PROCESS FOR PRODUCTION OF NATURAL HYDRAULIC LIME

The present invention relates generally to a process for production of natural hydraulic lime. More in particular, the invention concerns a production process, suitable to give marls, marly limestones and their mixtures the shape and compactness suit for undergoing a cooking process in traditional layers furnaces for producing hydraulic lime, or equicurrent or countercurrent vertical furnaces .

The lime is a material used mainly in construction building sector, but also in other uses, known since the ancient times: the Romans and the Phoenicians had learned to use lime as a material for buildings construction, mixed with sand to form mortar. Vitruvius, in his work "De architectura" , describes its production starting from white stones, cooked in special ovens, to give rise to strongly hygroscopic caustic lime (calcium oxide, CaO) .

The caustic lime was then turned off (calcium hydroxide, Ca(OH) ₂ ), throwing it in special tanks filled with water. Initially, the lime was used in the form of air lime, i.e. lime able to harden only when in contact with

air.

Subsequently, the caustic lime was mixed with pieces of cooked clay (crockery, bricks, etc.) or with pozzuolana, a sand rich in silica, so as to alter the characteristics of strength and water resistance, in order to allow its hardening also in environments outside the contact with air, typically under water: air mortars and lime-based hydraulic limes were thus born. The raw material for the production of lime is limestone, a sedimentary rock rich in calcium carbonate (CaCO ₃ ) extracted from apposite quarries. Such a material, coarsely crushed, is introduced into special ovens or furnaces, where it is gradually heated up to 800-1000 ⁰ C so as to exit from the bottom of the furnace .

At this phase, a chemical calcination reaction occurs, which leads to the release of carbon dioxide and the production of calcium or caustic lime. After cooking, the stone fragments reduce their weight by about 40%, due to the atoms of carbon and oxygen lost, and take a porous consistency, typical of the caustic lime, which is sold as it is or reduced to powder for various uses, such as treatment of water (to reduce acidity) , purification (to remove

phosphates and other impurities) , paper manufacture (to dissolve wood fibres) .

Lime is also used as bleach and whitener, as disinfectant for environments, as correction of acid soils in agriculture and as desiccant and absorber of carbon dioxide in chemistry.

In any case, the caustic lime is preserved in perfectly airtight containers because of its high hygroscopicity . The final lime, or slaked lime, is obtained through a turning off reaction of the material, resulting in a violent release of heat and the weathering of the stone due to the expansive effect of the transformation from oxide to calcium hydroxide. The slaked lime is used as construction material (joined to sand) , as raw material for plasters (mixed with cement) , in leathers tanning, in petrochemical industry to produce additives for lubricants, for producing calcium stearate, for neutralizing and absorbing pollutants, for treating water used in food industry, for correcting acidity of soils, for preparing salts of calcium and magnesium in pharmaceutical industry, as medicament in odontologist and as a fungicide in agriculture. Today, as then, after centuries from the first use,

the lime is not only a basic element in building, but its applications have extended to many fields such as, for example, steel industry, refineries, agriculture and ecology industry, just to mention some of the main sectors .

The hydraulic lime, in particular, is, as the name suggests, a hydraulic binder and is therefore prepared to resist to water action,- it can be natural or artificial . Currently, artificial hydraulic lime is obtained starting from air lime, as said able to harden when in contact with air, and mixing it with potentially hydraulic materials, such as pozzuolana or blast furnace slag (loppa) . Natural hydraulic lime, however, is produced from impure calcareous sedimentary rocks, known as marl, typically composed by a predominantly clayish fraction and a carbonate fraction, generally given by calcium carbonate (calcite) or magnesium carbonate (dolomite) .

The marl comes from muddy sediments, of predominantly marine origin, produced in absence of current, in which the clayish component lays down for slow settling of particles of clay. The raw material of natural hydraulic lime (limestone

and clay up to 20%) is crushed and cooked at a temperature of about 1100 ⁰ C.

Decomposition of limestone (CaCO ₃ ) in lime (CaO) and carbon dioxide (CO ₂ ) , as well as clay decomposition, is achieved at around 900 ⁰ C, while the products of clay decomposition bind with lime (CaO) , giving rise to aluminates and silicates, at around 1000-1100 ⁰ C. The residual caustic lime undergoes the same turning off reaction which occurs for air lime in order to produce slaked lime or calcium hydroxide (Ca(OH) ₂ ) .

This type of production of the natural hydraulic lime comes from the fact that in 1850 the English John Smeaton discovered that some calcareous marls contain inside siliceous or clayish impurities and, therefore, these minerals could obtain the natural hydraulic lime, as already containing the features of hydraulicity, without the addition of external parts. However, nowadays the marls from which natural hydraulic lime is obtained, for simple cooking, are increasingly running out for the burnout of historical bodies or for the impossibility to cultivate such bodies for environmental reasons. Furthermore, materials in alternative to marls and which, in theory, possess the appropriate chemical feature for the production of natural hydraulic lime,

are often unsuitable to cooking due to physical reasons (inconsistency, impossibility to obtain appropriate sizes) , and end by being treated as waste materials. As part of the requirements mentioned above, the present invention, therefore, aims to remedy the aforesaid drawbacks .

In particular, a purpose of the invention is to provide a process for production of natural hydraulic lime, which allows to give marls, marly limestones and mixtures thereof, the shape and compactness suitable to undergo a process of cooking in traditional type layers furnaces for producing hydraulic lime or in vertical kilns. Another purpose of the present invention is to provide a process for production of natural hydraulic lime which allows to use materials which, otherwise, would be used in less valuable applications, recovering sub- screens of other productions, with consequent effective savings of natural resources.

Further purpose of the invention is to develop a process for production of natural hydraulic lime which uses marls mixtures and marly limestones of various types, in order to reproduce the chemical composition and the physical performance of the limes

used in the ancient times and whose bodies are one and for all exhausted, with obvious utility in various application fields, especially in architectural restoration. These purposes are achieved by a process for production of natural hydraulic lime according to the appended claim 1, to which they refer for the sake of brevity. The additional attached dependent claims relate to other more specific details of the process of the invention.

In advantageous way, contrary to processes used in the last century and nowadays, which tend to produce artificial hydraulic limes, combining limestones, such as air limes, and clayish or potentially hydraulic materials, such as pozzuolana or blast furnace slag in general (loppa) , the process according to the present invention does not aim to change the chemical composition of the material, but only the physical shape, in order to optimize the cooking thereof .

In particular, by means of the production process described in the present invention, through the use of selected raw materials and appropriate equipment, an artificially produced marl, suitable to be cooked

in ordinary lime kilns for producing natural hydraulic lime, is obtained.

Further purposes and advantages of the present invention will more greatly result from the description that follows, related to an illustrative, but not limited, embodiment of the process for production of natural hydraulic lime, according to the invention, with the aid of the attached drawing (figure 1) , also provided for pure way of explanatory but not limiting example, which shows an informal block diagram of the various phases of production of the natural hydraulic lime belonging to the process object of the present invention. The raw materials exclusively consist of natural origin materials, even though not necessarily in their original state, provided that the change of such a state results essentially from working processes, such as washing, crushing, screening, etc. By way of example : - any chemical nature and petrography rocks, such as limestones, limestone marls, marly limestones, clayish marls, marly clays, pozzuolana effect artificial materials; quarry or river sands ; - clays and limes;

products of working of previous materials, such as coarse crushed aggregates, washing limes, crushing powders, etc., provided they're not identified as waste, can be used as raw materials.

Sub- screens of chemical and/or iron and steel industries, products of petrochemical origin, waste of industrial origin, coming from processes of casting, working, smokes damp, burning, etc., are, however, excluded from the raw materials used in the production process according to the present invention.

According to the invention, the process provides that starting from the selected raw materials, the composition of mixtures of a series of components is carried out, making their weight dosage in order to obtain compounds with a hydraulic index substantially included between 0,2 and 0,55.

The hydraulic index, also known as hydraulicity modulus, allows to know in which ratio the different percentages of components are and is calculated as the ratio between the content of "acid" oxides (CaO and MgO) and "alkaline" oxides (Fe ₂ O ₃ , Al ₂ O ₃ , SiO ₂ ) , expressed as a percentage. The mixture may, for example, reproduce a particular

kind of no longer available marl in order to replicate a type of lime necessary, for example, for a restoration operation.

In other cases, the mixture used a body of material which cannot used as it is owing to particular lying conditions or physical state, for example, because too crushed, altered by weather agents or collapsed or other. In general, however, the mixture serves to produce a natural hydraulic lime with certain physical- mechanical properties .

After selection and before mixing, the raw materials are properly stored or stocked. In the case at issue, the process of production of natural hydraulic lime comprises drying in a rotating drum the raw materials used, prior to their mixing. After the mixing of raw materials or simultaneously with it, the reduction of raw materials into powders, with a fineness of 90 microns, occurs in a spheres mill.

The mixture of fine powders is, then, transformed, using a rotating drum or granulator, into granules of raw flour, having sizes equal to about 25-30 mm, which are subsequently dried, if necessary, on a conveyor belt made of net.

Alternatively, it is also possible to opt for the formation of briquettes, by using a rollers compacting press.

In any case, the powder mixture is transformed into product pieces (granules or briquettes) of marl, having mass and sizes so that they can be cooked in normal lime kilns.

At this point, the stage of calcination follows, i.e. the cooking of marl granules or briquettes in a layers vertical static furnace, with blowing of combustion air from the bottom, or in a double-vat equicurrent static regenerative furnace, or in a countercurrent static vertical furnace, using coal, petrolcoke, gas, combustible oil or wood sawings as fuel.

In any case, any type of furnace is used, the maximum cooking temperature is kept at values included between 900 ⁰ C and 1100 ⁰ C, lower than the sintering temperature . A crushing phase of the product, by means of a hammers mill which disaggregate the granules or briquettes, and the turning off of the intermediate product (caustic natural hydraulic lime) , by means of a single-stage or multistage hydrating device, adding a predefined and controlled amount of water, so as

to, however, turning off the magnesium and/or calcium oxide formed during cooking, are then carried out. The seasoning of the intermediate product then occurs, consisting in the seasoning of the slaked natural hydraulic lime, into a proper environment with adjusted and controlled humidity, for a time which allows to reach the volume stability, e.g. about 1 month . The following operation provides the grinding of the intermediate product in a spheres mill having a closed path with separator, or in a track or rollers mill, so as to obtain powders with finenesses corresponding with standard EN 459-1:2002, for example not superior to 90 microns. Finally, the storage of the finished product (natural hydraulic lime) in special silos is carried out. Regardless the type of mixture used, as far as physical, chemical and mineralogical features are concerned, the finished product is to all intents and purposes a natural hydraulic lime (NHL or "Natural Hydraulic Lime"), according to the definitions provided in the current Italian and European standards, and, depending on its characteristics, will be classified as NHL2, NHL3.5 or NHL5 and provided with CE marking, in accordance with the

relative legislation of relevance.

From the description just made, therefore, the salient features of the process for production of natural hydraulic lime, which is the object of the present invention, as well the resulting benefits, are clear.

It is, finally, clear that many variations can be made to the production process in question, without for this reason going out of the novelty principles inherent to the inventive idea, as it is clear that in the practical implementation of the invention, materials, shapes and sizes of the illustrated details could be any depending on the needs and the same could be replaced with others technically equivalent.