WO2018113894A1

WO2018113894A1 - Partially carbonatated slaked lime

Info

Publication number: WO2018113894A1
Application number: PCT/EP2016/002137
Authority: WO
Inventors: Stephan Dietz; Alexandra REINHARD; Marion WIRTH; Fabio Rencurosi; Frederic Delvaux; Jonathan ROCCA
Original assignee: Knauf Gips Kg
Priority date: 2016-12-20
Filing date: 2016-12-20
Publication date: 2018-06-28

Abstract

A process for the production of partially carbonatated slaked lime, comprising the steps of -providing slaked lime -providing a material comprising at least 1% by weight of anhydrite III -mixing said slaked lime and said material to obtain a slaked lime/material -mixture -partially carbonatating said slaked lime.

Description

PARTIALLY CARBONATATED SLAKED LIME

The invention relates to a process for the production of partially carbonatated slaked lime as well as the partially carbonatated slaked lime and its use.

Calcined calcium sulfate, especially plaster is a material of diverse industrial use.

Slaked lime (Ca(OH)₂) is one common ingredient in the production of plaster and related building materials.

In the past, production of plaster has been facilitated with progress in the fields of e.g., additives, especially those that enhanced machinery processing, grain size control and setting retarders. One major problem in the processing of plasters has always been to ensure steady characteristics of the starting material, especially in terms of setting times. These setting times during the production and processing of plaster depend e.g., on the age of the starting material and its time of exposure to air. DE 27 12 190 B1 discloses that the use of partially carbonatated slaked lime improves the maintenance of setting times in cases of extended storage times as well as reduces the amount of setting retarders necessary to exactly control user friendly setting times. This solution to the problem of setting time control, however, created problems with regard to processing of the partially carbonatated slaked lime. Carbonatation of slaked lime produces H₂0 molecules which as a liquid interact with the slaked lime or as a gas interact with the surrounding gas phase. One problem is the change in flow behaviour of the partially carbonatated slaked lime as deposits are formed from released water molecules and slaked lime. Gaseous H₂0 interferes with and complicates dedusting processes of the surrounding gas phase in the process of carbonatating slaked lime. It is the object of the present invention to provide a process for the production of partially carbonatated slaked lime which overcomes at least some of the disadvantages of prior art processes.

This object is solved by the process of the present invention. The process of the present invention is a process for the production of partially carbonatated slaked lime, comprising the steps of providing slaked lime

providing a material comprising at least 1 % by weight of anhydrite III - mixing said slaked lime and said material to obtain a slaked

lime/material mixture

partially carbonatating said slaked lime.

Provided is thus a process for the production of slaked lime that is partially carbonatated. This process comprises the steps of providing slaked lime and providing a material that comprises at least 1 % by weight anhydrite III. The provided slaked lime and the provided material are mixed, and a slaked lime/material mixture is thus obtained. The slaked lime is partially carbonatated. The term calcium sulfate material comprises all calcium sulfate materials independent of their hydration status, e.g. gypsum (CaS0₄ ' 2 H₂0), hemihydrate (CaS0₄ ^■ ½ H₂0, a- and/or β-form), and anhydrite (CaS0₄), and independent of whether their origin is natural or synthetic. As the person skilled in the art readily appreciates, anhydrite III refers to a substance of the formula CaS0₄ · x H₂O wherein 0≤ x < 0.5. It is typically obtained from calcium sulfate hemihydrate (CaS0₄ · 0.5 H₂0) at temperatures > 300 °C. As the material to be provided according to the present invention comprises at least 1 % by weight anhydrite III, impure stucco but also materials comprising anhydrite III contents of > 30% by weight or even as pure anhydrite III as available may be provided according to the present invention. Preferably, the anhydrite III content of the material is between 1% and 40% by weight; more preferably it is between 5% and 25% by weight. Especially, good results are achieved if the anhydrite content is at least 10% by weight.

As the person skilled in the art appreciates, stucco is a material that almost always contains certain amounts of anhydrite III. It typically comprises predominantly calcium sulfate hemihydrate (CaS0₄ · 0.5 H₂0) and anhydrite III (CaS0₄ ^■ x H₂0 wherein 0 < x < 0.5), the latter representing over-calcined stucco unintendedly produced. In most cases, this anhydrite III is considered an impurity. However, with respect to this invention this impurity is considered as being beneficial.

As the person skilled in the art readily appreciates, slaked lime refers to a substance of the formula Ca(OH)2. The partial carbonatation of slaked lime according to the present invention comprises the reaction of a part of the provided amount of slaked lime with carbon dioxide (C0₂). The reaction may typically be described by the equation:

Ca(OH)₂ + C0₂→ CaC0₃ + H₂0

In one embodiment of the invention the material provided in the process comprises at least 8% by weight anhydrite III, preferably at least 10% by weight anhydrite III, most preferably at least 20% by weight anhydrite III.

In another embodiment of the invention the slaked lime/material mixture comprises slaked lime and the material in a ratio of from 4:1 to 1 :4 by weight, preferably 2:1 to 1 :2 by weight, more preferably 1.5:1 to 1 :1.5 by weight, most preferably 1.1 :1 to 1 :1.1 by weight.

In a preferred embodiment of the invention, the slaked lime/material mixture comprises slaked lime and the material in a ratio of 1 :1 by weight. ln one embodiment of the invention the slaked lime/material mixture comprises slaked lime and anhydrite III in a ratio of from 80:1 to 1 :4 by weight, preferably 40:1 to 1 :2 by weight, more preferably 30:1 to 1 :1.5 by weight, most preferably 22:1 to 1 :1.1 by weight. In a preferred embodiment of the invention the slaked lime/material mixture comprises slaked lime and anhydrite III in a ratio of from 20:1 to 1 :1 by weight or 20:1 to 4:1 by weight.

In a further embodiment of the invention, partially carbonatating is terminated when 1% to 20% by weight slaked lime, preferably 5% to 20% by weight slaked lime, more preferably 10% to 15% by weight slaked lime based on the initial amount of slaked lime, are carbonatated.

In yet another embodiment of the invention said mixing and said partially carbonatating are performed simultaneously.

There are preferred embodiments of the process of the invention wherein said partially carbonatating is conducted after said mixing, with a delay of at least 1 minute, 5 minutes, 10 minutes or 60 minutes. In a further embodiment of the invention said partially carbonatating comprises providing a source of C0₂.

In one embodiment of the present invention said source of C0₂ is technically pure C0₂ or C0₂ gas with a purity of at least 3% by weight or at least 50% by weight, 90% by weight or 99% by weight.

If the purity of the used C0₂ source is lower it is useful to enhance the process temperature in order to achieve sufficient carbonatation in spite of low C0₂ concentrations. Preferably, the process temperature is for example above 30°C.

In another embodiment of the present invention said providing a source of C0₂ comprises injecting said source of C0₂ into a container comprising said slaked lime/plaster of Paris mixture. In yet another embodiment of the present invention excess C0₂ is withdrawn from the process.

Withdrawal of excess C0₂ is typically conducted when the partial carbonatation has reached a desired degree. Withdrawal of excess C0₂ may in this embodiment of the present invention serve to terminate the carbonatation of slaked lime. In alternative embodiments of the present invention, withdrawal of excess C0₂ may be initiated prior to termination of the carbonatation of slaked lime or may be pursued after termination of the carbonatation of slaked lime.

In a preferred embodiment of the invention, the grain size of slaked lime is such that less than 0.2% by weight have a grain size larger than 1 mm, preferably less than 0.2% by weight is larger than 0,5 mm, most preferably less than 0.2% by weight is larger than 0.2 mm.

Grain sizes are generally, unless states otherwise, determined in analogy to DIN EN 13279-2:2014, paragraph 4.1.3. For example, slaked lime is sieved on a 200 μιη sieve, the slaked lime remaining on the sieve corresponds to the amount of slaked lime with a grain size larger than 0.2 mm.

In a more preferred embodiment of the invention the material which is mixed with slaked lime is calcium sulfate hemihydrate originating from a natural or a synthetic source, e.g. from FGD gypsum, phosphogypsum, citrogypsum, fluorogypsum or titanogypsum, or a calcium sulfate material of a higher calcination grade.

Plaster of Paris may as well be used as the material according to the present invention. An embodiment of the invention is the partially carbonatated slaked lime, obtainable by the process according to the invention. Provided is thus the partially carbonatated slaked lime which is obtained from a process comprising the steps of providing slaked lime

providing a material comprising at least 1% by weight of anhydrite III mixing said slaked lime and said material to obtain a slaked lime/material mixture

partially carbonatating said slaked lime. Another embodiment of the invention is the use of the partially carbonatated slaked lime that is obtainable by the process of the present invention for the production of a calcium sulfate comprising product.

Provided is thus partially carbonatated slaked lime which is obtainable by conducting the process of the present invention and its use in the production of products comprising calcium sulfate. A typical use includes the use as an educt for the production of plaster.

One embodiment of the invention is the use of the partially carbonatated slaked lime for the production of a calcium sulfate comprising product wherein this product comprises calcium sulfate hemihydrate, especially stucco.

A preferred embodiment of the invention is gypsum-lime plaster comprising the partially carbonatated slaked lime that is obtainable by the process of the present invention, stucco and a retarder.

A more preferred embodiment of the invention is gypsum-lime plaster wherein the amount of retarder needed to achieve a predetermined setting time is reduced compared to a gypsum-lime plaster of identical composition but without the partially carbonatated slaked lime.

The process according to the present invention is suitable to be conducted in various types of containers as long as sufficient mixing of slaked lime, material comprising at least 1% by weight anhydrite III and provided C0₂ is ensured. Preferred containers comprise Eirich mixers, Lodige type mixers and screw-conveyors such as twin-shaft screw-conveyors which is a preferred container for the process according to the present invention. While improved characteristics such as setting times in the processing of plasters have been achieved in prior art, the processes to solve setting time control problems have created subsequent problems. Among these problems is the emission of liquid or gaseous H₂0 which interacts with the slaked lime or the surrounding gas phase, respectively.

Both the interaction with slaked lime and the surrounding gas phase are disadvantageous: an uncontrolled reaction with the intended product is naturally not desirable, particularly the flow behaviour of the partially carbonatated slaked lime is adversely affected due to the adsorption of H₂0 at the Ca(OH)₂ particle surfaces which results in a cohesive flow behavior of the bulk material; an H₂0 enriched gas phase results in greater difficulties with regard to dedusting the gas phase; and a significant amount of scavenging air is needed to remove water from the production containers.

Surprisingly, it has been found that the addition of a material comprising at least 1% by weight anhydrite III results in an improved process for the production of partially carbonatated slaked lime which exhibits less H₂0 emission and consequently less adverse effects on the partially carbonatated slaked lime or the surrounding gas phase without impairing the quality of the desired product.

While anhydrite III has been considered an impurity to avoid with regard to processes related to calcium sulfate materials, e.g. plaster, its use according to the present invention has surprisingly been found to provide great benefits.

When the material comprising at least 1 % by weight anhydrite III is provided according to the present invention, there is no adverse effect with regard to flow behavior observed. Since the partially carbonatated material does not clump or only to a very little degree production downtimes due to the necessity of cleaning the production devices are strongly reduced. Further, dedusting can be strongly reduced and scavenging air can be omitted as emitted water is bound by anhydrite III. Moreover, the temperature increase during the exothermic reaction is reduced as the total amount of reagents is increased. All these advantages are not borne by an impairment of the product characteristics as the partially carbonatated slaked lime obtained by the process according to the invention features a similar need of setting retarders as conventionally partially carbonatated slaked lime.

All references cited herein are incorporated by reference to the full extent to which the incorporation is not inconsistent with the express teachings herein.

The invention is illustrated by the following examples which do not restrict by any means the scope of protection conferred by the claims.

Example 1 : Preparation of carbonatated slaked lime

Experiments were performed with mixtures of 5 kg stucco comprising about 9.8% by weight of anhydrite III calcined in a rotary kiln and 5 kg slaked lime in a 50 L Lodige type mixer. The material was filled in and mixed prior to injection of technically pure C0₂ (samples a) - c)), or the mixer was flooded with technically pure ^~C0₂ prior to provision of stucco and slaked lime and mixing (samples d) - e)). An overview of experimental conditions is given in Table 1. Table 1 : Preparation of partially carbonatated slaked lime

Example 2: Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) was used to determine the ratio of carbonatafion obtained by the procedures according to Example 1. 20 - 80 mg of samples a) to e) as well as a reference sample without carbonatated slaked lime were finely comminuted and measured on a TGA Q 500 TA Instruments with the following, parameters:

T_end: 950 °C

Heating rate: 30 °C/min

tisotherm, 950 °C- 3 ITlin

TGA results are listed in Table 2.

Example 3: Determination of setting times

Setting times were determined as follows.

The. start of the setting process (initial setting) is defined as and determined according to DIN EN 13279-2:2014, paragraph 4.4.2. Briefly, a sample is put in a vicat ring and a cone is dropped on the sample. Initial setting is reached, when the cone enters into the sample in a depth of 22 +/- 2 mm.

The end of the setting process (final setting) is defined as and determined according to the following procedure. Once initial setting has been reached, the vicat ring is removed from the glass plate and repositioned on the glass plate upside down. The stirring bar is loaded with 1 kg and determination of the intrusion depth of the cone is conducted as for initial setting.

Final setting is reached when an intrusion depth of the cone is 15 mm +/- 2 mm as for determination of initial setting, a second intrusion of the cone to the same depth is needed to confirm final setting.

Setting times are given in Table 2. It is shown that setting is retarded in samples with carbonatated slaked lime compared to the non-carbonatated reference. Table 2: Analysis of partially carbonatated slaked lime (small scale)

Example 4: Large scale preparation of carbonatated slaked lime

75 kg Ca(OH)₂ and 90 kg stucco comprising about 11.2% by weight anhydrite III were mixed in a 200 L Eirich mixer. Technically pure C0₂ was injected for 23 min at a rate of 30 L niin. Samples were taken after f) 5 min, g) 10 min, h) 15 min and i) 23 min.

Example 5: Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TGA) was used to determine the ratio of carbonatation obtained by the procedures according to Example 4. Measurements of samples f) - i) as well as the non-carbonatated reference were conducted as in Example 2. Results are given in Table 3. The amount of carbonatated slaked lime increases with carbonatation time from 5 min to 23 min. Table 3: Analysis of partially carbonatated slaked lime (large scale)

Example 6: Setting retardation

The carbonatated slaked lime/ stucco mixture was added in an amount of 4% by weight to a plaster compound for manual application. As a reference, 2% by weight slaked lime and 2% by weight stucco plaster were added to the same plaster compound. The setting retarder tartaric acid was present in amounts of 0.11 % by weight (reference 1 ) or 0.08% by weight (reference 2 and samples f) - i)). All samples and references exhibited a water/plaster ratio of 0.56.

Determined were the start and end times of setting (initial and final setting). These parameters were determined as follows.

Results are given in Table 4. Mixtures according to the present invention exhibit similar values to those of reference 1 although the amount of setting retarder is reduced by >25%. Table 4: Parameters of slaked lime/mounting plaster mixtures

Example 7: Industrial scale carbonatation of slaked lime

On an industrial scale, 3 1 of a lightweight plaster compound for machine application with 0.10% by weight of the setting retarder tartaric acid and 1.47% by weight slaked lime as a reference and 3 t of the same lightweight plaster compound, however, with only 0.084% by weight tartaric acid and 2.94% by weight carbonatated slaked lime/stucco mixture were prepared. Apart from the mentioned differences the composition of the lightweight plaster compound was identical, the stucco content of the material mixture being compensated for. Determined parameters for the two samples were inital and final setting. Results are given in Table 5. It is shown that the mixture according to the present invention exhibits comparable values as the state of the art mixture, although the amount of setting retarder is reduced by 15%. Table 5: Parameters of industrial scale carbonatated slaked lime products

Claims

A process for the production of partially carbonatated slaked lime, comprising the steps of

providing slaked lime

partially carbonatating said slaked lime.

The process according to claim 1 , wherein said material comprises at least 5% by weight, preferably at least 10% by weight anhydrite III.

The process according to any one of claims 1 or 2, wherein said slaked lime/material mixture comprises slaked lime and the material in a ratio of from 4:1 to 1 :4 by weight, preferably 2:1 to 1 :2 by weight, more preferably 1.5:1 to 1 :1.5 by weight, most preferably 1.1 :1 to 1 :1.1 by weight.

The process according to any one of claims 1 to 3, wherein said partially carbonatating is terminated when 1 % to 20% by weight slaked lime, preferably 5% to 20% by weight slaked lime, more preferably 10% to 15% by weight slaked lime based on the initial amount of slaked lime, are carbonatated.

The process according to any one of claims 1 to 4, wherein said mixing and said partially carbonatating are performed simultaneously.

The process according to any one of claims 1 to 5, wherein said partially carbonatating comprises providing a source of C0₂.

7. The process according to claim 6, wherein said source of CO₂ is technically pure C0₂ or C0₂ gas with a purity of at least 50% by weight, 90% by weight or 99% by weight.

The process according to any one of claims 6 to 7, wherein said providing a source of C0₂ comprises injecting said source of C0₂ into a container comprising said slaked lime/material mixture.

The process according to any one of claims 6 to 8, wherein excess C0₂ is withdrawn from the process.

The process according to any one of claims 1 to 9, wherein the grain size of said slaked lime is such that less than 0.2% by weight of said slaked lime have a grain size larger than 1 mm, preferably larger than 0.5 mm, more preferably larger than 0.2 mm.

1 1. The process according to any one of claims 1 to 10, wherein said material is calcium sulfate hemihydrate originating from a natural or a synthetic source, e.g. from FGD gypsum, phosphogypsum, citrogypsum, fluorogypsum or titanogypsum, or a calcium sulfate material of a higher calcination grade.

12. Partially carbonatated slaked lime, obtainable by the process according to any one of claims 1 to 11.

Use of the partially carbonatated slaked lime according to claim 12 for the production of a calcium sulfate comprising product.

Use of the partially carbonatated slaked lime according to claim 13, wherein the calcium sulfate comprising product comprises calcium sulfate hemihydrate, especially stucco.

Gypsum-lime plaster comprising the partially carbonatated slaked lime according to claim 12, calcium sulfate hemihydrate, especially stucco, and a retarder.