection into the pig-iron or injected separately into the pig-iron.
The use of a calcium compound and/or magnesium, in combination with a gas-producing compound, has also been used to increase the amount of sulfur removal. It has been found that the gas-producing compound releases a gas upon contact with the molten pig-iron to create a turbulent environment within the molten pig-iron. The released gas primarily breaks down agglomerations of the desulfurization agent and disperses the desulfurization agent throughout the molten pig-iron. The gas-producing agent is typically a hydrocarbon, carbonate or alcohol which has a tendency to release various amounts of gas upon contact with the molten pig-iron. Use of these various gas-producing agents is well documented. (See Takmura U.S. Pat. No. 3,876,421; Meichsner U.S. Pat. No. 4,078,915; Gmohling U.S. Pat. No. 4,194,902; Koros U.S. Pat. No. 4,266,969; Freissmuth U.S. Pat. No. 4,315,773; Koros U.S. Pat. No. 4,345,940; Green U.S. Pat. No. 4,705,561; Rellermeyer U.S. Pat. No. 4,592,777; Meichsner U.S. Pat. No. 4,764,211; Meichsner U.S. Pat. No. 4,832,739; and Luyckx U.S. Pat. No. 5,021,086).
Desulfurization agents can contain various slag-forming agents. The importance of the slagging agent generally has been passed over for more immediate concerns about the economics of using various ingredients of the desulfurization agent. The composition of the slag can be important to retain the removed sulfur within the slag and not allow the sulfur to re-enter the molten pig-iron. Various slagging agents have been used for various purposes. In U.S. Pat. No. 4,315,773 a desulfurization agent comprising calcium carbide, a gas-involving compound, and fluorspar is disclosed. Fluorspar is used to modify the properties of the slag to prevent carbon dust production from igniting during the desulfurization. In U.S. Pat. No. 5,021,086, fluorspars are used to modify the characteristics of the slag increasing the fluidity of the slag during the desulfurization process.
There is a critical need to maximize sulfur removal in the pig iron at the lowest possible cost. Although magnesium is an excellent desulfurizer due to its very high reactivity with sulfur, much of the magnesium in the pig iron immediately vaporizes on contact with the pig iron and rapidly escapes from the pig iron by bubbling to the surface of the pig iron, allowing very little time for reacting with sulfur. Magnesium must dissolve into pig iron, forming a solution, in order for it to react efficiently with sulfur. Since magnesium is one of the more costly components of a desulfurization agent, various desulfurization agents have been developed to remove sulfur from the pig iron using components other than magnesium, such as calcium oxide and calcium carbide, as the principal desulfurizer, to reduce the cost of the desulfurization agent. Larger quantities of these desulfurization agents, in comparison to magnesium, are needed to remove sulfur in the pig iron, thus driving up the cost of the desulfurization process. In addition, the use of large quantities of desulfurization agent results in large slag formation which in turn results in a significant loss of iron in the slag. The loss of iron in the slag results in higher costs associated with the desulfurization process. As a result, there remains a need in the steel industry to desulfurize pig iron in an efficient and cost effective manner and to reduce the loss of iron during the desulfurization process.
SUMMARY OF THE INVENTION
The present invention relates to an improved desulfurization agent and a method of treating molten ferrous materials such as molten pig iron with a desulfurization agent that improves desulfurization efficiency.
In accordance with the principal feature of the present invention, there is provided a desulfurization agent which includes a reactive desulfurizing agent that actively reacts with sulfur in the molten iron, such as molten pig iron. Preferably, the reactive desulfurizing agent forms a compound with the sulfur that can be removed from the molten pig iron, such as migrating into a slag on the surface or to the bottom of the molten pig iron and/or forming into a gas and bubbling out of the molten pig iron. The reactive desulfurizing agent is at least partially coated with a heat absorbing agent. The heat absorbing compound is formulated to absorb heat around the reactive desulfurizing agent. In one embodiment, the heat absorbing compound is formulated to absorb heat about and/or closely adjacent to the reactive desulfurizing agent to increase the time the reactive desulfurization agent remains in the molten pig iron for reaction with sulfur and/or to increase the reaction rate of the reactive desulfurizing agent.
In accordance with one aspect of the present invention, the reactive desulfurization agent is partially or totally coated with the heat absorbing agent. The reactive desulfurization agent can be pre-coated with the heat absorbing mixture or coated with the heat absorbing mixture just prior to being added to the molten pig iron. In one specific aspect of the invention, a reactive desulfurization agent is sufficiently coated with the heat absorbing compound to reduce the rate of or prevent the vaporization of the reactive desulfurization agent prior to the reactive desulfurization agent reacting with a significant amount of sulfur in the pig iron

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