WO1997001613A1

WO1997001613A1 - Method for inhibiting dusting of expanded clay

Info

Publication number: WO1997001613A1
Application number: PCT/FI1996/000368
Authority: WO
Inventors: Eero Aitta; Simo Salanne; Hillevi Oikarinen
Original assignee: Kemira Chemicals Oy
Priority date: 1995-06-26
Filing date: 1996-06-25
Publication date: 1997-01-16
Also published as: FI101293B; FI955482A0; WO1997001612A1; FI955482A; FI101293B1

Abstract

The invention relates to a method for suppressing dust emissions from an expanded clay made up of porous particles, and to an expanded clay produced by the method. The essential idea of the invention is that the surface of the expanded clay particles is treated with a solution which contains an alkali metal salt or earth alkali metal salt, whereafter the particles are dried. The treatment may be carried out by immersing the particles in the solution or by spraying the solution onto the particles. Suitable salts include sodium and potassium formiates and sodium silicate. The amount of alkali metal salt or earth alkali metal salt introduced onto the surface of the particles may in expanded clay according to the invention be approx. 0.5-6 %, most preferably approx. 2.0-4.0 %, of the weight of the expanded clay.

Description

Method for inhibiting dusting of expanded clay

The invention relates to a method for inhibiting dust emissions from expanded clay made up of porous pellets. An additional object of the invention is an expanded clay which emits less dust than previously, produced in accordance with the method.

By expanded clay is meant porous clay pellets which have been fired in order to entrain air inside them. An expanded clay pellet is hard but breaks easily. Expanded clay is made from plastic clay. After preliminary working the clay is fired in a rotary kiln at a temperature of approx. 1150 °C. During the firing the clay expands and is molded into sintered rotating pellets under the rotary motion. The pellets are porous and full of small air voids. The particle size of expanded clay is typically approx. 3-7 mm, and it is capable of adsorbing water at a rate of approx. 10 % of its volume in 10 minutes. Owing to its light weight and good thermal insulation capacity, expanded clay is used as an insulation material in buildings. Further¬ more, expanded clay can be used for manufacturing lightweight concrete, from which various building elements such as blocs are manufactured. Besides pellet form, there exists expanded clay also in crushed form, which is prepared by crushing from oversized pellets separated in the screening of expanded clay pellets. The particle size in crushed expanded clay is approx. 4-8 mm.

In the manufacture of expanded clay and in the use of expanded clay on building worksites there is the problem of dust emis¬ sions from the expanded clay. The finely-divided silicate dust emitted from expanded clay constitutes an occupational health problem. Protection against dust has been provided by the use of dust masks. The use of such masks is, however, experienced as inconvenient, and they are not fully capable of providing protection from finely-divided dust. On the other hand, the dust produced in the manufacture of expanded clay is an en¬ vironmental problem for which a solution has been sought, for example, by using dust filters. Thus, dust emissions from ex¬ panded clay cause hazards both to people and to the environ¬ ment.

The most preferable solution to both of these problems is one by means of which emissions from dust from expanded clay con¬ sisting of small-sized particles, such as pellets or crushed material, can be prevented or substantially reduced. It is an object of the invention to provide specifically such a solution to the problem of dust emissions from expanded clay. The object is achieved through a solution which is characterized by the facts stated in the characterizing clause of Claim 1.

According to the invention, it is possible to produce on the surface of expanded clay a surface layer by immersing the ex¬ panded clay in a solution which contains a water-soluble alkali metal salt or earth alkali metal salt and by drying the ex¬ panded clay. The period during which the expanded clay par¬ ticles are in contact with the solution may be 5-60 seconds, preferably 10-20 seconds. Alternatively, the aqueous solution of an alkali or earth alkali metal salt can be introduced onto the expanded clay particles by spraying or atomizing. The con¬ centration of salt in the solution may be within a range of 5- 30 % by weight, preferably 10-20 % by weight.

The said water-soluble alkali or earth alkali metal salt may be a carboxylic acid salt, such as sodium formate (HCOONa) or potassium formate (HCOOK). The water-soluble compound may also be a silicate, such as sodium silicate (Na₂Si₃0₇, waterglass) or a silicofluoride, such as magnesium silicofluoride (MgSiFg).

The concentration of the sodium formate solution may vary with¬ in the range 5-30 %; however, the preferred concentration of the solution is within the range 10-20 %. The drying tempera- ture is preferably above 100 %. For waterglass the concentra¬ tion of the solution may vary within the range 5-30 %; a pre¬ ferred concentration of the solution is 10-20 %. The drying temperature is above 100 °C. The concentration of a magnesium silicofluoride solution may vary within the range 5-30 %, pre¬ ferably within the range 10-25 %. The drying temperature is 40- 100 °C, preferably 40-60 °C.

By means of sodium formate, dust emissions from granules can be reduced by 30-40 %. Economy is at its best with a 15 % solu¬ tion. Sodium formate is formed as a byproduct in the organic chemical process industry, and it has the advantages of a low price and commercial availability. Its corrosive action is not very detrimental, and also its toxicity is low.

Sodium silicate suppresses dust emissions by as much as 70 % when suitably added. Economy is in the same order as that of sodium formate. For example, in blocks of lightweight concrete the usability of sodium silicate is better than that of sodium formate.

Magnesium silicofluoride, which is formed as a byproduct in the preparation of phosphoric acid, is even more effective than the former, with a sufficient batching of the chemical. Its usabil¬ ity may, however, be limited by the possibility of formation of toxic gases (silicofluoride) and potential corrosion problems.

An expanded clay according to the invention, treated with a solution which contains an alkali or earth alkali metal salt, is in particular characterized in that the expanded clay par¬ ticles of fired clay have, introduced onto their surface, a water-soluble alkali metal salt or earth alkali metal salt in an amount of approx. 0.5-6.0 %, preferably approx. 1.0-4.0 %, and most preferably approx. 2.0-4.0 %, of the weight of the particles. The salt may be partially adsorbed into the par¬ ticles, while leaving particles at least partially porous. The performance of the invention was tested by carrying out tests, which are described in greater detail below. In the tests the expanded clay was on the one hand in crushed form and on the other hand in pellet form. The results of the tests are compiled in Tables 1 and 2.

Example 1 (Comparative Example)

Light expanded clay aggregate (Leca) was placed on a sieve, and any loose dust was rinsed off with distilled water. Thereafter the crushed material was dried in an incubator at 105 °C. Approx. 100 g of the dried crushed material was weighed onto a sieve. Thereafter the crushed material was transferred to a 10- liter drum, which was rotated for 15 minutes. The crushed material was poured onto a sieve, and fine dust was screened off. Thereafter the crushed material was weighed. The crushed material was returned to the drum, which was rotated for an¬ other 15 minutes, the total period of rotation thus being 30 min. Thereafter the crushed material was screened and weighed. The test was repeated 3 times; the mean result is given in Table 1. According to the results, on average 5.2 g out of a 100 g dried sample turned into dust in the first rotating. After the second rotating on average 10.6 % had turned into dust. In other words, after the first rotating, 5.2 % of the original dried and dust-free mass had turned into dust, and after the second rotating the corresponding value was 10.6 %. These percentage values were used as reference values in the following examples.

Example 2

The procedure was in other respects the same as in Example 1, but now the sieve-dried and weighed, approx. 100 g sample, to¬ gether with the sieve, was immersed into a 5 % sodium formate solution for approx. 15 seconds, whereby the crushed material was thoroughly moistened. Thereafter any excess solution was allowed to run off. The sample was dried at 105 °C and was thereafter weighed (100.7 g) . The sample was then transferred to a 10-liter drum, which was rotated for 15 minutes. The sample was poured onto a sieve, and any fine dust was screened off. Thereafter the sample was weighed, and the dust amount was calculated; it was 5.1 % of the original mass of the dried material. The sample was returned to the drum, which was rotat¬ ed for another 15 minutes. Thereafter the sample was screened and weighed. The result obtained was that now in total 10.2 % of the mass had turned into dust.

Examples 3 and 4

The procedure was in other respects the same as in Example 2, but 15 % and 30 % sodium formate solutions were used, and the drying was carried out at 105 °C. The results are shown in Table 1.

Examples 5, 6 and 7

The procedure was in other respects the same as in Examples 2, 3 and 4, but waterglass (sodium silicate) solutions of 5 %, 15 % and 30 % were used. The solution was prepared by using as the raw material a sodium silicate (Fluka 71957) which con¬ tained NaOH approx. 14 % and Si0₂ approx. 27 %. The drying was carried out at 105 °C. The results are shown in Table 1.

Examples 8, 9 and 10

The procedure was the same as in Examples 2, 3 and 4, but sodium silicate solutions of 5 %, 15 % and 30 % were used, and the drying was carried out at 300 °C. The results are shown in Table 1.

Examples 11, 12 and 13

The procedure was the same as in Examples 2, 3 and 4, but mag¬ nesium silicofluoride solutions of 5 %, 15 % and 30 % were used. The raw material used was magnesium silicofluoride, MgSiF₆*H₂0, which contained approx. 5.3 % Mg. The drying was carried out at 50 °C. The results are shown in Table 1. Table 1

Results of dust emission tests in Examples 1-13. The amount of dust is indicated in per cent of the original dust-free mass of the crushed material.

Example Solution Drying Dust amount (%) Adsorbed salt

No. °C 15 min 30 min amount (%)

1 Reference 105 5.2 10.6 _

2 HCOONa 5% II 5.1 10.2 0.7

3 HCOONa 15% II 3.0 7.2 1.8

4 HCOONa 30% II 3.0 6.7 4.1

5 Na₂Si₃0₇ 5% II 4.2 8.2 0.8

6 Na₂Si₃0₇ 15% II 2.4 5.1 1.1

7 Na₂Si₃0₇ 30% II 4.5 6.2 3.4

8 Na₂Si₃0₇ 5% 300 2.6 6.6 0.6

9 Na₂Si₃0₇ 15% II 2.2 3.2 1.8

10 Na₂Si₃0₇ 30% II 2.3 4.3 3.9

11 MgSiF₆ 5% 50 2.9 7.0 0.8 12 MgSiF₆ 15% II 2.7 4.7 2.9 13 MgSiF₆ 23% II 1.1 2.1 5.6

From the results in Table 1 it can be concluded that by using sodium formate the dust amount can at best be reduced by 30- 40 %. Economy is at its best when a 15 % solution is used. Waterglass (sodium silicate) suppresses dust emissions at best by as much as 70 %. Economy is in the same order as with formate. The greatest suppression (80 %) was achieved by using a 23 % magnesium silicofluoride solution, the use of which, however, involves the problems mentioned above.

Example 14 (Comparative Example)

Dust emission tests were carried out by using a larger batch of expanded clay. A batch of 2.370 kg was weighed of an expanded clay from which any loose dust had been screened off by using a Sweco sieve. The pellets were placed in a concrete mixer, which was rotated for 10 minuteε. Any loose dust was screened off from the pellets, and the pellets were weighed, whereupon the result obtained was that the mass had been reduced by 6.1 %. Thereafter, rotation in the concrete mixer was continued for 5 min (in total 15 min), whereafter the pellets were screened and weighed. The result obtained was that after 15 minutes of rotation the mass of the pellets had been reduced by 9.0 %. Thereafter, rotation was continued for a further 15 min (total period 30 min), whereupon, upon weighing, the mass of the pellets had been reduced by a total of 17 %. These percentage values were used as reference values in the following Example 6.

Example 15

The procedure was otherwise the same as in Example 14, but now the weighed amount of pellets was poured into a bucket contain¬ ing a 15 % solution of sodium formate. Thereafter the moistened expanded clay was poured onto a sieve to drain and was dried at 105 °C. After the drying, the pellets were weighed and placed in a concrete mixer, in which they were rotated for 10 min, thereafter for 5 min, and finally for 15 min. After rotation, screening and weighing were carried out. The result obtained was that after 10 minutes the mass had been reduced by 3.2 %, after 15 minutes by 5.6 %, and after 30 minutes by 12.0 %. The results are shown in Table 2.

Example 16

The procedure was in other respects the same as in Example 15, but a 15 % sodium silicate solution was used, and the drying was carried out at 105 °C. The results are shown in Table 2.

Example 17

The procedure was in other respects the same as in Example 15, but a 15 % sodium silicate solution was used, and the drying was carried out at 300 °C. The results are shown in Table 2. Example 18

The procedure was in other respects the same as in Example 15, but a 15 % magnesium silicofluoride was used, and the drying was carried out at 50 °C. The results are shown in Table 2.

Table 2

Results of dust emission tests carried out in a concrete mixer. The dust amount is indicated in per cent of the original dust- free mass of the pellets.

Ex. Solution Drying Dust amount %) Adsorbed salt

No. °C 10 min 15 min 30 min amount (%)

14 Reference 105 6.1 9.0 17.0 _

15 HCOONa 5% II 3.2 5.6 12.0 2.8

16 Na₂Si₃0₇ 15% II 2.7 4.3 8.8 2.2

17 Na₂Si₃0₇ 15% 300 2.7 4.0 8.2 2.3

18 MgSiF₆ 15% 50 1.5 3.0 6.4 4.8

The results in Table 2 show that formate-treated and silicate- treated samples were more or less similar even after a brief, vigorous rotating. Magnesium silicofluoride was better than the were former ones. The dust-binding capacity of formate de¬ creased at a faster rate than that of silicate, for which it remained quite even. Silicofluoride was throughout the time a better dust binder than were the others. When sodium silicate was used the expanded clay pellets adhered to one another more strongly after being moistened than when other chemicals were used.

Claims

1. A method for suppressing dust emissions from expanded clay made up of porous particles, characterized in that the surface of expanded clay particles is treated with a solution which contains an alkali metal salt or earth alkali metal salt, whereafter the particles are dried.

2. A method according to Claim 1, characterized in that the particles are immersed in an aqueous solution of an alkali metal salt or earth alkali metal salt for the duration of approx. 5-60 seconds, preferably approx. 10-20 seconds.

3. A method according to Claim 1, characterized in that the aqueous solution of an alkali metal salt or earth alkali metal salt is sprayed onto the surface of the particles.

4. A method according to any of the above claims, charac¬ terized in that the drying of the particles takes place on a sieve surface.

5. A method according to any of the above claims, charac¬ terized in that the concentration of the salt in the aqueous solution is approx. 5-30 % by weight, preferably approx. 10- 20 % by weight.

6. A method according to any of the above claims, charac¬ terized in that the salt is a water-soluble alkali metal salt or earth alkali metal salt of a carboxylic acid, such as sodium formate.

7. A method according to Claim 6, characterized in that the salt is sodium formate HCOONa or potassium formate HCOOK, and that the particle drying temperature is 100 °C or higher.

8. A method according to Claim 6, characterized in that the salt is a water-soluble silicate, such as sodium silicate Na₂Si₃0 , and that the particle drying temperature is 100 °C or higher.

9. A method according to Claim 6, characterized in that the salt is a water-soluble silicofluoride, such as magnesium silicofluoride MgSiFg, and that the particle drying temperature is 40-100 °C, preferably 40-60 °C.

10. A porous expanded clay treated according to any of the above claims, characterized in that the expanded clay particles of fired clay contain a water-soluble alkali metal salt or earth alkali metal salt introduced onto their surface in an amount of approx. 0.5-6.0 %, preferably approx. 1.0-4.0 %, and most preferably approx. 2.0-4.0 %, of the weight of the particles.