Patent Description:
Fertilizers are well known for agricultural and horticultural application. A number of nutrients are thereby supplied to the soil or growing medium of the plants. Nutrients such as nitrogen, phosphorous, potassium and sulfur are supplied in relatively large amounts, while many other elements are supplied in lower amounts, as micronutrients.

Potassium sulfate (K<NUM>SO<NUM>, also called Sulfate of Potash, or SOP) is a well-known fertilizer for providing potassium and sulfur. The fertilizer is most commonly used in granular form for providing granular fertilizer to the soil. Powdered, soluble forms are available as well, for fertigation and foliar spray.

The Mannheim process is a well-known process for producing potassium sulfate. Chemically, the process consists of two steps: (i) a reaction of potassium chloride and sulfuric acid to potassium bisulfate and hydrogen chloride as a by-product, and (ii) a reaction to potassium sulfate and hydrogen chloride. The steps are shown by reaction formula (<NUM>) and (<NUM>).

KCl + H<NUM>SO<NUM> → KHSO<NUM> + HCl     (<NUM>) Exothermic reaction.

KHSO<NUM> + KCl → K<NUM>SO<NUM> + HCl     (<NUM>) Endothermic reaction.

In order to achieve at least about <NUM>% chloride conversion, the Mannheim process generally uses a reaction temperature of the product of higher than about <NUM>, generally up to about <NUM>.

Generally, the potassium sulfate obtained from the Mannheim process has a moderate chloride content (<NUM>% to about <NUM>% by weight). The use of potassium sulfate with high chloride values such as ><NUM>% is less desirable for the agricultural and industrial markets.

In order to lower the amount of chloride, it is possible to use an excess sulfuric acid, such that more chloride can be liberated as HCl gas. For example, <CIT> describes that KCl is reacted with sulfuric acid at an equivalent ratio of <NUM>-<NUM> at temperatures from about <NUM> to about <NUM>.

The Mannheim process, as carried out currently, is performed in essentially the same manner as developed in Germany during the nineteenth century. It involves a furnace consisting of two parts, a combustion chamber at the top and a reaction chamber underneath. An example of a Mannheim furnace is described in e.g. <CIT>.

The granular potassium sulfate product has not been subject to changes for a long time. A process for the formation of a potassium sulfate product comprising micronutrients is disclosed in <CIT>.

It is an object of the present invention to provide an effective and efficient process for the production of an improved potassium sulfate using a Mannheim process.

It is another object of the invention to provide an improved solid potassium sulfate.

It is yet another object to overcome some of the issues related to adding micronutrients via for instance a simple blending. Micronutrients are often small in particle size and hence can segregate in a bulk blend.

To attain one or more objectives, the present invention provides a process for the preparation of a potassium sulfate product comprising micronutrients, said process comprising the steps of:.

Preferably the potassium sulfate is one that is prepared in a muffle furnace, via the Mannheim process. The potassium sulfate provided contains from <NUM> to about <NUM> wt% of "acidic compounds", calculated as sulfuric acid.

In the above the term "micronutrients" refers in particular to "plant micronutrients". Essential plant nutrients include primary nutrients, secondary or macronutrients, and trace or micronutrients. Micronutrients can include boron, zinc, manganese, nickel, molybdenum, copper, iron, chlorine, sodium or combinations thereof. Secondary nutrients can include, but are not limited to calcium, magnesium or combinations thereof. Throughout the invention, for the sake of simplicity, the term "micronutrients" as used herein refers to and micronutrients. Preferred in the use of the invention is the use of water soluble micronutrients. Examples include but are not limited to water soluble boron, water soluble copper, water soluble manganese and/or water soluble zinc. Sodium and chlorine are not tolerated by all crops or soils and can lead to salt stress. They are hence less preferred.

Micronutrients in the invention can be provided as salts, as chelates, as a natural organic complex and/or as an industrial metal bearing by-product (the latter if low in heavy metal content). A few examples of micronutrient sources that may be used are anhydrous borax, zinc sulfate monohydrates, manganese sulfates, sodium molybdate dehydrates, manganous sulfates etc. Micronutrients are often provided in the form of phosphates and/or sulfates. Boron can be provided under the form of boric acid or as e.g. an octoborate.

Micronutrients of the invention are added to a potassium sulfate feed stream when the temperature of the potassium sulfate is between about <NUM> and about <NUM>. More preferred temperatures follow below.

The micronutrients in the context of the invention are sodium, zinc, manganese, iron, molybdenum, copper, boron, or mixtures thereof (of any of these). Particularly preferred are zinc, manganese, iron, molybdenum, copper, boron, or mixtures thereof (of any of these). Even more preferred are zinc, manganese, iron, molybdenum, copper, or mixtures thereof (of any of these). It was found that suitable micronutrients with sufficient water solubility can be added to the feed stream of a potassium sulfate that is produced in a muffle furnace.

Micronutrients according to the invention can be added at various moments to the potassium sulfate feed stream. In one embodiment of the invention, the one or more micronutrients are added shortly after the potassium sulfate leaves the reaction chamber, in casu the muffle furnace, at the latest just prior to the granulation process. In another embodiment of the invention, the one or more micronutrients are added at the end of the production process, when the potassium sulfate has its final characteristics but before it leaves the reaction chamber, in casu the muffle furnace. The one or more micronutrients are added prior to the granulation process, at any stage between leaving the muffle furnace and reaching the granulation unit.

The potassium sulfate has a temperature of at least about <NUM>, preferably at least about <NUM> and more preferably at least about <NUM> but preferably not more than <NUM>°, preferably not more than about <NUM>, not more than about <NUM>.

Soluble micronutrients are generated in situ via a reaction with "acidic compounds" that are present in the potassium sulfate produced.

Provided herein is hence also a process for the preparation of a potassium sulfate product comprising micronutrients, said process comprising the steps of :.

The potassium sulfate provided, as indicated above, has a temperature of about <NUM> to about <NUM>, preferably at most <NUM>, more preferably at most <NUM> at the moment of adding these one or more metal compounds.

Examples of suitable metal compounds include zinc oxide, manganese oxide, iron oxide, molybdenum oxide, potassium hydroxide, iron hydroxide, iron carbonate, iron bicarbonate, sodium carbonate, sodium bicarbonate, sodium hydroxide, manganese carbonate, manganese bicarbonate, manganese hydroxide, copper carbonate, copper bicarbonate, copper hydroxide, or mixtures thereof (of any of these). Preferred metal compounds are zinc oxide, manganese oxide, iron oxide, molybdenum oxide, iron hydroxide, iron carbonate, iron bicarbonate, manganese carbonate, manganese bicarbonate, manganese hydroxide, copper carbonate, copper bicarbonate, copper hydroxide, or mixtures thereof (of any of these).

Preferred metals are: zinc, manganese, iron, molybdenum, copper, or mixtures thereof (of any of these). Even more preferred are zinc and/or manganese. Most preferred is zinc. A preferred metal compound in the present invention is an oxide. Preferably the metal compound(s) used comprises (or consists of) zinc oxide.

Preferably the amount of "acidic compounds" in the potassium sulfate is between about <NUM> and about <NUM> wt%, more preferably between about <NUM> and about <NUM> wt%.

In the above process the metal oxide and/or metal carbonate and/or metal bicarbonate and/or metal hydroxide advantageously is/are turned into the corresponding metal "sulfates", at least in part. The process of the invention allows to use cheaper compounds and/or allows to have a conversion into more soluble compounds that can be taken up by the plants. Metal sulfates in general are easily taken up by plants, and provide an advantage in particular when the soil is deficient in certain metals, certain micronutrients.

Preferred metal compounds according to the invention are metal oxides and/or metal carbonates and/or metal bicarbonates. Even more preferred are metal oxides and/or metal carbonates. Most preferred are metal oxides.

Preferably, the step of providing a potassium sulfate comprises the steps of:.

In a particle embodiment of the invention, also the KCl product provided is one that contains one or more micronutrients as described. Generally however, the KCl product used for economic reasons is not containing any extra micronutrients.

The present invention also relates to products obtainable by any of the processes of the invention. The invention furthermore provides for a solid potassium sulfate product, wherein the amount of potassium in the potassium sulfate is between <NUM> and <NUM> wt%, calculated as K<NUM>O, comprising particulates that contain both potassium sulfate as well as one or more micronutrients chosen from sodium, zinc, manganese, iron, molybdenum, copper, boron, or mixtures thereof wherein the potassium sulfate produced comprises between <NUM> and <NUM> wt% of metal salts, calculated as metal ions other than potassium. In a preferred embodiment of the invention, said particulates are particles, more in particular granules. Preferred lists of micronutrients are given above. Provided in particular is a solid potassium sulfate product comprising particles that contain both potassium sulfate as well as one or more metal sulfates wherein the metal is selected from the group consisting of zinc, sodium, manganese, magnesium, iron, molybdenum, copper, boron, or mixtures thereof (of any of these). The "sulfates" can be sulfates (SO<NUM>), bisulfates (HSO<NUM>), or a mixture of both. In it broadest sense, the term "sulfates" can also refer to pyrosulfates. It is preferred that sulfates (SO<NUM>) are present. Preferred lists of micronutrients and preferred lists of metal compounds are provided above.

The micronutrients as described are entrained within a potassium sulfate particulate, more in particular within a potassium sulfate granule.

An advantage of the processes and materials of the invention is that the one or more micronutrient components will be more or less uniformly distributed in the potassium sulfate product, which typically is in the form of a granule, possibly a spherical granule. Alternatively the potassium sulfate product of the invention is in powder form. It can also be in compacted form. A uniform application of micronutrients to a plant growing area in general leads to a better uptake by the plants. A uniform size distribution reduces or eliminates segregation during material handling and transfer.

The foregoing and other objects, features and advantages of the present invention will be apparent to those skilled in the art from the following detailed description and appended claims.

The term "acidic compounds" refers to acids as well as to products that in the presence of water can generate acids, and to mixtures of both.

The term "sulfuric acid" means sulfuric acid of at least about <NUM>% concentration, and preferably at least about <NUM>, <NUM>, <NUM>, or <NUM> % concentration or above, more preferably at least about <NUM>, <NUM>, <NUM>, <NUM> up to <NUM>% concentration, like for example about <NUM>% grade or <NUM>% grade.

The general term "sulfates" is used in the invention to refer to sulfates (SO<NUM>-<NUM>), to hydrogen sulfates and to mixtures of both. Most often, sulfates (SO<NUM>-<NUM>) and their mixtures with hydrogen sulfates are referred to.

Potassium sulfate refers to a compound comprising potassium and sulfate. The amount of potassium in the potassium sulfate, calculated as K<NUM>O, is at least about <NUM> wt%, preferably at least about <NUM> wt%. Typically the amount of sulfur in the potassium sulfate is at least about 48wt%, preferably at least about <NUM> wt%, calculated as SO<NUM>.

The potassium sulfate of the invention preferably comprises a certain amount of acidic compounds that are capable of converting the metal compound of the invention into a metal salt, either directly or indirectly.

The level of (remaining) acidic compounds in potassium sulfate can be measured, for example via an acid-base titration, measuring the amount of protons (H+) formed, and is calculated as amount of sulfuric acid (at <NUM>%).

Potassium sulfate with an excess acid at a temperature between about <NUM> and about <NUM> can be provided by heating potassium sulfate, for example as obtained from a Mannheim process. Alternatively, the product from a Mannheim process can be used after it is removed from the muffle furnace (in situ), before it is cooled to room temperature.

Preferably, the temperature of the reaction between the micronutrients and the acidic compounds comprising potassium sulfate is between about <NUM> and about <NUM>. In a preferred embodiment, this temperature is between about <NUM> and about <NUM>.

A Mannheim process uses an excess sulfuric acid over potassium chloride, which is preferably between about <NUM> and about <NUM>% on mole basis, preferably this excess is between about <NUM> and about <NUM>% on mole basis. Therefore, the molar ratio of sulfuric acid to <NUM> moles of potassium chloride generally is from about <NUM> to about <NUM>, preferably from about <NUM> to about <NUM>, more preferably from about <NUM> to about <NUM>.

The reaction between the acidic compounds in the potassium sulfate and the micronutrient metal compound generally is aided by mixing, and the zone where the reaction takes place is therefore for example provided with mixing devices, such as mixing screws; the chamber where this reaction takes place may itself be a tumbling device.

In this zone where this reaction takes place, the required metal compound, or mixtures of metal compounds in the form of a metal oxide, carbonate, bicarbonate, hydroxide or mixtures thereof, is or are added.

The metal compound preferably is in the form of a slurry of the metal compound in water. This aids in a fast and effective reaction. The amount of solids in the slurry generally can be between about <NUM> and about <NUM> wt%, preferably between about <NUM> and about <NUM> wt%.

A Mannheim process for producing potassium sulfate generally comprises the following steps.

The raw materials potassium chloride (KCl) and sulfuric acid (H<NUM>SO<NUM>) are dosed in the reaction chamber. For the next reaction step heat is provided, which is done in a muffle furnace by applying heat above the surface (dome) of the reaction chamber. This heat is supplied by burning a hydrocarbon source like gas or fuel.

In this way the contents of the reaction chamber, through its upper surface, is heated, generally to achieve a product temperature of between about <NUM> and about <NUM>, like for example about <NUM> and about <NUM>, or about <NUM> and about <NUM>.

In addition to applying heat, the content of the furnace is slowly mixed by continuous mixing with rakes, screws or the like.

Generally, the process of the present invention relates to a continuous industrial process for the production of potassium sulfate. The process can be stably practiced on a commercial scale. In some cases, the potassium sulfate after production will be neutralized, at least in part, with for instance lime or any other suitable means (see <CIT>). Today the standard potassium sulfate is compacted then broken or crushed and sieved. This is the more standard granulation step. Other forms of granulation that may be used are Physical/steam granulation and chemical granulation. Compaction granulation is generally preferred. Alternatively, spherical granules can be prepared using a spherical granulation step, using for instance a high shear mixer, a pan granulator or a granulation drum. Additionally the processing can contain one or more of the following steps: drying, grinding, polishing, sizing, packaging etc..

In an in situ preparation of the product of the invention, the reaction product from the muffle furnace will be transported to a reaction zone for the reaction with the metal compound(s) of the invention. Alternatively, a product at room temperature can be heated. In situ processing has the advantage, that no heating energy has to be applied.

Preferably the potassium sulfate used in the process of the invention has a temperature of between about <NUM> and about <NUM> when the metal compound(s) of the invention are introduced. Preferably the conversion from metal compound into metal sulfate takes place at a temperature from about <NUM> to about <NUM>, preferably from about <NUM> to about <NUM>. This often means cooling down the potassium sulfate coming out of the muffle furnace before its reaction with the metal compound(s) of the invention. Cooling can be done in any possible way, including natural convection and/or air cooling.

Alternatively, a potassium sulfate produced before by a Mannheim process as described, but now at lower temperature, may be heated preferably to between about <NUM> and about <NUM>, for example using waste heat present in the process.

Preferably, the conversion of metal compounds to metal sulfates takes place in the presence of water or at least some moisture.

An amount of metal compound is provided to achieve between about <NUM> and about <NUM> wt% of metal (calculated as metal ions) in the potassium sulfate.

The metal compounds, in the form of oxides, carbonates, bicarbonates, hydroxides or mixtures thereof are virtually insoluble in water. It is an advantage of the invention that the reaction at a temperature between about <NUM> and about <NUM>, preferably between about <NUM> and about <NUM>, provides for better soluble metal salts, principally in the sulfate form. Most metal sulfates have a high solubility in water, and in general metal sulfates are easily taken up by the plant. In this way, an efficient process is provided to obtain soluble salts of metals which are useful as micro-nutrients.

Preferably, the metal compound is in the form which is cheapest, which generally is the oxide. Furthermore, the use of oxides is advantageous, as the oxides generally have a (very) low solubility to an extent it deprives its nutrient value, while transferring the oxides into corresponding sulfates increases the solubility substantially.

As explained, the reaction of the invention is performed with a metal oxide, carbonate, bicarbonate, hydroxide or mixtures thereof, in which the metal preferably is chosen from zinc, manganese, iron, molybdenum, copper or mixtures therefrom. Preferably, cheaper oxides are used. These oxides are not considered as soluble in the agricultural area. Therefore, up to now it is considered necessary to buy relatively expensive soluble salts, which are mixed into other fertilizers to be effectively applied as micro-nutrient. This (bulk) blending process in addition provides several disadvantages as explained above.

The amount of metal compounds is chosen such, that the potassium sulfate so produced comprises between <NUM> and <NUM> wt% of metal salts, more in particular metal sulfates, the wt% being calculated as metal ions.

Preferably an amount of metal compounds is reacted to obtain between <NUM> and <NUM> wt% of metal salts, calculated as metal ions.

In a further embodiment, mixtures of metal compounds are used, such that the potassium sulfate so produced comprises between about <NUM> and about <NUM> wt% total metal ions, other than potassium. Preferably, the mixture comprises two to three different metal ions, each comprising between about <NUM> and about <NUM> wt%.

In mixtures, preferably two to three different micronutrients other than potassium are used. Preferably two to three different metal ions are then used. The different micronutrients can be provided each separately, via a different feed; or they can be provided to the potassium sulfate in the form of a mixture. A preferred list to choose the micronutrients from are zinc and/or boron and/or manganese. Particularly preferred are a combination of zinc and manganese; a combination of boron and manganese; or a combination of zinc, boron and manganese.

The reaction at elevated temperatures is largely a solid state reaction that provides metal salts, in particular metal sulfates, which are largely soluble in water. Preferably, the process according to the invention provides products, wherein the solubility of the metal ion is about <NUM>% or higher, about <NUM>% or higher, about <NUM>% or higher; preferably about <NUM>, <NUM>, <NUM>, <NUM>, <NUM>% or higher, about <NUM>, <NUM>, <NUM>, <NUM>, <NUM> % or higher; more preferably about <NUM>% or higher. This relative to the total amount of metals present.

The product of the invention relates to solid potassium sulfate comprising particles having both potassium sulfate and metal sulfates of micronutrients chosen from zinc, manganese, iron, molybdenum, copper, or mixtures thereof. Preferred lists of metals are given above. The reaction is largely a solid state reaction, which provides particles having both potassium sulfate, and the metal sulfate. Hence, the product of the invention is different from a physical mixture of potassium sulfate and micronutrients-metal sulfates.

The preferred metal compound is chosen from zinc, manganese or mixtures thereof. The most preferred metal compound is a zinc compound.

Preferably, the potassium sulfate of the invention comprises zinc, and the amount of zinc in the potassium sulfate is between about <NUM> and about <NUM> wt%, preferably between about <NUM> and about <NUM> wt%.

The product of the invention is a potassium sulfate wherein the amount of potassium is between <NUM> and <NUM> wt%, calculated as K<NUM>O. Preferably, the amount of potassium in the potassium sulfate is between about <NUM> and <NUM> wt%, calculated as K<NUM>O, and even more preferably between about <NUM> and <NUM> wt%.

Generally, the product of the invention is a potassium sulfate having an amount of sulfur, calculated as SO<NUM>, of about <NUM> wt% or more, preferably of about <NUM> wt% or more. Generally, the amount of sulfur, calculated as SO<NUM>, will be about <NUM> wt% or less, preferably about <NUM> wt% or less.

The amount of chloride in the potassium sulfate of the invention generally will be about <NUM> wt% or less, preferably about <NUM> wt% or less, and more preferably about <NUM> wt% or less.

In an alternative way, the product of the invention can be made by adding a metal compound chosen from an oxide, carbonate, bicarbonate, hydroxide, or mixtures thereof, in which the metal is chosen from zinc, sodium, manganese, iron, molybdenum, copper, or mixtures thereof together with potassium chloride at the initial step of the Mannheim process. In this way, a fully homogeneous product is obtained, with virtually all micronutrients as metal "sulfates".

The potassium sulfate obtained from the Mannheim process generally is in a powder form, and generally has a particle size of about <NUM> and lower. For use as a granular fertilizer, that can be easily spread over land, and mixed with other granular fertilizers, it is preferred to apply a compaction step, or a granulation step. Such granulation steps are common in the art.

In a preferred granular form, the potassium sulfate of the invention has > <NUM>% of the particles between about <NUM> and about <NUM>. More preferably, more than about <NUM>% of the particles (by weight) have a size between about <NUM> and about <NUM>. A range with at least about <NUM>% between about <NUM> and about <NUM> is most preferred. The granule can be a spherical granule.

The hardness of the granules of the invention preferably is about <NUM> or higher, more preferably about <NUM> or higher.

Preferably, the potassium sulfate of the invention when <NUM> gram is dissolved in about <NUM> water exerts a pH of between about <NUM> and about <NUM>, more preferably between about <NUM> and about <NUM>.

To the potassium sulfate of the invention, one or more of the following can be added: a compacting aid, a coloring agent, additives like waxes and/or one or more binding resins and/or grinding resins. The improved potassium sulfate of the invention can be used as fertilizer an sich, or in combinations with other fertilizers in the art, like conventional NPK fertilizers.

Although specific embodiments have been described, these are examples only and are not limiting upon the scope of the invention.

The amount of (remaining) acidic compounds in the potassium sulfate produced in a muffle furnace is measured by dissolving <NUM> milligram of potassium sulfate produced in a muffle furnace in <NUM> water, and titrating <NUM> of this solution with <NUM> molar NaOH, and calculating the amount of acidic compounds as H<NUM>SO<NUM> (assumed to be at <NUM>%) from the amount of H+ ions measured.

Hardness was measured with a Hardness meter (Type Indelco <NUM>-M).

Color was measured via a Colorimeter (type Minolta CR <NUM>).

The K, S, Cl and Na-content in examples was determined via XRF (X-ray Fluorescence) and recalculated as K<NUM>O, SO<NUM>, Cl and Na<NUM>SO<NUM>.

In order to measure the solubility of the Zn, <NUM> of sample was dissolved in <NUM> demiwater (at about <NUM>) for about <NUM> while stirring. Thereafter, the liquid was filtered, and the filtrate was analysed with ICP-MS (Inductively Coupled Plasma Mass Spectroscopy) for the amount of Zn. As ZnO is virtually insoluble, while ZnSO<NUM> is well soluble, it can be assumed that all dissolved Zn originates from ZnSO<NUM>. In a comparable way, the solubility of other metals can be measured. With this method the soluble metal concentration is determined versus the total (theoretic) metal concentration, and expressed as weight percentages.

Particle size analysis: the particles were screened over a sieve, and respective fractions were measured (weight basis).

K<NUM>SO<NUM> as obtained from a Mannheim process after the muffle furnace, produced with about <NUM>% excess sulfuric acid over KCl, was analysed for acidic compounds present (which were denoted as amount equivalent to H<NUM>SO<NUM>). The product contained near <NUM> wt% of acidic compounds, calculated as H<NUM>SO<NUM>.

To K<NUM>SO<NUM> from a Mannheim process as described in Preparation Example <NUM>, <NUM> wt% ZnO resp. <NUM> wt% ZnO were added and continuously mixed at ~<NUM>. After mixing for ~<NUM>, and further cooling to below ~<NUM>, the particulate product was packed in sacks of <NUM> and put in carton boxes. Boxes were transported to a compaction/breaking operation.

In this product according to example <NUM>, <NUM>% of the zinc appears soluble.

After granulation in a compaction device, the analysis yielded the following results.

Claim 1:
Process for the preparation of a potassium sulfate product comprising micronutrients, said process comprising the steps of:
- providing a potassium sulfate, that comprises from <NUM> to <NUM> wt% of acidic compounds, calculated as sulfuric acid (<NUM>%), wherein the amount of potassium in the potassium sulfate so produced is between <NUM> and <NUM> wt%, calculated as K<NUM>O, and wherein the potassium sulfate provided is at a temperature of between <NUM> and <NUM>, when adding the one or more micronutrients,
- mixing said potassium sulfate, prior to granulation, with one or more micronutrients selected from the group consisting of sodium, zinc, manganese, iron, molybdenum, copper, boron, or mixtures thereof, wherein an amount of metal compounds is reacted to obtain between <NUM> and <NUM> wt% of metal salts, calculated as metal ions other than potassium, and
- applying a granulation step.