WO2007137381A1

WO2007137381A1 - A process for recovery of potassium values contained in verdete slates

Info

Publication number: WO2007137381A1
Application number: PCT/BR2007/000128
Authority: WO
Inventors: Lino Rodrigues De Freitas; Marisa Nascimento; Ericksson Rocha E Almendra
Original assignee: Companhia Vale Do Rio Doce
Priority date: 2006-05-25
Filing date: 2007-05-25
Publication date: 2007-12-06
Also published as: BRPI0602252A; BRPI0602252B1

Abstract

comprising comminution (1), followed by pressure leaching (3), which may, or may not, be preceded by a baking stage (2); after the leaching stage (3), the resulting alkaline liquor is separated from the slurry by filtration (4), preferably vacuum filtration, and then is subjected to further processing for obtaining the target potassium salt; said process having a second variant involving a calcination step of the verdete slate (5) preceding the pressure leaching stage (3).

Description

"A PROCESS FOR RECOVERY OF POTASSIUM VALUES CONTAINED IN VERDETE SLATES"

The present invention relates to a process for solubilization of potassium contained in silicate-rich rocks of marine sedimentary origin - such as muscovite and glauconite - termed in Brazil as "verdete" slates. Liquors resulting from said solubilization process become intermediate products for the production of potassium salts to be later used as potassium fertilizers.

As known by those skilled in the art, plants, as well all living beings, need nutrients for their survival and growth. Fertilizers are substances added to soils, and sometimes to foliage, in order to provide nutrients for plant nourishment and growth. Commercially, the term fertilizer is used to designate inorganic or organic substances that provide plants with primary macronutrients, namely nitrogen, phosphorus and potassium.

Major potassium-based fertilizers are KCI, K₂SO₄ and KNO₃. Of these, potassium chloride is by far the most important, accounting for more than 95% of the world production of these substances.

World production of potassium compounds is, in the most part, associated with the extraction of ores from underground deposits containing significant amounts of soluble salts of this element, said deposits being known as evaporites. One of the few exceptions is the recovery of brines from the Dead

Sea, such as currently practiced by Israel and Jordan.

Production of potassic fertilizers in Brazil is limited to a single mine in Sergipe state, from which a mixture of sylvite (KCI) and halite (NaCI) is extracted. The potassium chloride is beneficiated by a flotation-based process, so that in the end a product containing 95% KCI is obtained. Besides this mine in Sergipe, there are only two known evaporite deposits in Brazil, both located in Amazonas state, in an area of difficult access and devoid of infrastructure.

In the year 2004, Brazilian consumption of potassium chloride was approximately 7.0 million tons, while domestic production was only a little more than 640,000 tons, which means that the country imported more than 90% of its demand for this type of fertilizer. This picture is likely to get worse in view of the continuing increase in Brazil's agricultural production resulting from the increasing standard of living of its population.

In such scenario, development of processes for recovering potassium from alternative sources of this element assumes great importance, not only for Brazil, but also for other developing nations.

A number of occurrences of silicated rocks with relatively high potassium contents have been discovered in Brazil. The most notable of these occurrences are the deposits of marine sedimentary origin located in Cedro do Abaete County, in Minas Gerais state. Those deposits contain occurrences of a rock known as verdete slate, in which the main potassium-bearing minerals are glauconite (a hydrated potassium-iron silicate) and muscovite (a monoclinic, basic potassium aluminosilicate of the mica group), with an average K2O content of 12%. In studies performed in the past (LEITE, P. C, 1985, "Effect of heat treatment as applied to Abete verdete, Araxa phosphate rock and magnesium limestone", M. Sc. thesis, Federal University of Lavras, Brazil; and VALARELLI, J.V. and co-workers, 1993, "Use of Cedro do Abaete verdete slates in production of potassic thermophosphate and related economic efficiency", Proceedings of the Brazilian Academy of Science, volume 65, pp. 343-375), a number of pyrometallurgical processes were proposed for recovering the potassium contained in samples of verdete slate from Cedro do Abaete, Brazil.

Such processes, however, have some drawbacks, among which the high temperatures (1400-1500⁰C) required for melting of the potassic rock stand out.

In the present application, a hydrometallurgical process is described for solubilizing the potassium contained in glauconite and muscovite rich verdete slates, using alkaline pressure leaching.

The present invention will be described with reference to the drawings attached hereto, in which: Figure 1 shows a simplified process diagram of a variant of the potassium recovery process; and

Figure 2 illustrates the process diagram of another variant of the potassium recovery process. The process comprises a comminution stage (1), so as to provide an ore particle size of 100% below 0.074 mm, followed by pressure leaching (3) of the ground feed material using a solution of sodium hydroxide (NaOH). Such leaching (3) may, or may not, be preceded by a baking stage (2) in which the feed material is mixed with the leaching solution at room temperature for a number of hours; after baking, the mixture is fed to the pressure leaching operation (3). After leaching (3), the resulting alkaline liquor, into which most of the potassium initially contained in the verdete slate has been dissolved, is separated from the slurry by filtration (4), preferably by vacuum filtration, and is subjected to further processing for obtaining the target potassium salt - preferably potassium sulfate (K₂SO₄) - to be later used as potassium fertilizer.

In another variant of the process, calcination (5) of the verdete slate is performed prior to the pressure leaching stage (3). Such calcination (5) is carried out for at least 2 hours at a temperature within the 800-900⁰C range and its objective is to increase potassium extraction levels during NaOH attack, therefore increasing efficiency of the overall process.

The nature and scope of the present invention can be better understood from the examples presented below. It bears pointing out that said examples are illustrative only, and shall not be viewed as limiting the scope of the process here proposed. EXAMPLE 1 - a 20 g sample of verdete slate from Cedro do

Abaete, Brazil, containing 56.4% SiO₂, 15.3% AI₂O₃, 11.4% K₂O, 7.0% Fe₂O₃ and 2.8% MgO was mixed with 27 mL of a 6 M NaOH solution (approximately 240 g/L NaOH). The resulting slurry was then fed to a cylindrical zirconium vessel, the open top of which was sealed by a concave zirconium cover. A four-blade zirconium impeller to agitate the slurry was inserted into the vessel through a special connection on the vessel's cover, with the aid of a suitable device capable of maintaining the seal, thus assuring the high pressure inside the vessel. Next, the vessel/impeller/pulp set was placed inside a furnace, the temperature of which was set at 200⁰C, and the slurry was continuously agitated for 15 hours. After cooling the reactor down in order to lower its internal pressure, solid-liquid separation by vacuum filtration was performed, followed by washing the filter cake with deionized water at 50⁰C. After separation, 200 ml_ of a solution containing 5.2 g/L of K was recovered, which corresponds to an extraction of 55.4% of the potassium initially contained in the verdete slate. The use of a calcination operation prior to NaOH attack makes it possible not only to significantly reduce the residence time, but also to substantially improve efficiency of pressure leaching, as shown in the following example.

EXAMPLE 2 - a zirconite crucible containing the same verdete slate sample used in example 1 was placed inside the chamber of a muffle furnace previously heated to 900°C. After a residence time of 2 hours at 900°C, followed by cooling to room temperature, a 20 g sample of the calcined product was withdrawn and mixed with 27 ml_ of a 6 M NaOH solution (approximately 240g/L NaOH). Next, the resulting slurry was subjected to leaching for 3 hours at 200⁰C, in a device similar to the one used in the previous example, and then subjected to the same filtration and washing stages as described hereinabove. In the end, 325 ml_ of an alkaline solution containing 5.0 g/L of K were recovered, which corresponds to an extraction of 85.7% of the potassium initially contained in the verdete slate. Such solution is the starting material for recovery of K₂SO₄ to be used as potassium-based fertilizer. Although a preferred embodiment of the proposed process has been described and illustrated, it bears pointing out that other solutions can be accomplished without departing from the scope of the present invention.

Claims

1 -"A PROCESS FOR RECOVERY OF POTASSIUM VALUES CONTAINED IN VERDETE SLATES", characterized by the fact that it comprises ore comminution (1 ), followed by pressure leaching (3) of the ground ore, which may, or may not, be preceded by a baking stage (2); after the leaching stage (3), the resulting alkaline liquor is separated from the slurry by filtration (4), preferably vacuum filtration, and then is subjected to further processing for obtaining the target potassium salt; said process has a second variant involving a calcination operation (5) preceding the pressure leaching stage (3). 2 -"A PROCESS FOR RECOVERY OF POTASSIUM VALUES

CONTAINED IN VERDETE SLATES", according to claim 1 , characterized by the fact that the slate used as starting feed material is subjected to fine grinding, preferably to a particle size of 100% below 0.074 mm.

3 -"A PROCESS FOR RECOVERY OF POTASSIUM VALUES CONTAINED IN VERDETE SLATES", according to claims 1 and 2, characterized by the fact that the alkaline pressure leaching stage is preceded by a baking stage, in which the ore is thoroughly mixed with the leaching agent, while keeping the mixture at ambient temperature, for a period of between 1 and 24 hours, preferably 4 hours. 4 - "A PROCESS FOR RECOVERY OF POTASSIUM

VALUES CONTAINED IN VERDETE SLATES", according to claims 1 , 2 and 3, characterized by the fact that in the baking stage use is made of an aqueous solution of sodium hydroxide in a concentration range of 1 to 10 moles per liter (between 40 and 400 g/L NaOH), preferably 6 M (240 g/L NaOH). 5 -"A PROCESS FOR RECOVERY OF POTASSIUM VALUES

CONTAINED IN VERDETE SLATES", according to claims 1 , 2, 3 and 4, characterized by the fact that the alkaline pressure leaching stage is performed at a temperature within the 100-300⁰C range, preferably at 200⁰C.

6 -"A PROCESS FOR RECOVERY OF POTASSIUM VALUES CONTAINED IN VERDETE SLATES", according to claims 1 , 2, 3, 4 and 5, characterized by the fact that in the alkaline pressure leaching stage the leaching agent utilized is an aqueous solution of sodium hydroxide in a concentration of 1 to 10 moles per liter (40 to 400 g/L NaOH), preferably 6 M (240g/L NaOH).