Ultra-fine blast furnace slag could reduce one of the main weaknesses of cements containing blast furnace slag, that bei
LOESCHE METHOD FOR MANUFACTURING ULTRA-FINE BLAST FURNACE SLAG
Blast furnace slag grain with reaction seam of CSH phases (taken by B. Möser, Bauhaus University, Weimar, FIB)
Dr. Holger Wulfert, Dr. Winfried Ruhkamp, Andre Bätz, Paul Erwerth Material Tests: Prof. Ludwig, Bauhaus University, Weimar, FIB
1. Initial situation Due to the high CO2 emissions that are associated with the production of Portland cement clinker, a central objective of the cement manufacturer is to reduce the clinker proportion by using composite materials. One of the most important composite materials is blast furnace slag. The amounts of blast furnace slag that accumulate are used almost exclusively today as a composite material in cement or a concrete additive in concrete. The usual fineness range of the blast furnace slag is between 3,500 and 6,000 Blaine. To be able to exploit the performance potential of blast furnace slag fully, material producers have expressed again and again the wish for a suitable preparation technique for creating the finest blast furnace slag. Ultra-fine blast furnace slag could reduce one of the main weaknesses of cements containing blast furnace slag, that being too little early strength for many applications (Diagram 1). As is already apparent today, the ultra-fine blast furnace slag could be used for many different specialised products. Until now extremely finely ground blast furnace slag (> 10,000 Blaine) has been produced in ball mills (mostly in batch operation) in a very energy-intensive way. Generating such a high degree of fineness requires an enormous amount of energy. The consequent high cost of the corresponding products has so far prevented a greater market penetration, with the result being that binder systems based on the finest blast furnace slag have so far only been used in niche areas.
Against the backdrop of the situation described above, Loesche has developed a more energyefficient production of the finest blast furnace slag. The concept includes initially manufacturing a blast furnace slag of typical fineness (4,500-6,500 Blaine) on a vertical roller mill, from which the finest blast furnace slag is separated in an extremely effective and economical manner. This low-cost production of the finest blast furnace slag will enable a wider market access for the corresponding products. 2. Tests for attaining ultra-fine BFS on pilot scale Blast furnace slag produced in large-scale Loesche grinding plants usually has finenesses between 3500 and 6000 Blaine, max. 7000 Blaine. Investigations on grinding products from an industrial vertical roller mill showed that, depending on the fineness, grinding products contain ultrafine material components with degrees of fineness of >10,000 Blaine, in the range of approx. 10 to 20 %. A way has been found to be able to use known and tested equipment and technologies to attain the ultra-fine content without significantly increasing the specific power consumption. For this, the product is drawn out of the product silo and fed into a separate plant in order to attain the ultra-fine product. As a core part of this type of technology, a specialised cyclone is used for the finest dust. The basic structure of such a system is illustrated in a simplified flowchart in figure 1.
GBFS-mortar mix 50% / 50% with CEM I 52,5 R 100 7d
2d
90
28 d
86.9 77.8
Compressive strength [MPa]
80 70
65.0
61.7
60 50
42.5
38.2
40
36.5
30 20
19.6
20.1
10 0
2
BFS with 3,600 Bl.
BFS with 5,000 Bl.
BFS with 17,000 Bl.
Diagram 1: Compressive strength development as a function of BFS-fineness
Feed silo
Multicyclone system
Filter
Fan
M
M
M
Grit Ultra fine
M
Disperser
Supply
M
Figure 1: LOESCHE Cyclone-plant for producing of ultrafine materials
It is known that the fineness of a final product determines the geometric dimensions of the cyclone to be used. Because of the relatively small dimensions, generating larger amounts of the product requires working with a larger number of cyclones, which are consolidated into so-called multi-cyclone batteries.
• Proof of the general operational capability and control capability of the multi-cyclone facility regarding the achievable fineness and the fine material yield. • The use of a disperser, which transfers the grinding material from the grinding plant’s silo back into a gas/solid matter suspension without any agglomerates forming.
In the first experimental tests, the following points were the focus:
9147
9397
9362
Feed to cyclone: Blaine Feed to cyclone: P50 Portion of ultra fine material
cm²/g
6000
5000
4500
µm
6.0
6.8
8.5
%
18
14
10
Ultra fine mat.
cm²/g
12000
11000
12000
Fines: P50
µm
2.1
2.2
2.1
Grit: Blaine
cm²/g
4650
4050
3650
µm
7.0
8.2
11.4
Grit: P50
Table 1: Proportion of ultra-fine material and “grit” at different degrees of fineness in feed
3
Feed to cyclone: Blaine
cm²/g
5000
4500
Feed to cyclone: P50
µm
6.8
8.5
Portion of ultra fine material
%
14
9
10
5.5
cm²/g
11000
16000
12000
17000
µm
2.2
1.7
2.1
1.6
cm²/g
4050
3950
3650
3850
µm
8.2
8.3
11.4
10.7
Ultra fine mat.: Blaine Ultra fine mat.: P50 Grit: Blaine Grit: P50
Table 2: Manufacturing of various ultra-fine material at equal fineness of the feeding material
The gas/solid matter mixture is transported through the plant with the suction of a fan. The grinding product with a selectable fineness, between 4500 and 6500 Blaine wherever possible, is stored in a bin and supplied to a disperser facility via a rotary gate and speed-controlled screw conveyor. From here the gas/solid matter mixture reaches the multicyclone.
100
1.00 CEM
90 Compressive strength (N/mm2 ) / norm prisms
The ultra-fine material leaves the cyclones via the immersion tube and is transported to the filter. The material from the lower stream of the multi-cyclone apparatus, called “grit”, has a fineness in the range of conventional blast furnace slag and can be used as such (Tab.1 and 2).
0.50 BFS meal with 4000 BLAINE 0.70 BFS meal with 4000 BLAINE
80
Development of compressive strength compared to CEM I 42,5 R (1.00) and mixtures with 50 and 70 Ma.-% BFS meal (12,600 Blaine)
70 60
53.1
50 40
55.8 45.1
43.1 36.0
32.4
30 20.2 20
15.1 6.5
10 0
2d
7d Curing time [d]
28 d
Diagram 2: Compressive strengths of blended cements with blast furnace slag from a Loesche blast furnace slag grinding plant
4
Tables 1 and 2 show paired values: • Fineness of the ultra-fine material and • of the remaining “grit”.
3. Cycloning results and material tests of the ultra-fine blast furnace slag In the Loesche test center many grinding and cycloning tests have been carried out with many different types of blast furnace slag over the past year. The throughput of the cycloning plant’s fine material is 20 to 80 kg/h. The greater the fineness of the feed material (grinding material), the higher the proportion of ultra-fine material. The higher the proportion of ultra-fine material, the lower its fineness. These correlations are shown in tables 1 and 2. The degrees of fineness are stated in Blaine. The authors are aware that the stated values may contain errors, since the Blaine procedure for high degrees of fineness no longer produces any exact values. Thus the given values have a purely indicative value. For these degrees of fineness it is common to characterise the material with P50 values that have been determined by laser granulometry with a CILAS device. Of course the user is immediately faced with the question of how the “remaining blast furnace slag” proportion can be used.
100
This makes it clear that there is no waste product and the grit can be used completely normally as a composite material. Diagrams 2 and 3 show compressive strengths of blended cements with typical degrees of blast furnace slag fineness (4000 Blaine) and of ultra-fine blast furnace slag (12600 Blaine), attained from the 4000 Blaine material, that has been produced in a blast furnace slag grinding plant with a Loesche mill. With technology of this type, the binder industry in question is made able to produce ultra-fine blast furnace slag in a cost-efficient way. If prices are significantly below those of the products currently available on the market, a dynamic development of the demand in various applications is to be expected. The first manufacturing pilot plant is expected to go into operation in 2018.
1.00 CEM 0.30 BFS meal 12600 Blaine
90 Compressive strength (N/mm2 ) / norm prisms
The degrees of fineness of the “grit” vary within the ranges of commonly used blast furnace slags.
0.50 BFS meal 12600 Blaine
80
Development of compressive strength compared to CEM I 42,5 R (1.00) and mixtures with 30, 50 and 70 Ma.-% BFS meal (12,600 Blaine)
69.2
70 59.7
60
73.5
63.6 58.9 53.1
50 40
78.8
0.70 BFS meal 12600 Blaine
43.1 36.0
33.2
30
30.5 25.0
20 10 0
2d
7d Curing time [d]
28 d
Diagram 3: Compressive strengths of blended cements with ultra-fine blast furnace slag, attained from a grinding product from a blast furnace slag production plant
5
M
M M
M
M
M
M M
M
M
M
M
M
M
M
Figure 2: Integrating a cycloning system into a Loesche blast furnace slag grinding plant
6
M
M
M
M
M
M
M
M
7
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