This section covering sources of blending comprises five pages:

Introduction and Definitions
Raw materials
Process control
Blending equipment
Prevention of segregation

Introduction and Definitions

1. Overview
2. Definitions

Overview

This section of FertInform provides a technical introduction to the good practice of producing blended solid fertilisers. It is largely based on the Handbook of Solid Fertiliser Blending: Code of Good Practice for Quality, published by the European Fertilizer Blenders Association. However the approaches and practices that it covers are applicable and relevant throughout the world. This section covers the topics of the physical and chemical properties of raw materials for blended fertilisers, storage good practices, aspects of process control, types of blending equipment, the prevention of segregation, and appropriate quality control.

The blending of solid granular materials to produce a wide range of compound fertilisers has been successfully practised for over 40 years. In some countries blends form by far the major proportion of compound fertilisers sold.

Blended fertilisers differ from complex fertilisers in several ways, the impact of these being subjects of debate within the industry.  These are:

1. Blended products are more versatile, as their formulation can be tailored to meet the precise needs of individual soils, crops, and even fields. By reducing nutrient wastage, this helps to avoid the application of excess nutrients which may enter the environment.
2. Blended fertilisers have lower production costs per tonne, due to a combination of generally lower raw material costs and a lower energy cost.
3. Blended fertilisers face a higher risk of segregation of the components during handling or spreading, if the best practices laid out in this resource are not consistently adhered to.
4. Blended fertilisers may create a hazard if incompatible raw materials are mixed, which can turn the blend into a product with a greater risk of self sustaining  decomposition or detonation (e.g. AN based products and other products containing trace elements like copper, etc but also mixing of two C grades causing an end product being a B grade, etc.) Therefore combining raw materials into a blend requires knowledge of this aspect and how to avoid accidents.
5. Mixtures comprised of building blocks with very different CR (Critical Humidity) can lead to a deterioration of quality.
6. Producing a blend requires extra quality control of the chemical content of the final product, since mistakes or fraud can create a product with off spec composition. This requires blenders to have proper analytical equipment to check chemical composition.

Definitions

Throughout this section the general terms ‘blend’ and ‘blending’ are used to denote the product and the process of mixing granular materials. Blended fertilisers may be supplied to farmers in bags or in bulk. The term ‘bulk-blend’ is reserved for situations where the blend is handled (stored, transported) in bulk rather than in bags.

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Blends do not necessarily consist of mixtures of straight fertilisers or single compounds. A blend of two or three granular complex fertilisers will still be a blended fertiliser.

A number of other specific terms are used and these are defined as follows:

Blended fertiliser

Fertiliser obtained by dry physical blending of various raw fertilisers, without any chemical reaction.

Complex fertiliser

Compound fertiliser obtained by chemical reaction, by liquid solution or, in the solid state, by granulation and having a declarable content of at least two of the major nutrients.

Note 1: For solid granules, each particle contains all the nutrients approximately in their declarable content.

Note 2: Some companies use the term “uniform” to mean a complex fertiliser and to indicate the product is not a blend.

Compound fertiliser

Fertiliser having a declarable content of at least two of the nutrients nitrogen, phosphorus and potassium, obtained chemically or by blending, or both.

Note: With these definitions, mono and di-ammonium phosphates and potassium nitrate are not “straights” but are NP and NK complex fertilisers respectively.

d₅₀ (Mean particle size)

That size such that half the particles, by mass, are larger than that size and half are smaller.

Granular fertiliser:

Solid fertiliser formed into particles of a predetermined mean size by granulation. Note: In some countries, this term can be (wrongly) used to mean complex fertilisers.

Granulometric spread index (GSI)

Measure of the spread of particle sizes and a means of expressing the granulometric spread.

Increment

Representative quantity of material taken from a sampling unit.

Lot

Total quantity of material, assumed to have the same characteristics, to be sampled using a particular sampling plan.

Mean particle size (d₅₀)

That size such that half the particles, by mass, are larger than that size and half are smaller.

Particle size

Dimension which corresponds to the smallest sieve aperture size through which a particle will pass if presented in the most favourable attitude.

Particle size analysis by sieving

Division of a sample by sieving into size fractions.

Raw material

Solid, granular material used as a component in a blended fertiliser.

Note: Some of these materials are not the basic source materials which provide the nitrogen, phosphate and potash. In these cases, they are often known as intermediates or pre-mix.

Segregation

Differential movement of particles within a mixture due to differences in their size, shape or density, resulting in their separation.

Sieving

Process of separating a mixture of particles according to their sizes by one or more sieves.

Size guide number (SGN)

100 times the d₅₀ measured in millimetres

Straight fertiliser

Qualification generally given to a nitrogenous, phosphatic or potassic fertiliser having a declarable content of only one of the plant nutrients nitrogen, phosphorus or potassium.

Note: It is possible for a straight fertiliser to be a blend. For example a mixture of granular ammonium nitrate and granular ammonium sulphate would be a straight nitrogen fertiliser.

Test sieving

Sieving with one or more test sieves.

Definitions of other technical terms may be found in EN 12944 Fertilisers and Liming Materials, Vocabulary, Part 1.

References

EN 12944 Fertilisers and Liming Materials, Vocabulary, Part 1

Links to related IFS Proceedings

558, (2005), Introduction to Guidelines for the Production and Handling of Blended Fertilisers, S Pockelé, O Miserque