Evolution of Refractory Shotcreting Technology
Shotcrete is the fastest growing technology for refractory installation. Compared to casting or gunning, the installation by shotcrete is 5 to 10 times faster and the quality of installed mass is similar to that obtained by casting. Due to the technological advantages of this installation technique, shotcrete virtually has seen exponential growth in the Indian market.
Shotcrete, like castable, is mixed with water in a controlled manner. The mixed material is pumped through a double piston pump and is conveyed through a combination of steel and rubber pipelines. At the nozzle, an admixture and air are added to allow the material to thicken and stick on the targeted surface. As expected, degree as well as rate of thickening depends on the quantum of added admixture. Shotcrete is a highly versatile installation technique and has proven to be successful virtually in all industrial applications. Material installed by shotcreting yields desired performance in virtually all sections of cement industry, blast furnaces, reheating furnaces, torpedo ladles, DRI kilns, and in numerous non-ferrous segments like alumina kilns, power, waste heat generators and incinerators.
Figure 1 illustrates the basics of shotcrete installation. Shotcrete material can be classified as a pumpable. Shotcrete material, like any castable, is mixed with requisite amount of water in an appropriate mixer to pumpable consistency. Subsequently the material is pumped to the installation location. Prior to its discharge from the nozzle, the material is mixed with an additive, which acts as thickener. Subsequently it is sprayed pneumatically on the installation surface. Owing to conveying of wet mass, shotcreting process is also known as wet gunning.
For cement industry, shotcrete has emerged as one of the most sought after technology due to its superior installation rates, lower rebound loss and better lining life, thereby benefitting the end user by means of lower downtime and extended refractory life. Figure 2 reports monolithic refractories installation rates by various installation techniques. Requisite features of refractories, except for those for burning and transition zones, are high resistance to abrasion and resistance to alkali attack. Refractories with alkali resistance suit extremely well to resist build up in the riser ducts as well as kiln inlet areas. Such products, in general, contain silicon carbide. As a fall out of presence of silicon carbide, alkali-resistant products for the cement industry also has high resistance to thermal shock, which make them suitable for kiln hood and throat wall. Calderys India has been successful in developing shotcrete formulations with features required for the cement industry. ACCSHOT B and ACCSHOT AFM/CRC are abrasion and alkali resistant shotcrete products, respectively from Calderys India.
Development of shotcrete material for Indian condition
The major challenge for the development of shotcrete formulations has been to make it compatible with Indian weather condition. It is well known that the material for shotcreting, unlike gunning formulations and like castable, is premixed with specified amount of water for pumpable consistency. Owing to this mixing process, the variability as well as dependence of installed mass quality on the skill of installer (gunman), observed for gunning material, is virtually eliminated. The obvious requirement for shotcrete, hence, is the mix should remain pumpable for reasonable duration so that the material does not get choked in the conveying conduit during pumping process.
Choking is the biggest hindrance for installation of material by shotcreting. This not only causes installation interruption leading to loss of time but also is one of the major causes of material wastage. Like any castable, the working time of shotcrete reduces with the increase of ambient/mix temperature. Especially during Indian summer, when the ambient temperature exceeding 45 degree Celsius is a common occurrence, maintaining the pumpability of the mix for long enough duration, so that the material does not set in the conduit during pumping, has been the major challenge during the development of shotcrete material.
Apart from maintenance of adequate working time, even at ambient temperature of 45 degree Celsius or so, the other challenges, during shotcrete product development, had been:
Elimination of segregation of fines during pumping (Figure 3), which illustrates that improperly-graded material causes segregation of fines during pumping.
Limiting the thickening additive requirement at the nozzle to its minimal level, as well as
Minimal variation in thickening additive requirement even with the change in ambient temperature
Calderys India has successfully overcome these challenges through inûhouse R&D activity.
Comparative properties of shotcrete and other mechanically-installed monolithic refractories are illustrated in Figures 4 and 5. It is evident from the data that characteristics of shotcrete materials, developed by Calderys India, are virtually comparable to those of Low Cement Castables (LCC) in all respect, including their CaO content. The latter implies that the refractory properties of installed Shotcreted material would be similar to that of LCC. In short, shotcrete yields features and properties of LCC at significantly higher installation rate.
Calderys India's success with shotcreting technology can primarily be attributed to development of product compatible for Indian ambient condition as well as the skilled human resource for installation. The acceptability of Calderys India's shotcrete material, virtually in all industrial segments, emphasises our success with shotcrete technology. In this paper, we give a brief coverage of the shotcreting job carried out, in various areas of cement industry, by Calderys India.
Calderys's experience with Shotcrete
In India, Calderys has emerged as a key solution provider for all industry segments including cement. Table I, based on our experience, reports the comparison amongst various application methods of monolithics. Table II reports some major shotcrete installation jobs by Calderys India. In the following section some of them have been covered in greater detail.
Refractory lining of a new preheater at a prestigious cement plant in Maharashtra: This job involved refractory lining of a newly-installed preheaterûcalciner connecting duct. The estimated time requirement for the completion of refractory lining by 75 MT of castable has been 22 days, which included eight days for mechanical work. As per the project schedule, however, only 15 days had been allocated to this job. Since the reduction of time requirement for the mechanical job was not possible, the customer was in the lookout for a solution through refractory. In short, we had approximately seven days for the completion of the refractory job, which included erection of scaffolding, welding of 4,500 anchors, fixing of 250 m2 Calcium Silicate blocks, refractory and finally removal of the scaffolding. It was estimated that if the refractory installation was done by using castable, it would require 15 days and hence, installation by Shotcreting was proposed, to which the customer agreed. We had been successful in installing 75 MT of Shotcreting materials in the duct, which included constrained environment like of less die in 15 hours. The total refractory installation job could, thus, be completed within seven days, as desired by the customer, in effect there has been a saving of seven days.
Refractory solution for kiln hood, throat wall and cooler take off duct at a prestigious cement plant in Madhya Pradesh: The primary issue at this unit had been poor refractory life in kiln hood, throat wall and cooler take off duct (ToD). Refractories in these locations did not last even for a year. The customer approached Calderys India for extension of refractory life in the aforementioned segments, simultaneously allocated less time for completion, i.e., only three days.. It was communicated to customer that castable lining would require approximately seven days, whereas the same, if carried out by Shotcreting, would require three days leading to reduction of kiln downtime by approximately four days. Installation by shotcreting was agreed upon.
While analysing the refractory condition of the kiln hood, throat wall and ToD, it was realised that inadequate abrasion resistance of the refractory used is the root cause its poor performance. It, hence, was recommended that we use our ACCSHOT B for this job and the customer agreed with the proposal.
The job involved anchor welding as well as calcium silicate blocks fixing, which required two days. Subsequently 70 MT of ACCSHOT B was installed in nine hours. The revamping job could, thus, be completed in less than three days. The same job, if installation was carried out by using castable, would have required seven days, thus, leading to saving of more than four days. Figure 6 illustrates the condition of throat wall before and after the completion of the job.
Inspection was carried out in January 2018, i.e., after 13 months of operation and the refractory condition was found to be excellent. The kiln is still in operation and thus, shotcreted material met customer's requirement of improved refractory performance. Subsequent to successful performance of ACCSHOT B in the high abrasion areas, this cement plant has placed a repeat order for ACCSHOT B, which will be completed within September 2018.
Breakdown converted to annual shutdown at prestigious cement plant in Chhattisgarh: Due to breakdown in raw meal section the plant operation had to be stopped in the first week of September 2017. Since the annual shutdown was planned in the end of the same month, the unit was not ready with all the refractories at the plant site. The major refractory repair, during the annual shutdown, was planned for throat wall cooler bull nose and kiln hood side wall. Based on the performance of shotcreted material at their unit, customer opted for shotcrete material.
Calderys India could organise shotcrete material, shotcrete machine and the installer in short notice. Material recommended for all the affected sections was ACCSHOT B. Total installed quantity was ~ 70 MT. All the materials could be installed in 18 hrs, compared to over 7 days, if the work had been carried out with castable. Figures 7 and 8 illustrate the condition of the refractory linings of cooler bull nose and kiln hood wall before and after the repair carried out by shotcrete materials.
Throat wall and cooler first grate was repaired by ACCSHOT B at a prestigious cement plant unit in Andhra Pradesh: Apart from installation of refractory, the job involved stainless steel anchor welding, ceramic anchor fixing, placement of calcium silicate blocks and shotcrete installation. Sixty MT of shotcrete material was installed in 12 hours as compared to four days if the same had been done with castable.
Refractory installation by shotcreting has gained popularity among the Indian refractory users due to its incomparable benefits - faster rate of installation, lesser possibility of human error due to reduced manpower involvement, lower rebound loss, excellent service from the installed lining, etc. Calderys India has gained enough experience in shotcrete Installation, through inûhouse developed products, virtually in all industry segments including cement. Calderys' R&D team has ensured that the product formulations are robust enough to withstand Indian summer heat during installation.
About the authors
IN Chakraborty, S Sinha, RK Laharia, A Bose and S Upadhyay of Calderys India Refractories Limited, Nagpur.