DK148794B - TOUR CEMENT COMPOSITION - Google Patents

Description

148794

The present invention relates to a cement composition packaged for shipment to the final consumer and containing cement and 0.01 to 10% by weight of ferrous sulfate.

It has been known for a long time that cement can contain chromium compounds which, when mixed with water, appear as Cr dissolved in the water and that this dissolved chromate is contraindicated from a health point of view, since it can cause people who work with the cement-water mixture get eczema when the mixture comes into contact with their skin. According to Fregert, Professional. In dermatosis 21, 171 (1973), the contraindicated concentration is 1-2 µg of chromate. g of cement and above.

It is known to reduce the concentration of dissolved chromate in a cement-water mixture and in wet concrete by the addition of ferrous sulfate during the mixing process in which the mixture is prepared. The ferro sulphate's ♦ 6 +3 15 ferro ion reduces Cr to Cr, which is only slightly soluble in cement-water mixtures.

Swiss Patent No. 534,643 discloses a process for the manufacture of cement, with the aim of reducing the content of soluble chromate in the cement to add ferrosulphate in a quantity of 0.1%. According to the Swiss patent, the ferrous sulfate can be added during cement manufacture to the clinker, gypsum or in the cement mill itself, but the patent does not contain information on the state of which the ferrous sulfate is added. It has been found that the stated amount of 0.1% is too low when the ferrous sulfate is to be exposed to the extreme stresses in a cement mill in which the chromium neutralizing ability of the ferrous sulfate will be reduced.

Thus, it is not clear whether the finished cement composition contains the iron sulphate at all in the desired Fe (II) SO 2, x 1 2 O form.

In U.S. Pat. No. 3,425,892, it is recommended that 0.1 to 5% of ferrous sulfate be added to its cement during its manufacturing process for the purpose of retarding the cure rate of the cement, and it is stated that the retarding agent may be added to powdered Portland cement, for example. like dry powder. However, the United States Patent Specification does not state whether the cement has a water-soluble content of 2 148794 and consequently does not relate at all to the reduction of water-soluble chromate in cement mixtures, so that the reducing ability of the sulphate is not affected, nor is there any information as to whether the sulphate is the material available for shipment is still available in the desired FeODSO x x H 2 O form.

From US Patent No. 2,307,270, a building material containing cement and sulfate of divalent iron is known, but this is not a cement having a water-soluble chromate content. The sulphate is added as a substitute for gypsum with the purpose of regulating the cement bonding. Accordingly, the patent does not relate at all to the reduction of water-soluble chromate in cement mixtures, so the reducing ability of the sulfate is not mentioned.

Contact Dermatitis, 1979, pp. 39-42, describes laboratory experiments with the addition of ferrous sulfate to cement for the purpose of reducing the water-soluble content of chromate in cement. The publication discloses partly an experiment in which ferrous sulfate dissolved in water was added to chromate-containing cement and partly an experiment in which 0.35% by weight of crystalline ferrous sulfate was mixed with 10 and 5 g of cement, respectively. Immediately after mixing, 5 ml of water was added and a determination of water-soluble chromate was made in the mixture, but no chromate was found. Since the detection limit was 0.01 µg Cr / ml, it can be concluded that the chromate content had fallen below this limit. The publication also describes an experiment on reducing chromate in cement by means of a stored aqueous solution of pure ferrous sulfate. The ferrous sulfate was dissolved in water at concentrations of 10, 20 and 30% by weight. These ferrous sulfate solutions were stored at room temperature in glass containers with and without lids. These solutions were added daily for 7 days to cement, and it was found that the solutions in the glass containers bearing 30 lids, after 7 days, still retained their chromate-reducing ability, while the solutions stored in the open containers after 5 days had reduced their chromate reducing ability.

It has so far been common to add the chromate neutralizing ferrous sulfate to the cement along with water and aggregate at the construction site itself in the preparation and mixing of the wet concrete. However, adding the ferrous sulfate at the construction site involves a risk that the dosage of the ferrous sulfate will not be correct, for example due to inaccurate weighing or due to the use of a ferrous sulfate solution which has been stored for too long or on a inappropriate way.

In the present invention, there has been provided a cement composition of the kind mentioned initially which, when mixed with water, gives an immeasurable or low concentration of Cr6 + (in this description also called "chromate") dissolved in the water.

The cement composition of the invention is characterized in that the ferrous sulphate is dry and particulate and that the cement contains a contraindicated amount of water-soluble chromate from a health point of view. The ferrous sulphate may be added to the starting material fed to a cement mill at one or more suitable locations which, in the flow direction, lie before a storage vessel. Thus, the whole dosed stream of ferrous sulfate or at least a portion thereof can be added to the starting material before the cement mill, in which the ferrous sulfate during the milling process taking place therein is very thoroughly mixed with the starting material. It is also possible to add the ferrous sulphate or at least a portion thereof to the ground starting material at a location which, in the flow direction, lies after the cement mill and before the storage container, for example at the inlet of a pneumatic conveying device for transporting the ground cement material to the storage container. . The ground cement material and the particulate ferrous sulfate added to the cement mill can be sufficiently mixed by passing the pneumatic conveyor.

Since the ferrous sulfate is dry and particulate and thus free-flowing, it can also be mixed with the cement by passing it together with the cement through a special mixing device of any suitable type. Provided that it is possible to obtain an effective mixing of the components of the cement, the amount of ferrous sulfate added after the cement mill may be somewhat smaller than when the sulfate 35 is added before the cement mill. In that case, one can go that far down

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The FIG. 1 illustrated cement-producing plant has a silo 10 having at the bottom three funnel-shaped outlets 11 and having a filter above the top 12. The silo contains dry free-flowing, particulate ferrous sulfate FS, for example of the type sold by Melchemie BV, Arnhem, The Netherlands. under the trademarks MELSTAR and FERROMEL-20. Appropriate portions of ferrous sulfate may be withdrawn from the outlets 11 by selectively operable vibration devices 13 and collected in corresponding receptacles 14. Dosing devices 15 are disposed at the bottom of receptacles 14 and each of the metering devices may comprise a screw conveyor powered by an electric motor. 15 ', which can be controlled by an electronic control device 16, as will be described in more detail below. Each of the metering devices 15 brings 15 a continuous, controlled, dosed stream of ferrous sulfate to flow through an air lock 17 'and into a pneumatic conveying device 17 comprising a conveyor line 18. The lines 18 are connected to three different mills 19a, 20 and 21. in three cement-producing plants, which are otherwise separated. A starting material comprising 20 cement clinkers C, gypsum G and optionally other components is provided with an inlet 22 for each of the mills. In the drawing, two supply devices 23 and 24 are shown, which are provided with corresponding measuring devices 23 'and 24', which serve to supply a continuous stream of cement clinker and gypsum respectively, and the transport lines 18 open 25 in the inlet 22 so that it is dosed. stream of ferrous sulfate produced by each of the metering devices 15 is added to the starting material at the cement mill inlet 22.

Each of the containers 14 and the associated dosing device 15 form a unit supported by a weighing cell 25. This cell 30 produces weight signals representing the actual total weight of the associated unit. It is to be understood that the rate at which the weight of each of the units is reduced corresponds to the rate at which the ferrous sulfate is supplied to the transport device 17 by means of the corresponding measuring device 15. Therefore, the control device 16 can control the delivery rates of the dosing devices 15 depending on the signals which is received from the weighing cells 25 and by corresponding 6,148,794 signals received from the supply devices 23 and 24 which feed starting material to the cement mills, thereby ensuring that the starting material fed to each of the cement mills 19a, 29 and 21 is continuously added to a desired weight percent. ferrous sulfate. When the 5 amount of ferrous sulfate contained in one or more of the containers 14 has reached a predetermined minimum, which means that the corresponding weighing cell 25 has determined a predetermined minimum weight for the corresponding unit, the control device 16 can establish a connection between the silo 10 and the container 14 and activating the corresponding vibrating unit 13 so that from the silo 10 a new portion of ferrous sulfate is filled into the corresponding container 14.

Apart from the system described above for supplying dosed streams of ferrous sulphate to the respective cement mills, the cement producing plants shown in the drawing are of known types. In the upper part of FIG. 1 shows a two-stage system with two separate mills 19a and 19b connected in series. As mentioned above, the ferrous sulfate can be fed into the inlet 22 of the cement mill 19a through the conduit 18.

However, supply of ferrous sulfate may instead or further be added to the inlet of the last mill 19b through a conveyor line 18 '20 and / or to the mill 19b's outlet through a conveyor line 18 "indicated by dotted lines in Fig. 1.

The milled cement material leaving the cement mill 19a is lifted by a lift 26 and fed to the inlet of the cement mill 19b through a conduit 27. Ventilation air from the cement mills 19a and 19b is passed through the conduits 28 and 29 to an electrostatic precipitator 30 and is then discharged into the atmosphere. Dust material separated from the air by the precipitator 30 is fed to an outlet conveyor 31 which is also connected to the outlet of the cement mill 19b. The finished cement from the outlet conveying device 31 can be transported to a storage silo 33 30 by means of an appropriate mechanical or pneumatic conveying device 34. If desired, milled cement from the mill 19a can be passed around the mill 19b by means of a conveying device 32 if it is desired to operate the plant. with only one mill.

In case the ferrous sulphate is applied to the plant at two or more sites, different ones may be added at different locations. types of chromate reducing agents, including various forms of ferrous sulfate and ferrous sulfate-containing products, such as particulate ferrous sulfate with elisr without property-enhancing particle coatings or coatings. For example, at the inlet 22, ferrous sulfate may be added, and 5 other suitable chemical compounds may be added between the mills 19a and 19b or the mill 19b and the storage silo 33. The ferrous sulfate may instead be added in particulate form with coated particles as described above at the inlet 22, and dried particulate ferrous sulfate with non-coated particles can be added at one or both of the other 10 sites or vice versa.

FIG. 2 shows a cement grinding plant with a single cement mill 20, and parts corresponding to those of FIG. 1 is provided with the same reference numerals. The milled material leaving the cement mill is lifted by the elevator 26 and passed to an air separator 44 through a conveying device 35. The separator separates finer particles from coarser particles. The coarser particles are returned to the inlet of the mill 20 through a return conveyor 36 while air or gas and the finer particles are fed to a cyclone 37. In the cyclone 37, the solid material is separated from the air recycled through a conveyor device 38 while the finer ones particles of the solid material are passed to an air outlet 39 through a conveyor 40. The finished product is conveyed from the product outlet 39 to a suitable storage location, e.g., a silo 33, by means of a pneumatic conveyor 34.

25 During operation, the mills 19-21 are rotated together with the cement material and the grinding balls contained therein. The mechanical energy thus supplied to the mills is converted into heat energy. In order to keep the temperature of the turbines at an acceptable level, usually within a temperature in the range of 120-140 ° C, a coolant, usually coolant, is usually sprayed inside the turbines by suitable spraying devices 43. Consequently, the atmosphere is inside the cement mills very damp.

In a plant of the embodiment shown in FIG. 2, the ferrous sulphate can be fed through the conduit 18 to the conduit 36, which returns coarse material from the separator 44. However, the ferrous sulphate may instead, or further, be conveyed to the transport device 40 through a conduit 41 and / or the mill 20 through a conduit 42 which is indicated by dashed lines.

It will be understood that any of the embodiments of FIG. 1, the cement mills 5 20 and 21 may form part of a grinding plant just like any of the ones in the upper part of FIG. 1 or in FIG. 2 or they may form part of a plant of any other known type. Furthermore, the ferrous sulphate can be applied to the grinding plant in a dry state at any convenient location before the storage container to ensure a thorough mixing with the other cement components.

EXAMPLE 1 In a cement manufacturing plant of the embodiment shown in FIG. 2, fast-curing cement was ground in a cement mill, and a chromate-reducing substance containing 96% FeSO 2 yh 2 O and provided with an antioxidant coating ("MELSTAR") was dosed to inlet 22 of mill 20. The finished cement product was transported approx. 350 m by means of the pneumatic conveying device 34 from the cement mill 20 to the storage silo 33. ' The amount of ferrous sulphate added was 0.47% by weight of cement. At the beginning of the experiment, the cement contained 8.2 ppm water-soluble chromate, which is a typical value. Samples were taken every hour immediately after the product left the cement mill.

During the sampling for 7 hours after the start of the experiment, virtually no water-soluble chromate could be detected in the cement samples. The dosage of ferrous sulfate was then reduced to 0.40%. For the last 3 hours of the experiment, a small but measurable content of water-soluble chromate in the cement was detected in the samples taken immediately after the product left the mill. In comparison with laboratory experiments, it was found that to obtain the same satisfactory degree of reduction of the water-soluble chromate in the production plant, 0.47% of ferrous sulfate calculated on the cement had to be used compared with 0.35% in the laboratory experiment.

A sample taken from the silo 33 immediately after the end of the experiment showed a low value of water-soluble chromate that was of the same order of magnitude as the value for the last 3 hours of production. This indicates that the reduction capacity of the 0.4% ferrosulphate in the cement that had been transported approx. 350 m of a pneumatic conveyor, was not impaired by the strong contact with the air during transport.

The amount of water-soluble chromate in the cement was determined by extracting a cement sample (3 g) with a 20% solution of sodium sulfate and measuring the amount of dissolved chromate in the extract by a colorimetric diphenylcarbazide method using a 5 cm cuvette in the spectrophotometer. to ensure maximum sensitivity.

The results of the above experiments are shown in Table I below:

Table I

Sampling time Water-soluble

Date Time chromat, ppm Cr 20 28.02.80 11.00 8.2 28.02.80 12.00 <0.1 28.02.80 14.30 <0.1 28.02.80 15.00 <0.1 28.02.80 16.00 <0.1 25 28.02.80 18.00 <0.1 28.02.80 19.00 <0.1 28.02.80 · 20.00 <0.1 28.02.80 21.00 0.5 28.02.80 22.00 0.2 30 28.02.80 23.00 0.1

Average 0.3

Silo (sample taken from silo after completion of production) 0.1 10 148794

Within a period of 3 months after storage of the cement mixture in the storage silo, samples were taken from the bottom of it in connection with the delivery of cement from the silo, and the content of water-soluble chromate in the samples was determined. The content of water-5 soluble chromate in the cement as a function of the storage time in the silo is shown in Table II. In the first 11 days, the content of water-soluble chromate is increased to 1 ppm Cr, and in the following 2 months the content of water-soluble chromate varies between 1 and 2 ppm Cr.

This indicates that there is a slight decrease in the reducing capacity of the ferrous sulfate, but the ferrous sulfate is still able to reduce 75-90% of the original water-soluble chromate content in the cement even after 2 to 3 months of storage.

It is noted that the theoretical amount of ferrous sulfate that would stoichiometricly correspond to the reduction of the initial concentration of water-soluble chromate in the cement is only 0.013% FeSO ^^ F ^ O. In other words, the amount of ferrous sulfate added during the aforementioned experiment represents an excess of more than 30-fold, which is believed to result from other chemical reactions taking place. It is envisaged that the use of ferrous sulfate having a more effective protective coating or coating, and / or the addition of ferrous sulfate at various stages during production, may reduce the amount of ferrous sulfate necessary to obtain an appropriate chromate reducing effect.

Table II

25 Date Storage time, days Water-soluble chromate, ppm Cr 29.02.80 0 0.3 10.03.80 11 1.0 30 27.03.80 28 1.0 16.04.80 48 1.0 20.05.80 82 0.9-2.1 EXAMPLE 2

An experiment similar to that described in Example 1 was carried out in a cement producing plant of the same type. The amount of ferrosulphate (FeSO4, 71 ° O) added was 0.6¾ based on the amount of 5 cement. At the beginning of the experiment, the cement contained 10.0 ppm Cr of water-soluble chromate. A sample was taken every hour immediately after the product left the cement mill, and the samples for each 8-hour period were mixed together to form a representative sample for that period. The amount of water-10 soluble chromate in the cement was determined as described in Example 1. The results obtained are reported in Table III.

Table III

Sampling time Water-soluble

Date Time chromat, ppm Cr 15 _ 07.10.80 14-22 <0.1 07.10.80 22-06 <0.1 08.10.80 06-14 <0.1 08.10.80 14-22 <0.1 20 08.10 .80 22-06 <0.1 09.10.80 06-14 0.1 09.10.80 14-22 <0.1 09.10.80 22-06 <0.1 10.10.80 06-14 <0.1 25 10.10 .80 14-22 <0.1

It appears that the content of water-soluble chromate in almost all the samples was less than 0.1 ppm Cr, i.e., the amount was below the measurement limit. Within a period of 63 days after storage of the cement in the storage silo, portions were taken from this and 30 delivered to a specific customer, and at the same time a sample was taken from each portion. The water-soluble chromate content was determined in the manner described above for each of the cement samples and, further, for a sample from each of the batches after they were mixed with water in a concrete mixer. The results obtained are listed below in Table IV: 12 148794

Table IV

Date Storage time, Water-soluble chromate ppm Cr days Samples of Samples of cement wet concrete 5 _ 08.10.80 0 <0.1 <0.1 27.10.80 19 <0.1 <0.1 28.10.80 20 <0.1 < 0.1 29.10.80 21 <0.1 <0.1 10 30.10.80 22 <0.1 <0.1 31.10.80 23 <0.1 <0.1 05.11.80 28 <0.1 <0 , 1 13.11.80 36 1.5 <0.1 19.11.80 42 <0.1 <0.1 15 26.11.80 49 <0.1 <0.1 03.12.80 56 0.4 <0.1 10.12 .80 63 0.4 <0.1

It can be seen that in the portions of the wet concrete mixture no content of water-soluble chromate could be determined, while in three 20 of the sampled cement samples smaller quantities were found. This is probably due to the fact that the cement samples taken are less representative and more affected by random variations than the portions that had been thoroughly mixed in a concrete mixer prior to the determination of the chromate content. 1 2 3 4 5 6 EXAMPLE 3 2 In a cement producing plant similar to that of FIG. 1, 3 ferrous sulfate was applied to three different parallel-bonded cement mills 4 from a common silo on the one shown in FIG. 1 illustrated. Two of the 5 cement grinding plants each contained two serially connected 6 cement mills, and the third grinding plant contained a single cement mill.