Options for Grinding Slag and Pozzolan for Use as Construction Materials

1. Introduction

Granulated Blast Furnace Slag (GBFS) contains a high level of moisture, it is hard to grind and very abrasive.² Natural pozzolans may not be as hard to grind as GBFS but may contain high moisture levels and be abrasive.² Both of these materials require grinding systems designed to deal with high moisture, hardness and abrasiveness.

GBFS can be utilized in the cement making process as an additive in the kiln feed to increase clinker production; however, it is better used as replacement for cement in concrete.² Pozzolans are also used as an additive to improve the properties of concrete. This paper addresses the various drying and comminution systems in use to economically produce additives for concrete production.

2. Characteristics of GBFS and Natural Pozzolan

Figure 1. Granulated blast furnace slag.

Granulated Blast Furnace Slag (GBFS) is a steel making by-product from furnaces creating intermediate pig iron. Once quenched, it becomes glassy and a reactive cementitious material. Main components of GBFS are CaO, SiO₂, Al₂O₃, and MgO, with CaO and SiO₂ composing the largest percentage, with lesser components like Fe₂O₃ comprising the remaining amounts. Slag cement can be effective in mitigating alkali-silica reaction by reducing the total alkalis by binding alkalis in the concrete hydration reaction.²⁰ Like many pozzolans, slag cement consumes by-product calcium hydroxide from the Portland cement component to form additional calcium silicate hydrate (CSH).¹⁵

Figure 2. Natural pozzolan. ²¹

Natural pozzolans are volcanic ash or pumice, kaolin, a type of clay, and diatomaceous earth consisting of reactive silicon dioxide and aluminum oxide, with additional iron oxide. Pozzolanic materials, when finely ground and in the presence of water and dissolved calcium hydroxide (Ca(OH)₂), form strength developing compounds.¹⁸ However, most applications for natural pozzolan use in concrete is to prevent excessive expansion due to alkali-silica reaction, which requires precise measurements depending on its properties, the reactivity of the aggregate and the alkali loading of the concrete.¹⁹

3. Cement Types

With concerns over environmental impacts of cement manufacture, decreasing the clinker content in concrete requires increasing the utilization of supplementary cementitious materials like GBFS and pozzolans. Shown below, are cement types in common use with varying degrees of clinker component.

Table 1. FLS mill feed composition of different cement types.

As noted previously, pozzolanic materials require a certain amount of clinker component to be reactive. The goal is to lower clinker content to the minimum possible level while still developing the concrete strength requirements depending on the application.

4. Comminution Overview

Comminution theory focuses on the relationship between energy input and the particle size produced from a given feed size.¹ Tube Mills, more commonly called Ball Mills or Rod Mills, use impact and attrition breakage mechanisms as the means of breakage. This compares with the compression mechanism utilized in both High-Pressure Grinding Rolls (HPGR) and Vertical Roller Mills (VRM) to break down the particles.

The general equation E=-K.dx⁄xⁿ in which the energy used is related to change in particle size, a relationship between the energy required for breaking the material and particle size.¹ E is the net specific energy, dx/x is the change in particle size from the initial particle size to the final particle size, n is an empirical and K is a constant. Substituting “2” as the exponent creates the Von Rittinger equation, where energy consumed is proportional to the new surface area produced.¹ Utilizing the exponent as “1” creates the Friedrich Kick equation, where energy consumed is proportional to the reduction achieved in volume of the particles.¹ However, inputting “1.5” in the exponent of the general equation results in the Bond equation, which is the most common method to determine the work index in a ball mill. For more practical calculations using 80% passing the 100 micron mesh, the Bond Ball Mill Work Index (BBMWi) is widely used to measure the grindability of a mineral, described in kilowatt hours per ton (kWh/t).¹ In reference to cement clinker grinding, the following are typical energy requirements:

• Ball Mills – 38 kWh/t
• HPGR plus ball mill – 30-34 kWh/t
• VRM – 28-32 kWh/t

Note. Sourced from Comminution Handbook (1).

Each mill discussed in this paper presents both advantages and disadvantages for grinding of GBFS and pozzolans.

5. Drying Methods

GBFS has an initial moisture content of 35%, but it is usually received at the grinding plant at moisture levels as high as 15%.⁵ Natural pozzolans can have a moisture content as high as 25%.⁵ These moistures are too high for mills to grind and therefore drying is required. It helps to store these raw materials under cover to prevent precipitation from adding moisture.

Table 2. Typical properties of feed materials.

Note. Adapted from Loesche Mills for Cement and Granulated Blast Furnace Slag E 2016, pg. 3.

Thermal stressing in vertical roller mills when drying and grinding very moist materials has led to development of improved designs to cope with these conditions.⁴ The most common way to generate heat for drying raw materials in the mill is by utilizing a hot gas generator (HGG) to produce high temperature gases to drive off the moisture.

Figure 3. Unitherm HGG.

Note. Adapted from Unitherm HGG Brochure.

Table 3 shows a comparison of yearly cost for heating raw material as an operating cost.

Table 3. Cost of operating HGG for drying materials.

Note: The above cost is based on $3.50/MM kJ

It should be noted that GBFS devitrifies at approximately 700°C, thereby losing its hydraulic properties and thus excessive temperatures should be avoided.¹⁷ Using excess process heat from the cement plant, if available, or storing material in covered storage reduces the cost of drying. Material drying occurs in the air suspension between table and classifier in a VRM.⁷ For Ball Mill drying, a separate dryer is required ahead of the mill. In some cases, if the moisture is not too elevated, the drying may be done in the ball mill classifier as shown in Figure 4 below.

Figure 4. Example of a ball mill grinding plant with a HGG.

External drying is required for an HPGR, due to a limited amount of moisture permissible in the feed for size reduction.

6. Vertical Roller Mills

In a vertical Roller Mill (VRM), interparticle comminution takes place in the filled gap between the grinding table and the rollers.³

This can be illustrated using the FLS OK Mill shown below.

Figure 5. FLSmidth OK vertical roller mill.

Many advances in vertical roller mill technology have been made, due to the adoption of this milling system for clinker and slag grinding as well as production of pozzolanic blended cements.⁴

Vertical Roller Mills have gaining more popularity in new projects due to possible 40% less energy consumption than ball mills.⁹

For materials such as slag and pozzolan which are received as fine materials <5mm for slag and between 10mm and 50mm for pozzolan, the VRM requires lower power consumption.⁵

GBFS contains iron oxide, which is further enriched in the circulating load and must be removed from the circuit.² A metal detector and a magnetic separator are used to remove as much iron as possible from the mill circuit.

Grinding is a very energy intensive process which accounts for a significant amount of production cost and even small efficiency improvements can have impacts on production costs.⁴ This is why advances in classifier technology have been vital in increasing production efficiency. Such technologies as the ROKSH separator from FLSmidth also provides higher drying capacity for wet materials.¹⁰

Figure 6. FLS ROKSH separator.

Other separation technology, like the Loesche LDC Series and Gebr. Pfeiffer SLS classifiers, can reduce fineness down to 10µm.^{9 5}

Figure 7. Loesche LDC series classifier (a) and Gebr. Pfeiffer SLS classifier (b).

Since GBFS and pozzolan require a large surface area for reactivity, the fineness of the ground material plays a vital role, thus the efficiency of the classifier is essential.

The abrasiveness of GBFS and pozzolan requires high wear resistant liners. For VRM mills grinding very abrasive materials, such as slag, hard-facing is an economical alternative to changing wear parts and is suitable for high-chrome castings, optimizing the grinding process and saving refurbishment costs.¹⁰

Note. Adapted from FLS OK Mill Brochure.

Innovations in material science has also led to reduction in the loss of weight of wear parts and thus increase their life. While welded liners can be used many times, ceramic liners provides double the amount of life.¹¹ Metal matrix composites can be applied to either roller, tires, tire segments, table liners or double cast tires.⁴ These options enable mill designers the ability to design specifically to the material being ground, allowing plant operators flexibility to select wear materials to last the length of one operating campaign.⁴ Some companies, such as FLSmidth, are pursuing ceramics, with the recyclable ceramic wear segments according to data provided below.

Table 4. Example of wear life on FLS OK mill.

Note. Adapted from FLS OK PRO Plus Ceramic Wear Segments Brochure.

7. Tube Mills

The true workhorse of the cement industry in terms of grindability, the Ball Mill has been around for many years and still constitutes a large number of grinding systems in place today. Disadvantages are the energy consumption in kWh/t of around 30-40 kWh/t for clinker, is typically louder than other comminution machines and may require longer downtime periods for maintenance.⁴ Ball mills come in two compartments with ball charge size of around 50mm-90mm for grinding the coarser material and the second compartment containing 50mm and smaller for grinding the finer material that passes the screen separating the two compartments.¹ Advantages of ball  mill system are the internal heat generation which helps with drying, potentially lower capital cost than roller mills high run factors, and easier to operate.¹

Figure 8. Ball mill typical arrangement.

Ball mills have undergone considerable changes in the last few decades with trends of increased mill sizes, high efficiency separators and innovative internal designs.⁴ The efficiency and output are primarily dictated by ball charge for coarse and fine grinding optimization.⁴ This optimization has been accomplished mostly in part by the internals design on maximum angular lift and the accurate trajectory of the ball charge.⁴

Figure 9. Ball mill grinding process.

Some factors in ball mill operation are crucial for maintaining optimal production.  Wear-resistant liners protect the mill shell while providing lift to the ball charge.⁴ Liners will also provide a classification of the ball charge, primarily in the second compartment to promote classification of ball sizes with the larger balls at the back and the smaller balls at the front.⁴ With improvements in liner material such as high chromium steel, wear has been reduced.

Figure 10. Ball mill liners.

The charge media typically ranges from 30-35% of mill volume and the use of classifying liners further reduces the internal volume of the mill by roughly 10%.⁴ Between production, ball charge percent loading and kWh relationship should be observed carefully as these factors are pivotal and not mutually exclusive of one another.⁴ Since GBFS and pozzolan will need to be ground to a finer particle size, smaller media of around 12mm is utilized. The ball size is dictated by the hardness of the material and its feed size distribution.¹

Grinding aids in a ball mill can proficiently impact production cost.⁴ Three major aspects in grinding aid would be decrease “pack-set,” increase flowability and reduce moisture in the silo.⁴ Pack-set is the agglomeration of mineral coating on the media which reduces the crushing effect.⁴ Using an additive increases flow by reducing ball coating and increasing the separator efficiency by allowing a better “cut.”⁴ Agglomeration on ball charge can be seen in figure 11 below.

Figure 11. Agglomeration of ball mill media.

8. High Pressure Grinding Rolls

High pressure grinding rolls (HPGR) technology was first utilized in the grinding of clinker and raw material in the mid-1980s and has quickly proved be an economical choice in comminution process.³ It utilizes the same compression method as the VRM to break the particles to the desired fineness. The HPGR compresses the material into a cake that includes both fines and coarse material that needs later to be deagglomerated.

Figure 12. High pressure grinding rolls grinding method.

Note. Adapted from Polysius Polycom HPGR Brochure.

An HPGR can reduce kwh/t energy consumption when working in conjunction with another mill, usually a tube mill.³ A savings of around 1.8-2.5 kwh/t for clinker and 2.5-3.8 kwh/t for GBFS can be obtained.³

Moisture content of feed material to the HPGR needs to be low so as slip does not occur in the operating gap of the two rolls, as excessive moisture will result in lower throughput due to this phenomenon.²² HPGR were used for softer materials for years because highly abrasive minerals caused high wear on the expensive rollers.¹ Using a HPGR for grinding slag and pozzolan has become possible due to newer designs to reduce wear.¹ Hard facing is the optimal solution as it provides adequate protection and wear time without requiring to replace the rolls.

As noted above, HPGR is added to existing mill systems, typically a ball mill to reduce its overall power consumption. As a primary grinding system, it requires additional equipment downstream for deagglomeration. Polysius’ Polycom High Pressure Grinding Roll can illustrate this further (see Figure 13 below).

Figure 13. Example arrangement of finish grinding (a), combination grinding (b) and finish grinding (c).

Source: POLYSIUS POLYCOM HPGR BROCHURE

(a)

For primary grinding, there is a high output of fines achieved even with relatively low energy input, illustrating one of the main advantages of the HPGR.¹⁷ Installation of the grinding roll as primary mill increases ball mill output by about 15%, with energy saving close to 5-10% of total energy requirement based on conventional ball mill grinding systems with a specific energy requirement of 30 kWh/t.¹⁷

(b)

Combination grinding with a HPGR being placed before the ball mill system can reduce the cost of energy required to grind abrasive material in the ball mill. This semi-finish grinding can also greatly reduce cost of energy and be one of the efficient methods when considering a HPGR.¹

(c)

A HPGR has two main advantages: the simple mechanical design of the plant and the attainable energy reduction.¹⁷ However, the compacted cakes that are typically formed when using a HPGR will require a downstream disintegration by a hammer mill or ball mill.¹⁷

9. Conclusion

Utilizing the latest technologies, slag and pozzolan grinding is far less energy intensive than it used to be and can provide GBFS and ground pozzolanic mineral in large quantities and required fineness.² To reduce the overall energy cost of grinding in an existing ball mill grinding plant, adding a HPGR into the system may be a great option to consider; however, the lower capital cost and easier operation, the tried and true ball mill system provides efficient grinding of fine material that for grinding GBFS and pozzolan could be  a good option if considering blending with clinker. With advances in wear resistance, drying methods, classifier technology and energy efficiency, the VRMs are good options for slag and pozzolan grinding.

References

1. Alban Lynch, Comminution Handbook (2015).

2. Dipl. Ing. Eberhard W. Neumann, Processing Slag in Cement Making (2003 NSA Spring Meeting).

3. S. Komar Kawatra, Advances in Comminution (2006).

4. Innovations in Portland Cement Manufacturing, Bhatty

5. Loesche Mills For Cement and Granulated Blast Furnace Slag E 2016

6. https://www.unitherm.at/images/downloads/catalogue/english/EN_2016-09_HGG_Catalogue_web.pdf

7. Cement Plant Operations Handbook 6^th edition

8. https://fctcombustion.com/hot-gas-generators

9. Gebr. Pfeiffer Brochure Minerals Lime Industry

10. FLS OK Mill Brochure

11. FLS Webinar OK Mill November 2020

12. Use of pozzolans in concrete – concrete countertop institute

13. National pozzolan association – how natural pozzolans improve concrete

14. Separate slag grinding in different milling systems, Donald A. Longhurst

15. Slag cement association (SCA), Slag cement facts

16. Effect of ball size milling performance

17. Dipl.-Ing. Walter H. Duda, Cement Databook

18. Dr. Woywadt, Grinding with MVR World Cement November 2018

19. ASTM C618-19, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete

20. ASTM C989-989M, Standard Specification for Slag Cement for Use in Concrete and Mortars

21. Siddique, Rafat, and Paulo Cachim. Waste and Supplementary Cementitious Materials in Concrete: Characterisation, Properties and Applications. Woodhead Publishing, an Imprint of Elsevier, 2018.

22. Daniel Saramak, The Effect of Feed Moisture on the Comminution Efficiency of HPGR Circuits

This article was contributed by Russell Reimer, Process Engineer, with the collaboration of Francisco M. Benavides, Principal Consultant at PEC Consulting.  Mr. Reimer holds a Bachelor of Science in Chemical Engineering from Oklahoma State University, Stillwater, Oklahoma. This article also appeared in the September 2022 issue of the International Cement Review.