GB2140454A - Anti-caking agents - Google Patents

Abstract

An anti-caking agent, which is particularly suitable for use with prilled ammonium nitrate fertilizers, comprises: (1) an oxidized paraffinic wax which has been partially saponified with magnesium oxide; (2) free magnesium oxide; and (3) a polyethylene wax. The invention also provides a prilled ammonium nitrate fertilizer composition containing the anti-caking agent.

Description

SPECIFICATION Anti-caking agents This invention is concerned with improvements in and relating to anti-caking compositions, particularly for providing anti-caking properties to prilled ammonium nitrate fertilisers.

Prilled ammonium nitrate fertilisers are commonly produced by prilling a concentrated ammonium nitrate solutions which crystallizes at a temperature of, for example, about 180"C. During the course of the prilling operation, the prills are cooled through the crystal transition in a tower, and then dried and screened to appropriate particle size.

Prilled ammonium nitrate fertilisers are very sensitive to moisture, and to reduce this sensitivity, particularly in the case of high nitrogen containing grades of fertiliser, a fixed or bound water content of a maximum of 0.2% by weight is generally aimed for. However, reduction of the fixed or bound water content to this level generally leads to an increase in hardness of the prills and this may be undesirable since the prills may then have a higher tendency to fracture during and after manufacture.

Fracturing of the prills has the effect of increasing the proportion of small particles and thus increases the specific surface area of the product. In humid storage conditions, this wide particle size range leads to increased contact between the particles thus promoting the rate of moisture up-take causing accelerated degradation and subsequent product loss.

In order to overcome these problems, it is known to add anti-caking materials, especially clay particles, to the prilled fertiliser so that particles of the anti-caking agent are interposed between the prills to prevent contact between them. Ammonium nitrate prills may have smooth or highly porous surfaces. The smooth prills have the lowest contact area and, therefore, there are fewer pores present to accommodate clay particles and, as a result, there may be an increase in product compaction. Rough or porous prills retain more particles and more clay is used so that there may be a consequent unacceptable increase in clay dust.

It has also been proposed, in order to improve the anit-caking properties and stability of prilled ammonium nitrate, to add magnesium oxide to the prills by adding a slurry of magnesium oxide to the melt concentrated solution used to produce the prills. During crystallisation of the prills, the magnesium oxide becomes fixed in the crystals and serves to improve the resistance ofthe prills to degradation. However, in practice, the additon of magnesium oxide in the melt phase can interrupt or otherwise distort the crystallisation behaviour of the ammonium nitrate, thereby possibly leading to instability of the product in storage.

It has now been found, in accordance with the present invention, that certain anti-caking compositions, as hereinafter defined, may be used to coat preformed prilled ammonium nitrate fertiliser and to give a product having improved anti-caking and storage stability properties.

Accordingly, the invention provides, in accordance with one emgbodiment thereof, an anti-caking composition comprising: (i) an oxidised paraffinic wax which has been partially saponified with magnesium oxide; (ii) free magnesium oxide; and (iii) a polyethylene wax.

The first component [component (il] of the anti-caking composition of the invention (hereinafter simply referred to as the "partially saponified oxidised wax") is prepared by the partial saponification of an oxidised paraffinic wax with magnesium oxide.

Oxidised paraffinic waxes are well known materials. They are commonly prepared by the air and chemical oxidation of a paraffinic refined or crude wax by blowing air through a melt of the wax at a temperature of from 100 to 200"C in the presence of an oxidation catalyst. The products (oxidate acids) themselves are complex mixtures containing unreacted paraffins and starting materials and oxidised products such as acids, lactones and esters. The degree of oxidation of an oxidised paraffin wax is commonly indicated by its acid number (the number of mg of potassium hydroxide required to neutralise a one gram sample of the oxidate) and for use in the present invention, the oxidised wax suitably has an acid number of from 25 - 100, preferably from 30 - 75, most preferably from 35 - 50.

The partially saponified oxidised wax is suitably partially saponified to a degree such that it has an acid number of from 5 to 40, preferably from 10 to 30, most preferably from 20 to 30. The partial saponification is suitably effected by reacting the oxidised wax, preferably in the form of a melt (for example at a temperature of from 70 to 105"C) with magnesium oxide. Since the anti-caking composition of the invention also contains, as components (ii), free magnesium oxide it is most convenient to react the partially oxidised wax with an excess of magnesium oxide over that required to partially saponify the oxidised wax to the desired level and carry out saponification for a time sufficient to effect the desired degree of partial saponification as evidenced, for example, by the acid number of the material.The total amount of magnesium oxide used in such a process will, of course, depend upon (a) the desired degree of partial saponification in the oxidised wax and (b) the amount of free magnesium oxide to be present in the final composition. In this latter connection, a composition in accordance with the invention suitably contains free magnesium oxide in an amount of from 30 to 36 % by weight, preferably from 26 to 36% by weight, most preferably from 30 to 36% by weight, based on the weight of partially saponified oxidised wax.

The third component of the composition of the invention is a polyethylene wax and this is suitably one having a melting point of from 80 to 1 15"C, preferably from 100 to 1 15"C and a viscosity at 140"C of from 400 to 800 centistokes, preferably from 500 to 700 centistokes. The total amount of polyethylene wax present in the anti-caking composition of the invention is suitably from 2 to 8 % by weight, preferably from 3 to 5 % by weight, more preferably from 2 to 3 % by weight, based on the weight of partially saponified oxidised wax.

The polyethylene wax may be introduced into the composition after partial saponification of the oxidised wax with magnesium oxide but, more preferably, is dispersed in the composition during the course of the partial saponification reaction which is suitably carried out with stirring whereby to produce a uniform dispersion of the components of the composition.

As discussed more fully below, for use in the anti-caking treatment of an ammonium nitrate fertiliser, the composition of the invention is suitably in granularform suitably having a particle size of from 1 to 10 mm, preferably from 0.01 to 0.15 mm.

The anti-caking composition of the invention is most suitably applied to a prilled ammonium nitrate fertiliser by tumbling the prilled fertiliser with granules of the caking composition, in a suitable apparatus capable of providing a tumbling action, for example a rotating drum. It has been found that under such conditions, the composition is transferred to the surface of the prills of ammonium nitrate fertiliser to form a more or less continuous coating thereon. The anti-caking composition of the invention is suitably used in an amount of from 0.01 to 0.02 % by weight, preferably 0.01 to 0.15 % by weight, based on the weight of prilled ammonium nitratefertiliser.

In order that the invention may be well understood the following examples are given by way of illustration only.

Example An anticaking agent based on an oxidised paraffinic wax having the properties listed below was prepared as described below.

Properties of oxidised paraffinic wax.

(Hypax 450L manufactured by applicants) Drop Melting point 55"C Colour Tan Acid no. (mg KOG/gm( 45-55 Saponification No. (mg KOH/mg) 13-43 centistokes To 550 parts by weight of the molten oxidate (80"C) in a stirred vessel was added 400 parts of partly calcined magnesium oxide, having an mgO content of between 93-95% surface area of between 10-20 m2/gram and average particle size of approximately 53 microns.

The reaction temperature was increased to 90"C one hour and 50 Kg of a polyethylene wax having the properties listed below was added with continuous stirring.

Specification for polyethylene wax Melting point 105-1 14C Penetration at 250"C 3-5 Density at 20"C 0.90-0.92 Viscosity at 140"C (cSt) 500-700 Colour Gardiner 1-2 Average molecular weight 3000 The reaction was allowed to continue for a further hour until the mass was in a finely dispersed phase having the following properties.

Colour Tan Melting point 88 C Relative density 1.6- 1.8 The finished product was then granulated to an average particle size of between 4 - Smm.

the product, herewith referred to as Hypax 450L MgO, was tested as an anticaking agent for prilled ammonium nitrate.

Test No. 1 Type of fertiliser Na4No3 20.0.0 Nitrogen content 19.92% Moisture content 0.47 Hypax 450L MgO was added in granular form from a hopper having a controlled discharge rate to provide a dosage level of 0.267% wt of fertiliser. the temperature of the prills entering the rotating coating drum was 80"C. All the Hypax 450L MgO readily transferred to the prills by abrasion forces within the 3 minute residence period.

The coated fertiliser was then transferred to storage and dropped from a height of 50 meters into receiving silos.

The treated prills were tested after 48 hours outside storage and found to have a resistance to caking of 1.3Kg/CM2, i.e. this pressure was required to break a cake formed by compressing in a cylinder.

Test No. 2 Type of fertiliser Nh4No3 34-0-0 Nitrogen content 34% Sulphate content 5.39% Ph 3.1 Hypax 450L MgO was added in granular form from a hopper having a controlled discharge rate to give a dosage rate of 0.14% of fertiliser. The temperature of the prills entering the rotation dum was between 80-82"C. All the Hypax 450L MgO readily transferred to the prills by abrasion forces within the 3 minute residence period.

The coated fertiliser was then transferred to storage and dropped from a height of 50 meters into receiving silos.

The treated prills were tested after 48 hours outside storage and found to have a resistance to caking of 1.3 kg/CM2, i.e. this pressure was required to break a cake formed by compressing the granules in a cylinder.

For comparative assessment an untreated and a clay/amine wax treated fertiliser having the same nitrogen content of 34% was tested and were found to have caking values of 6.3 and 2.2 kg/CM2 respectively.

Claims (6)

1. An anti-caking composition comprising: (i) an oxidised paraffinic wax which has been partially saponified with magnesium oxide; (ii) free magnesium oxide; and (iii) a polyethylene wax.

2. An anticaking composition as claimed in claim 1 containing from 30 to 56% by weight, based on the weight of partially saponified oxidised paraffinic wax, of free magnesium oxide.

3. An anticaking composition as claimed in claim 1 or claim 2 containing from 2 to 8% by weight, based on the total weight of the composition, of polyethylene wax.

4. An anticaking composition as claimed in claim 1 substantially as hereinbefore described with reference to the example.

5. A prilled ammonium nitrate fertilizer composition containing, as anticaking agent, an anticaking agent as claimed in any one of the preceding claims.

6. A fertilizer composition as claimed in claim 1 substantially as hereinbefore described.