FR3083145A1 - PROCESS FOR THE GRINDING OF A MINERAL MATERIAL IN THE PRESENCE OF AT LEAST ONE GRINDING AGENT - Google Patents

Abstract

The invention relates to a method for grinding a mineral material, which method comprises: - a step of supplying said mineral material to be ground and at least one grinding agent, - a step of grinding said mineral material, in the presence of said at least one grinding agent. And, according to the invention, said at least one grinding agent is chosen from vegetable organic materials, to form a vegetable grinding agent. During said grinding step, said at least one vegetable milling agent is mixed with said mineral material in a mass proportion of less than 5% of the total mass.

Description

Technical field to which the invention relates

The present invention relates, in general, to the field of processes for grinding mineral materials.

It relates more particularly to the processes for grinding mineral materials, in the presence of at least one milling agent.

Technological background

Grinding is an operation consisting in dividing a solid, generally to increase its specific surface and therefore its reactivity.

Such grinding operations are essential in particular in the manufacture of cement, for example during the grinding of raw materials or during the final grinding of the clinker.

However, in practice, these grinding operations can be expensive because of the energy consumption required.

In this sense, the high energy consumption for grinding clinker represents an item of expenditure which affects the overall cost of the cement.

Solutions exist to improve the energy efficiency of grinding operations.

The use of grinding agents, added in small quantities, is a common solution to improve the efficiency of grinding mineral materials.

This improvement in yield then results in a reduction in energy consumption and / or in an increase in the final surface area of the product generated by the grinding operation.

Such milling agents can for example consist of organic compounds, such as phenols, alkanolamines or glycols.

The mechanism of action of these milling agents is not fully understood. The development of new milling agents thus generally results from a completely empirical approach.

In addition, conventional milling agents are generally "petro-sourced" organic products. There is thus a need for efficient, non-polluting and low-cost milling agents.

In view of the above, there is always a need for new grinding processes, in the presence of at least one grinding agent, the grinding efficiency of which would be improved.

Object of the invention

In order to remedy the aforementioned drawbacks of the prior art, the present invention provides a method for grinding a mineral material, in the presence of at least one grinding agent.

More particularly, a method is proposed according to the invention comprising:

- a step of supplying said mineral material to be ground and at least one milling agent, and

- A step of grinding said mineral material, in the presence of said at least one milling agent.

Said at least one grinding agent is chosen from vegetable organic materials, to form a vegetable grinding agent. And, during said grinding step, said at least one vegetable milling agent is mixed with said mineral matter in a mass proportion of less than 5% of the total mass.

The inventors demonstrate that such milling agents improve the efficiency of grinding mineral materials.

Interestingly, a low level of vegetable milling agent makes it possible to increase the volume surface achieved for a fixed energy expenditure; it can even allow an increase in the maximum attainable volume surface.

Still in general, the addition of the vegetable grinding agent can make it possible to obtain a certain fineness more quickly (and therefore save energy), but also to increase the maximum achievable surface area.

Such a grinding process according to the invention, in the presence of said at least one vegetable milling agent, thus makes it possible:

- an increase in the quantity of material produced during grinding, for the same energy consumption and the same fineness, or

- an increase in the fineness of the product for the same energy consumption.

Other non-limiting and advantageous characteristics of the grinding process according to the invention, taken individually or in any technically possible combination, are the following:

- Said at least one vegetable milling agent is incorporated in a mass proportion less than or equal to 1%, advantageously from 0.1% to 1% (or even between 0.1% and 1%), preferably 0.5% at 1% (or even between 0.5% and 1%) of the total mass;

- Said at least one vegetable milling agent is chosen from vegetable organic materials comprising at least one of the following components: lignin, cellulose, a parietal polysaccharide (advantageously one at least from a hemicellulose, a pectin, a hydrocolloid);

- Said at least one vegetable milling agent is chosen from lignocellulosic materials, advantageously unrefined lignocellulosic materials, more preferably from a by-product of forestry or the wood industry (for example branched wood, chips, sawdust , bark, etc.), a co-product of agriculture or the food industry (for example straws, rice husk, bran), a perennial plant (for example myscanthus, switchgrass or switchgrass, short rotation coppice as by example willow, poplar, eucalyptus), an algae or a by-product of seaweed exploitation (for example green algae like sea lettuce, brown algae like fucus or laminaria source of alginates, red algae like certain rhodophyceae source of agaragar and carrageenans, micro-algae cultivated for energy and chemistry);

- Before said grinding step, said at least one vegetable milling agent has a size less than or equal to 1 cm, preferably less than or equal to 0.5 cm, or even less than or equal to 1 mm; the step of supplying said at least one vegetable milling agent advantageously comprises a grinding / separation operation of a vegetable organic raw material;

- Said at least one vegetable milling agent has a water content of less than 50%, advantageously following a drying operation;

- Said mineral material to be ground is chosen from hydraulic binders (advantageously from aluminous cements, Portland cements), clinkers, rare earths, silica derivatives (for example quartz) and metalliferous ores;

- Said grinding process comprises an operation of mixing said at least one milling agent with said mineral material, which mixing operation is carried out at the start of said grinding step or during said grinding step;

- before the grinding step, said mineral material to be ground has a size less than or equal to 1 cm, preferably less than or equal to 0.5 cm, or even less than or equal to 1 mm;

the grinding step is carried out dry, advantageously by means of a grinding media mill (balls, agitated balls, vibrating balls, etc.);

- The grinding step is continued until a ground mineral material is obtained having a volume surface greater than or equal to 1000 m ² / L, advantageously from 1000 to 3000 m ² / L.

The invention also provides a process for the manufacture of a hydraulic binder, which process comprises:

- a step of supplying a clinker (for example a step of manufacturing this clinker),

a step of grinding said clinker to obtain said hydraulic binder, which step of grinding is implemented according to the grinding process according to the invention.

The present invention also relates to the use of vegetable organic materials as a vegetable grinding agent in a process for grinding a mineral material, which vegetable grinding agent is incorporated in a mass proportion of less than 5% of the total mass.

The invention also relates to the ground mineral material, resulting from a grinding process according to the invention. This ground mineral material comprises at least one vegetable milling agent incorporated in a mass proportion of less than 5% of the total mass; and said ground mineral material has a surface area greater than or equal to 1000 m ² / L, advantageously from 1000 to 3000 m ² / L.

Detailed description of an example of realization

The description which follows with reference to the appended drawings, given by way of nonlimiting examples, will make it clear what the invention consists of and how it can be carried out.

In the accompanying drawings:

- Figure 1 shows the variation of the electric power of a vibrating mill (expressed in W), as a function of the mass of powder in a bowl (expressed in g);

- Figure 2 shows the evolution of the volume surface as a function of the grinding mass energy for quartz in the presence of straw - a: pure quartz, b: quartz + 0.7% straw;

- Figure 3 shows the evolution of the volume surface as a function of the grinding mass energy for the cement in the presence of straw - a: pure cement, b: cement + 0.7% straw;

- Figure 4 represents the differences in energy consumption for the production of cement in the presence of straw (0.7% by mass) compared to the grinding of pure cement for several fineness - abscissa: volume area in m ² / L - a: grinder ball, b: vibrating mill;

- Figure 5 represents the differences in energy consumption for the production of ground sand in the presence of straw (0.7% by mass) compared to the grinding of pure sand for several fineness - abscissa: volume area in m ² / L - a: ball mill, b: vibrating mill and c: agitated ball mill;

- Figure 6 represents the differences in energy consumption on cement for different straw rates (straw mass content in%), in comparison with the grinding of pure cement - a: 1500 m ² / L, b: 1600 m ² / L and c: 1700 m ² / L;

- Figure 7 represents the surface increase (%) for the cement + 0.7% of straw, in comparison with the grinding of pure cement, for different mass electric consumption (kWh / t) on different grinders (a: ball mill and b: vibrating mill);

- Figure 8 represents the surface increase (%) for sand + 0.7% of straw, in comparison with the grinding of pure sand, for mass electrical consumption (kWh / t) on different grinders (a: ball mill , b: vibrating mill and c: agitated ball mill).

Method for grinding a mineral material

The present invention relates, in general, to a process for grinding a mineral material in the presence of at least one vegetable milling agent.

By "grinding process" is meant an operation of the trituration type consisting in dividing a mineral (solid) material, advantageously to increase its specific surface and therefore its reactivity.

The grinding process according to the invention comprises the following successive steps:

- a step of supplying said mineral material to be ground and at least one milling agent,

a step of grinding said mineral material, in the presence of said at least one milling agent,

Supply of mineral material to be ground

A first step in the grinding process according to the invention therefore consists in providing the mineral material to be ground.

The term “mineral material” advantageously means a solid natural compound of inorganic substances.

This mineral material to be ground is advantageously chosen from hydraulic binders, clinkers, rare earths, silica derivatives and metalliferous ores.

A hydraulic binder is a binder that forms and hardens by chemical reaction with water. It is used in construction and in the road industry.

Hydraulic binders advantageously include aluminous cements, Portland cements.

An aluminous cement is a cement resulting from the grinding, after extensive cooking or not until fusion, of a mixture mainly composed of alumina, lime, iron and silica oxides, in proportions such as cement obtained contains at least 30% of its weight of alumina.

Portland cement is a cement resulting from the spraying of a clinker formed mainly of hydraulic calcium silicates to which it can be added calcium sulphate, limestone, water and grinding agents.

This Portland cement notably includes artificial Portland cement (CEM I) and addition cements (CEM II, III, IV and V).

Clinker is a constituent of cement, which results from the firing of a mixture composed of approximately 80% limestone and 20% aluminosilicates.

In the case of rare earths, the mineral material to be ground is advantageously chosen from bastnasite, monazite, apatite, chéralite, eudialyte, loparite, phosphorites, xenotime, thorite.

Silica derivatives include in particular quartz, sandstone, quartzite from weakly cemented or unconsolidated sands.

Metalliferous ores are minerals that contain one or more metallic elements.

These metalliferous ores include oxides (bauxite for aluminum), sulfides (galena for lead, sphalerite for zinc), carbonates (malachite for copper, siderite for iron), silicates (garnierite for nickel and magnesium), or gold (as a native element or as a minor constituent of other minerals).

Before the grinding step of the process according to the invention, said mineral material to be grinded is advantageously in the form of a granular mineral (or divided solid).

This mineral raw material, to be ground, is advantageously composed of particles having a size less than or equal to 1 cm, preferably less than or equal to 0.5 cm, or even less than or equal to 1 mm.

By "size" is advantageously meant a dimension of the particles composing said mineral matter, resulting from a dimensional analysis by sieving (see for example the standard NF EN 933-1).

The size of a particle, and more generally of a constituent of a granular mixture, advantageously corresponds to the length of its primary axis, that is to say the longest straight line which can be drawn between one end of this constituent and an opposite end.

The size of a particle, and more generally of a constituent of a granular mixture, also corresponds to a characteristic dimension of the particle.

In the case where particles would present shapes very far from the sphere (either due to their elongation or their flattening), it is necessary to consider several characteristic dimensions.

The size of a constituent used in the composition of a granular mixture, for which the particle size distribution has a well-defined peak, is advantageously the average of the characteristic sizes of the particles of this constituent.

In general, the mineral raw material advantageously comes from the following manufacturing operations: extraction from a quarry, then crushing and finally homogenization.

Supply of said at least one milling agent

The first step of the grinding process according to the invention also includes the supply of said at least one milling agent.

The term "milling agent" means a product intended to be mixed in small quantities with the mineral material to be ground so as to improve the yield of the stage of grinding.

According to the invention, said at least one milling agent is chosen from plant organic materials, to form a component called "plant milling agent".

In other words, vegetable organic materials are used as a vegetable milling agent in the process of grinding a mineral material.

The term “vegetable organic material” advantageously means an organic material made from plants.

Said at least one vegetable milling agent is advantageously chosen from vegetable organic materials comprising at least one of the following components: lignin, cellulose, a parietal polysaccharide.

Lignin is a biomolecule, from the family of polyphenolic polymer macromolecules, which is one of the main components of wood with cellulose and hemicellulose.

Cellulose is a carbohydrate made up of a linear chain of D-glucose molecules (between 15 and 15,000).

The parietal polysaccharide is advantageously chosen from at least one hemicellulose, one pectin, one hydrocolloid.

Hemicellulose is a biopolymer that makes up the plant cell wall and is one of the components of wood. It has a bridging role between the cellulose fibers, but also with other matrix compounds.

Pectins are polysaccharides characterized by a backbone of α-D-galacturonic acid and small amounts of α-L-rhamnose (more or less branched).

By "hydrocolloid" is meant families of biopolymers extracted from plants, consisting mainly of polysaccharides and other compounds (proteins, ions), which are generally extractable with water and which have texturing properties (thickeners, gelling agents).

According to a preferred embodiment, said at least one vegetable milling agent is advantageously chosen from lignocellulosic materials, advantageously unrefined lignocellulosic materials.

"Lignocellulosic material" means a material composed of lignin, hemicellulose and cellulose (in variable proportions).

By "unrefined lignocellulosic materials" is meant a lignocellulosic material which has not undergone pretreatments to separate the constituents thereof, namely for example;

- physical pretreatments other than grinding (for example heating by heat exchanger or by irradiation with microwaves, as well as sonication and papermaking refining) and / or

- chemical pretreatments (for example soda, ammonia and / or ozone).

Alternatively, the lignocellulosic materials consist of refined lignocellulosic materials.

In this case, the lignocellulosic material undergoes pretreatments to separate the constituents thereof, namely for example physical pretreatments (for example heating by heat exchanger or by microwave irradiation, as well as sonication and paper refining) and / or pretreatments chemicals (e.g. soda, ammonia and / or ozone).

In practice, unrefined lignocellulosic materials advantageously consist of a co-product generated during an industrial process.

A co-product is a material, intentional and inevitable, created during the same manufacturing process and at the same time as a main product.

Thus, the unrefined lignocellulosic materials are advantageously chosen from:

- a by-product of the logging or wood industry,

- a co-product from agriculture or the food industry,

- a perennial plant,

- an alga or a by-product from the exploitation of algae.

For example, the co-product of the logging or wood industry consists of:

- a rameal wood, also called “fragmented rameal wood” or “fragmented ramial wood (BRF)”, that is to say an uncomposted mixture of grinding (fragmentation) residues of wood twigs (branches), mainly produced deciduous trees,

- wafers, that is to say the result of grinding by mechanized machines (shredders with knives) of the remnants of a forestry or wood operation of small diameter or of low quality, appearing for example in the form of small pieces of wood about 2 * 2 * 5 cm,

- sawdust, small particles and fine shavings from sawing wood,

- bark.

The co-product of agriculture or the food industry consists for example of:

- straws, co-product of the production of cereal seeds and represented by the part of the stem (or stubble) of certain grasses, called "straw cereals" (wheat, barley, oats, rye, rice), cut during the harvest ,

- bales or berries, a co-product derived from the processing of cereals and made up of all the glumes or glumella which contain the grain (obtained by threshing or shelling),

- bran, a co-product consisting of the envelope of the cereal caryopsis after separation of the almond (from, for example, wheat, rice or oats).

The perennial plant is for example chosen from:

- miscanthus,

- switchgrass or panic panic (Panicum virgatum),

- a short rotation coppice or TCR, such as willow, poplar or eucalyptus.

An alga or a by-product of exploitation of algae is for example chosen from:

- green algae {Chlorophyta, Streptophycophyta, Euglenophyta,

Chlorarachniophyta), like sea lettuce,

- brown algae (also called Phaeophyceae or Pheophyceae), such as fucus or laminaria, source of alginates,

- red algae (or rhodophytes), like certain rhodophyceae, source of agar-agar and carrageenans,

- microalgae, cultivated for energy and chemistry.

Before the grinding step, said at least one vegetable milling agent preferably has a size less than or equal to 1 cm, preferably less than or equal to 0.5 cm, or even less than or equal to 1 mm.

By "size" is advantageously meant a dimension of the particles making up said at least one vegetable milling agent, resulting from a dimensional analysis by sieving (see for example the standard NF EN 933-1).

The size of a particle, and more generally of a constituent of a granular mixture, advantageously corresponds to the length of its primary axis, that is to say the longest straight line which can be drawn between one end of this constituent and an opposite end.

The size of a particle, and more generally of a constituent of a granular mixture, also corresponds to a characteristic dimension of the particle.

In the case where particles would present shapes very far from the sphere (either due to their elongation or their flattening), it is necessary to consider several characteristic dimensions.

The size of a constituent used in the composition of a granular mixture, for which the particle size distribution has a well-defined peak, is advantageously the average of the characteristic sizes of the particles of this constituent.

Preferably, the size of the particles forming the vegetable milling agent is similar to the size of the particles forming the mineral material to be ground, that is to say a maximum difference of a factor of 10 (ratio of the size of the particles forming l vegetable grinding agent on the size of the particles forming the mineral material to be ground).

Preferably, the maximum difference is a factor of 3, more preferably a factor of 2.

For this, the step of supplying said at least one vegetable milling agent advantageously comprises a grinding / separation operation applied to the vegetable organic raw material intended to form the vegetable milling agent.

This grinding / separation operation is adjusted to obtain the desired size of the particles forming said vegetable milling agent.

The separation operation is for example chosen from sieving, turboseparation or tri-electrostatic (see for example the document Rajaonarivony et al., 2017, “Electrostatic separation of mineral and vegetal powders with a custom built corona separator: application to biorefinery of rice husk ", EPJ Web of Conferences, Volume 140, 2017, or Barakat et al.," Electrostatic Separation as an Entry into Environmentally Eco-Friendly Dry Biorefining of Plant Materials ", Journal of Chemical Engineering & Process Technology, 2017, Vol 8 (4): 354).

Still according to an advantageous characteristic, said at least one vegetable milling agent has a water content of less than 50%, advantageously following a drying operation.

For example, the water content is less than 25%, more preferably less than 10%.

The advantage of this drying is to obtain a controlled humidity level and also to facilitate grinding.

Grinding stage

The function of the grinding stage is to divide the mineral matter (solid) and to decrease the size of the particles, and therefore to increase its specific surface.

The "specific surface" is therefore one of the ways of measuring the size of the particles resulting from grinding.

It indicates the relationship between, on the one hand, the real surface of an object and, on the other hand, the quantity of matter of the object (in general its mass, even its volume).

This specific surface can be expressed as a mass area (surface units per unit mass), or even as a volume area / volume surface (surface units per unit volume).

The volume surface can be determined from the following equation (1):

S _v = Tf = ^ (1) in which

R, represents the radius of the i ^th grain size class, and a, represents the volume fraction of this class.

The calculated surface area is expressed in m ² / L.

The grinding step is advantageously carried out in a grinder.

This grinding step is advantageously carried out dry, preferably by means of a grinding media mill (also called a grinding mill with free grinding bodies).

The grinding media mill is for example chosen from:

- a ball mill, advantageously constituted by a cylindrical or cylindro-conical drum with a horizontal axis and filled (for example to a third of its volume) by the grinding charge (steel or cast iron balls, flint rollers, sticks, tetrahedrons or hard steel cylinders),

- a agitated or attrition ball mill, advantageously a chamber in the center of which is a variable geometry agitator (formed either of propellers, rings, solid, notched or perforated discs) to set in motion balls of grinding which occupies between 50 and 85% of the apparent volume of the grinding chamber as well as the suspension to be treated (the grinding balls can be made of steel, glass, alumina or ceramic),

- a vibrating crusher, comprising for example a grinding bowl producing vibrations in the form of an arc of a circle on a horizontal plane in which balls strike the material to be ground situated on the rounded external faces.

Such mills are for example presented in the article Blazy et al., Engineering technique - j3051 - Fragmentation - Technology.

During the grinding step, said at least one vegetable milling agent is mixed with the mineral matter in the grinder (before or during its operation).

This mixing operation is obtained by incorporating said at least one vegetable milling agent into the grinder containing the mineral material to be ground.

Said at least one vegetable milling agent is added in a mass proportion of less than 5% of the total mass (that is to say advantageously the sum of the mass of the milling agent plus the mass of the mineral matter to grind).

The "total mass" advantageously corresponds to the total mass of the materials mixed in the mill during the grinding step.

This total mass is at least equal to the value obtained by adding, on the one hand, the mass of the mineral matter and, on the other hand, the mass of said at least one vegetable milling agent.

Preferably, said at least one vegetable milling agent is incorporated in a mass proportion less than or equal to 1%, advantageously from 0.1% to 1%, preferably from 0.5% to 1%, of the total mass.

The operation of mixing said at least one vegetable milling agent with said mineral material to be ground can be implemented:

- at the start of the grinding stage, or

- during this grinding step, between the start and the end of said grinding step.

The grinding step is continued for a time sufficient to obtain mineral material in the desired state.

In this sense, the grinding step is advantageously continued until a ground mineral material is obtained having a surface area greater than or equal to 1000 m ² / L, advantageously from 1000 to 3000 m ² / L, more preferably from 2,000 to 3,000 m ² / L.

Method for manufacturing a hydraulic binder

The present invention also relates to a method for the manufacture of a hydraulic binder.

The corresponding method advantageously comprises the following steps:

a step of supplying a clinker, for example in the form of a step of manufacturing this clinker,

- A step of grinding said clinker to obtain said hydraulic binder.

The clinker manufacturing step includes for example the following steps:

- the preparation of a raw, wet or dry,

- raw cooking, for example at high temperature (1450 ° C.) in a rotary calcination oven (cooking process also called "clinkering"), and

- rapid cooling (quenching).

At the exit of the oven, the particle size of the clinker is too coarse for its reactivity to be sufficient; the clinker is in the form of nodules about ten millimeters in diameter.

Grinding the clinker develops the hydraulic properties of the cement and gives it its main rheological properties.

The grinding step is carried out according to the grinding process defined above.

In particular, the grinding step of this clinker is carried out in the presence of said at least one vegetable milling agent according to the characteristics developed above.

Also during this step, different natural or artificial minerals can be added simultaneously to adjust the composition of the mixture.

Gypsum (3 to 5%) can be added to the clinker. This gives artificial Portland cement (CEM I).

Cements with additions (CEM II to V) are obtained by the addition, during the grinding step, of additional mineral elements contained in materials such as:

- blast furnace slag,

- fly ash from power plants,

- limestone fillers (aggregates),

- pozzolans (natural or artificial).

Crushed mineral matter

The present invention also relates to a ground mineral material, resulting from a grinding process according to the invention (collected at the end of the grinding process according to the invention during a collection / recovery step, advantageously at the outlet of the mill).

This ground mineral material then advantageously comprises at least one vegetable milling agent incorporated in a mass proportion of less than 5% of the total mass.

Again, the "total mass" advantageously corresponds to the total mass of the mixed materials (at least mineral matter and vegetable milling agent).

The particles forming the ground mineral material have a surface area greater than or equal to 1000 m ² / L, advantageously from 1000 to 3000 m ² / L.

Application

The possible applications of the present invention are numerous and include all the industries grinding minerals.

We can cite in particular the cement industry, where the effectiveness of the concept has been demonstrated on aluminous cement. The grinding process according to the invention could be extended to the production of Portland cement.

The pigment industries, or those producing metal powders, may also be interested.

Examples

Materials and methods

Raw materials

One plant (soft wheat straw) and two minerals (quartz sand and aluminous cement) are used here.

Wheat straw

Pre-grinding on a knife mill (SM 100 from Retsch) is carried out for wheat straw.

The straw is crushed successively in the grinder with opening grids 6, 4, 2 mm and then three times at 1 mm to avoid excessive heating.

The straw is loaded into the mill manually and the mill rotates at 1500 rpm (rpm).

The straw is then sieved to keep the fraction between 0.25 and 0.5 mm.

This fraction, called “straw” in the rest of the document, is then dried for 24 hours in an oven at 65 ° C.

The aim of this drying is to obtain the same humidity level for all the samples of crushed straw and also to facilitate grinding.

Before use, a humidity measurement is made, it is between 3 and 4%.

Quartz sand

The sand comes from the Société Nouvelle du Littoral, it is sieved between 0.315 and 1 mm to obtain a grain size equivalent to straw.

The sand being very little subject to water intake, no passage in the oven is necessary before its use.

Aluminous cement

As with sand, the cement does not undergo any passage in the oven. It is also sieved between 0.315 and 1 mm.

Grinders used

Two different mills (FAURE® ball mill, Retsch® MM400 mill) were used for grinding cement (in the presence of straw) and for sand (in the presence of straw).

A third mill (stirred ball mill), additional, was used for grinding sand (in the presence of straw).

These three shredders were selected for their very differentiated operation, each having a quantity of product, an energy consumption, and a very distinct operating principle.

The MM400 Retsch mill (vibrating mill) consists of two vibrating bowls, 50 mL each. The grinding medium used is a steel ball 25 mm in diameter. For all the measurements, the vibration frequency used is 20 Hz.

The FAURE ball mill consists of a 3 L tank filled with steel balls (3 kg) of diameters 25, 20 and 15 mm with equal mass; the speed of rotation of the tank is 81 rpm ' ¹ .

Finally, the FEMAG agitated ball mill is composed of a 3 L tank in which are present small diameter steel balls (6 mm) set in motion by an agitator shaft whose speed is 300 rpm ' ¹ , with a quantity of balls representing 60% of the filling rate of the tank (i.e. 7.5 kg).

Grinding protocol

For a given crusher, the different grindings are carried out at constant filling of the grinder.

The densities considered to calculate the volumes of powders are the apparent densities of the powders at 10 pm, ie 1608 kg.rrT ³ for cement, 1696 kg.m ' ³ for sand and 0.221 kg.m' ³ for straw.

The volume of powder inserted in the MM400 mill is 8 mL, in the ball mill is 273 mL and in the agitated ball mill is 740 mL.

In the results presented here, the straw mass represents from 0.7% to 0.16% of the total mass (5% to 1% of the total powder volume).

Throughout the grinding, several samples are taken. The grinding is then stopped when it is considered that the size no longer changes significantly.

Particle size measurement

In order to characterize the powders obtained, particle size analyzes are carried out on a MALVERN® laser granulometer, which gives access to the generated surface.

Laser granulometry is based on the principle of light diffraction. The powder passes in front of a laser beam, the deviation of the beam and the light intensity is then measured.

Using Mie theory (taking into account the refractive and absorption indices of the particles), the powder size distribution (or "PSD" for "particle size distribution") is determined.

From the PSD, the volume surface is determined from the following equation (1):

S _v = Tf = ^ (1) in which

R, represents the radius of the i ^th grain size class, and a, represents the volume fraction of this class.

The calculated surface area is expressed in m ² / L.

The powder is dispersed in ethanol with ultrasound to deagglomerate the powder in suspension in ethanol. The ultrasound is activated for 3 minutes and the measurement is carried out 3 minutes after the end of the ultrasound to remove the air bubbles present.

Power consumption and energy gain

On the three grinders, the electrical power used by the grinder is measured during grinding.

Whatever the product, the ball mill consumes 69 W. The agitated ball mill consumes 909 W for all grindings, except for sand / straw grinding where the mill consumes 885 W. The MM400 mill sees its consumption vary with the mass of product reported.

Knowing the mass of powder, and the relationship between the mass of powder and the electric power, the electric power of each grinding could be calculated (Figure 1).

Knowing the grinding time and the mass of mineral introduced, the mass electric consumption is calculated for each point of the grinding kinetics.

This mass electric consumption is then calculated at fixed volume surfaces.

This value indicates the amount of energy expended to produce 1 kg of mineral at the desired volume surface.

Interpolation of the evolution of the volume surface

The volume area measured with the laser granulometer is plotted as a function of the mass mineral energy for each grinding.

This evolution is then interpolated by the following function (2):

S _v = 5 _r (l-aexp (- ^)) (2) with Sf, a and τ, the parameters of the function determined from the interpolation,

Sv the volume surface, and

E _m the mass energy of grinding.

In the following, it is from these interpolations that all the energy and surface gains are calculated.

Calculation of differences in energy consumption

From the interpolations (see the previous paragraph) of the evolution of the volume surface with mass energy, it is possible to calculate a difference in energy consumption to reach a given surface.

This difference D (in%) is calculated as follows by means of the following formula (3):

^ adjuvant \ ^ur _c'P cc \ NCC \ ^ v) zq \ - yl ^ur (q \ \ ° / adjuvantç wherein ^ ^ _es t | ^ gj _{ner e} mass of grinding in the presence of the grinding agent,

E ^ ^ur (S _v ) is the mass energy of grinding in the absence of the milling agent.

A negative difference will therefore mean that less energy has been spent on reaching the same surface with grinding aid. This therefore corresponds to a reduction in energy consumption thanks to the addition of plant biomass.

Results

Evolution of the volume surface

An example of evolution of the volume surface as a function of the grinding mass energy is presented in FIGS. 2 and 3.

The kinetics are interpolated with the function (2) described above.

As can be seen, the fineness of the powder increases during grinding and then stabilizes at a maximum value.

The addition of straw makes it possible, from a certain fineness, to obtain this fineness more quickly (and therefore to save energy) but also to increase the maximum achievable surface area.

Energy consumption difference

Figures 4 and 5 show the differences in energy consumption for grinding cement and sand, by adding straw up to 0.7% by mass on the different grinders used for different volume surfaces (1400 m ² / L corresponds to a d50 of 10pm and 2600 m ² / L corresponds to a d50 of 4.3pm).

The energy gain, which varies between 15% (grinding cement / straw at 1400 m ² / L on ball mill) and 31% (grinding sand / straw at 2600 m ² / L on FEMAG mill), is dependent on the material and the grinder used but overall increases with increasing fineness.

Smaller amounts of straw were also tested for cement / straw crushings on the MM400 mill (Figure 6). These results show that it is possible to reduce the amount of straw and still have significant energy savings. It seems that the best results are obtained for 0.57% of straw.

Increase in maximum area

A low straw rate increases the volume area reached for a fixed energy expenditure and even increases the maximum area achievable.

Figures 7 and 8 show the volume area gains observed at different energy costs for grinding in the presence of a biomass content of 0.7%.

It can be seen that, overall, the surface gain increases during grinding. It varies between 4% at the start of grinding (at 1000 kWh / t at a d50 of 13 pm, i.e. an area of 2200 m ² / L for sand) up to 30% at the end of grinding (at 8000 kWh / t at a d50 of 3.5 pm, i.e. an area of 2730 m ² / L for the sand).

Conclusion

Adding small amounts of straw to the cement and sand is responsible for improving the grindability of the mineral, thereby reducing energy costs and increasing the final surface area.

These effects are observable on two different minerals with contrasting chemical properties. This allows the concept to be extended to other materials.

In addition, the process according to the invention has proven its effectiveness on mills with very different functions.

The very small amount of straw added is an advantage since it has very little effect on the properties of the final powder, as does a traditional grinding aid.

The results presented here highlight the role of grinding agent (or grinding aid) that straw plays for grinding sand and aluminous cement on different grinders.

Furthermore, these results can be fully extrapolated to other lignocellulosic biomasses (some tests have also been carried out on pure cellulose) as well as to other minerals.

Claims (12)

1. Method for grinding a mineral material, which method comprises: - a step of supplying said mineral material to be ground and at least one milling agent, a step of grinding said mineral material, in the presence of said at least one milling agent, characterized in that said at least one milling agent is chosen from vegetable organic materials, to form a vegetable milling agent, and in that that during said grinding step, said at least one vegetable milling agent is mixed with said mineral material in a mass proportion of less than 5% of the total mass. 2. Grinding method according to claim 1, characterized in that said at least one vegetable milling agent is incorporated in a mass proportion less than or equal to 1%, advantageously from 0.1% to 1%, preferably from 0, 5% to 1%, of the total mass. 3. Grinding method according to any one of claims 1 or 2, characterized in that said at least one vegetable milling agent is chosen from vegetable organic materials comprising at least one of the following components: lignin, cellulose , a parietal polysaccharide. 4. Grinding method according to any one of claims 1 to 3, characterized in that said at least one vegetable milling agent is chosen from lignocellulosic materials, advantageously unrefined lignocellulosic materials, more preferably: - a by-product of the logging or wood industry, - a co-product from agriculture or the food industry, - a perennial plant, - an alga or a by-product from the exploitation of algae. 5. Grinding method according to any one of claims 1 to 4, characterized in that, before said grinding step, said at least one vegetable milling agent has a size less than 1 cm, preferably less than 0.5 cm , or even less than 1 mm. 6. Grinding process according to any one of claims 1 to 5, characterized in that said mineral material to be ground is chosen from hydraulic binders "clinkers, rare earths, silica derivatives and metalliferous ores. 7. Grinding method according to any one of claims 1 to 6, characterized in that said grinding method comprises an operation of mixing said at least one milling agent with said mineral material, which mixing operation is carried out: ~ at the start of said grinding step, or »during said grinding step. 8. Grinding method according to any one of claims 1 to 7, characterized in that, before the grinding step, said mineral material to be ground has a size less than or equal to 1 cm, preferably less than or equal to 0.5 cm »Even less than or equal to 1 mm. 9. Grinding method according to any one of claims 1 to 8, characterized in that the grinding step is carried out dry, advantageously by means of a grinder with grinding media. 10. Process for the manufacture of a hydraulic binder, which process comprises r - a step of making a dicker. • a grinding step of said clinker to obtain said hydraulic binder, characterized in that said grinding step is carried out according to the grinding process defined in any one of claims 1 to 9. 11. Use of vegetable organic materials as a vegetable grinding agent in a process for grinding a mineral material, which vegetable grinding agent is incorporated in a mass proportion of less than 5% of the total mass. 12. Ground mineral material, resulting from a grinding process according to any one of claims 1 to 9, which ground mineral material comprises at least one vegetable milling agent incorporated in a mass proportion of less than 5% of the total mass, and which ground mineral material has a surface area greater than or equal to 1090 m ^a / L, advantageously from 1000 to 3000 m * / L ·