Soda-kireç-silis camında, antrasit ve sodyum sülfat miktarı ile sıcaklık profilinin ergime ve afinasyon üzerine etkileri
The impact of anthracite, sodium sulphate and temperature profile on melting and fining of soda-lime-silica glass
Yazar: ASLI MANAV
Danışman: PROF. DR. ÖMER SERDAR ÖZGEN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Seramik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Seramik Mühendisliği = Ceramic Engineering

Özet:
Enerji tüketiminin yoğun olduğu cam üretim prosesinde en fazla enerji kullanımı fırınların ergitme tankında meydana gelmektedir. Günümüzde enerji tüketimini azaltmanın yanı sıra ürün kalitesini de artırmaya yönelik yapılan çalışmalar, harmandan ergiyiğe dönüşüm prosesinin geliştirilmesinde de önemli rol oynamaktadır. Cam üretiminde ergime ve afinasyon aşamaları, ürün kalitesini ve ergiyiğin fırında kalma süresini belirleyen ana proses adımları arasında yer almaktadır. Ergiyiğin fırında kalma süresinin ve sıcaklığının hammaddelerin tamamen çözünmesi, çözünmüş gazların cam ergiyiğinden salınması ve ergiyiğin homojenleşmesi için yeterli olması gerekmektedir. Gaz kapanımlarının (habbe) ve çözünmüş gazların ergiyikten uzaklaştırılmasına yardımcı olmak amacıyla harmana afinan maddeler ilave edilmektedir. Son ürünün kalitesi açısından cam üretiminde önemli bir role sahip olan sülfat bileşikleri, günümüzde soda-kireç-silis camında en yaygın kullanılan afinan maddelerin başında gelmektedir. Cam içindeki çözünürlüğü düşük olan sodyum sülfat, ilk sıvı silikat fazının oluştuğu sıcaklıklarda silika tanecikleri ve ergiyik faz arasındaki arayüzeylerde birikmekte ve bu sayede, katı harman taneciklerinin çözünmesini hızlandırmaktadır. Ergimeyi kolaylaştırıcı etkisinin yanı sıra, yüksek sıcaklıklarda silika ile girdiği reaksiyon sonucu oluşan konveksiyon hareketleri sayesinde habbelerin ergiyikten uzaklaşmasını hızlandırarak afinasyonu desteklemektedir. Bununla birlikte, 1450oC'de dekompoze olmakta ve açığa çıkan afinasyon gazları ile habbelerin büyüyerek yüzeye yükselmesine yardımcı olmaktadır. Ancak ürün kalitesini arttırmak amacıyla harmandaki sülfat miktarında yapılan küçük bir artış, özellikle yüksek sıcaklıklarda ergiyik üzerinde aşırı bir köpük oluşumuna ve yüksek SO2 salımına sebep olabilmektedir. Köpük tabakası, endüstriyel cam üretiminde alevlerin cam harmanına radyal ısı akışını ciddi oranda azaltan son derece etkili bir ısı yalıtıcısı olarak davranmaktadır. Bu durum, daha yüksek bir enerji tüketimine ve düşük cam kalitelerine yol açmaktadır. Tüm bu nedenlerden dolayı optimum bir sodyum sülfat miktarının belirlenmesi gerekmektedir. Harmanın redoks durumuna bağlı olarak sülfat afinasyonu farklı mekanizmalarla gerçekleşmektedir. Sülfat bileşiği, oksidan harmanlarda daha yüksek sıcaklıklarda bozunurken, harmanda indirgen madde bulunması durumunda daha düşük sıcaklıklarda bozunmaktadır. Harmana fazla miktarda ilave edilen indirgen madde sülfatın büyük bir kısmını düşük sıcaklıklarda tüketmekte ve son kalan çözünmemiş silika tanelerinin çözünebilmesi için ergiyikte yeterli miktarda sodyum sülfat kalmamaktadır. Düşük karbon içeriğinde ise, mevcut kalan sülfatın termal dekompozisyonu nedeniyle 1450°C'den yüksek sıcaklıklarda ikinci bir habbe oluşumu meydana gelebilmektedir. İndirgen maddenin ergime ve afinasyon üzerindeki olumlu etkisinden yararlanabilmek için karbonun, sodyum sülfatı tüketmeden geriye kalan bir miktar sülfat ile çözünmeden kalan silika tanelerinin çözünmesine olanak sağlaması beklenmektedir. Bu çalışmada, dünyada üretilen camların büyük bir bölümünü oluşturan soda-kireç-silis camının ergime ve afinasyon mekanizmalarını radikal olarak etkileyen karbon/sodyum sülfat ilişkisi sıcaklığa bağlı olarak incelenmiştir. Bu kapsamda ilk olarak ana cam bileşimi ve sodyum sülfat miktarı sabit tutularak 4 farklı antrasit miktarı için deneysel çalışmalar gerçekleştirilmiştir. Ergime ve afinasyon performanslarına bağlı olarak en iyi sonucu veren antrasit miktarı tespit edilmiştir. İkinci aşamada, belirlenen antrasit miktarı sabit tutularak 3 farklı sodyum sülfat miktarı içeren harmanlar ile deneysel çalışmalar yapılmış, buradan optimum karbon/sodyum sülfat değerine ulaşılmıştır. Üçüncü aşamada, belirlenen optimum değeri içeren harmana farklı sıcaklık profilleri uygulanmıştır. Deneysel çalışmaların tamamı Yüksek Sıcaklık Gözlem Sistemi'nde gerçekleştirilmiştir. Ergime ve afinasyon sırasında açığa çıkan gazlar FTIR ile ölçülmüş olup, CO2, CO ve SO2 gazları sıcaklığa bağlı olarak incelenmiştir. Görüntü analizi sayesinde harmanların köpük davranışı (köpük kalınlığı) ve habbe davranışı (habbe sayısı) takip edilmiştir. Elde edilen deneysel verilere dayanarak, çalışma koşullarında optimum proses parametrelerinin belirlenmesi sağlanmıştır. Bu doğrultuda başlangıçta kullanılan sodyum sülfat miktarı (0.694g/100g cam) ile antrasit (0.015g/100g cam) miktarının optimum değeri sağladığına karar verilmiştir. Seçilen kompozisyona uygulanan sıcaklık profillerinden, maksimum sıcaklığı 1540oC olan fırın profilinde en iyi değerler elde edilmiştir.

Abstract:
Glass is one of the oldest materials which  has been used for thousands of years. It is a thermodynamically unstable material with high viscosity and amorphous structure. Generally, it can be defined as an inorganic product of fusion that solidifies without crystallization from liquid state. Its chemical resistance, transparent and rigid structure are some of the most important properties that enable it to used in a myriad of products. Although it can be obtained by alternative methods (i.e., sol-gel and vapor deposition technique),  the majority of glass is produced by melting batch components at an elevated temperature. The main steps in glass production are melting, forming and cooling processes. Most of industrial glasses are melted on a large scale in continuous furnaces. Regardless of final product, glass manufacturing processes share the same procedure in general terms. First, raw materials are selected according to the desired glass composition. The composition of soda lime silicate glasses, the most commonly produced glass type, generally consists of sand, soda, feldspar, dolomite, limestone, cullet and small quantities of fining agents, coloring and reducing/oxidizing agents. Then the ratios of raw materials to be used in the batch are calculated,  weighed and mixed these raw materials to procure homogeneous an initial batch. The batch is continuously charged into furnace. It undergoes a series of chemical reactions and physical changes during heating. Thus batch-to-melt conversion takes place with increasing temperature. In order to obtain a homogeneous melt, further processes are required, including the removal of bubbles known as fining. The glass must be cooled to be processed and shaped as the final product. The batch passes through three main processes in the furnace tank: melting, fining and homogenization. The melting of the glass batch is explained by a three-stage transformation. The first stage of the melting process includes solid state reactions which involve only in solid and gas phases. In the second stage, the melting reactions of the glass batch are accelerated with the formation of the first liquid phase. The first two stages of the reactions between the raw materials take place at temperatures below 1100 ° C. Beyond this point, the melt approaches the final composition and the grains which have a high melting point are slowly dissolve. The dissolution of these grains in the melt is controlled by diffusion as in fining and homogenization. Fining process starts during melting, and also homogenization process is largely achieved during fining. For this reason, it is not possible to separate melting, homogenizing and fining zones in the furnace with sharp limits. Glass production is an energy-intensive process. The most energy usage occurs in the melting tank of the furnaces. Aproximately 70% of the total energy used in glass production is consumed during glass melting and fining steps which determine the product quality and the residence time of the melt in the furnace. The temperature and the residence time of the melt must be sufficient to dissolve the raw materials completely, release the dissolved gases from the glass melt and homogenize the melt. The quality of many commercial glass products varies depending on the performance of melting systems. Nowadays, increasing competition and changing economic conditions force glass producers to improve energy efficiency and product quality. Efforts to reduce energy consumption in a cost-effective manner and to improve product quality also plays an important role in the development of batch-to-melt conversion. In industrial glass melting furnaces, small amounts of chemical agents are added to the batch to support the melting and fining stages. Sulphate compounds are one of the most commonly used chemicals in the production of soda-lime-silicate glass. The solubility of sodium sulfate in the glass is very low. Therefore, it accumulates in the interfaces between the silica particles and the melt phase at the temperature at which the first liquid silicate phase occurs. Thus, it accelerates the dissolution of the solid batch particles. In addition to the melting effect, it accelerates the removal of the bubbles from the melt due to the convection movements formed by the reaction with silica at high temperatures. It decomposes at 1450 ° C and fining gases are released. It helps the bubbles to grow with the fining gases and rise to the melt surface. However, a small increase in the amount of sulfate in the batch  to improve product quality can lead to high SO2 release and excessive foam formation on the melt especially at high temperatures. The foam layer acts as a highly effective heat insulator which significantly reduces the radial heat flow to the glass batch  in industrial glass production. This leads to a higher energy consumption and lower glass qualities. For all these reasons, it is necessary to determine the optimum amount of sodium sulphate. Depending on the redox state of the batch, sulphate fining is carried out by different mechanisms. While the sulphate compound decomposes at higher temperatures(~1400 °C)  in oxidant batches, it decomposes at lower temperatures  (~1000 °C)  in reduced batches. If a large amount of reducing agent is added to the batch, most of the sulphate is consumed at low temperatures. In this case, there is no sufficient amount of sodium sulfate in the melt to dissolve silica particles that still undissolved. If there is not enough carbon content in the batch, a second bubble formation may occur at temperatures above 1450 ° C due to the thermal decomposition of the remaining sulphate. It is important to determine the optimum amount of carbon to achieve the positive effect of reducing agent on melting and fining processes. It is expected that carbon will not consume sodium sulphate completely, and remaining sulphate allows the dissolution of undissolved silica grains. In this study, it is aimed to determine the carbon / sodium sulphate equilibrium and furnace temperature profile which is the most suitable for the energy consumption and glass quality criterias. In this context, the carbon / sodium sulphate relationship, which radically affects the melting and fining mechanisms of soda-lime-silica glass, has been investigated depending on the temperature. The main composition of soda-lime-silica glass designed for glass packaging products with the anthracite (0.015 g/ 100 g glass) was considered as the starting composition in experimental studies carried out in three stages. Firstly, the main glass composition and the amount of sodium sulphate (0.694 g /100 g glass) were kept constant. The experimental studies were performed with four batches with and without anthracite (0, 0.015 g, 0.031 g, 0.048 g /100 g glass). The amounts of anthracite to be added to the prepared batch were determined according to colorless glass redox conditions. Depending on melting and fining performance, the optimum amount of anthracite was determined. In the second part of the study, the main glass composition and the amount of anthracite determined was kept constant. The experimental studies were carried out with batch containing three different sodium sulphate amounts. The less (0.550 g/100 g glass) and more  (0.850 g/ 100 g glass) amounts of sodium sulfate were added to the two batches than the stable batch (0.694g / 100g glass). Thus, the optimum carbon / sodium sulphate value was obtained. In the third stage, two different temperature profiles were applied to the batch which gave the optimum carbon / sodium sulphate value. All of the experimental studies were performed in High Temperature Observation System which is an experimental setup where all stages of the melting and fining processes of batch under high temperature  are carried out in a transparent silica tube. The gases released as a result of chemical reactions in melting and fining  processes were measured with FTIR, and CO2, CO and SO2 gases were examined depending on the temperature. Foam behavior (foam thickness) of the batch and bubble behavior (bubble number) were followed by image analysis.  Based on the experimental data obtained, optimum process parameters were determined in working conditions. It was determined that the amount of sodium sulphate (0.694g / 100g glass) and anthracite (0.015g / 100g glass) used at the beginning provided the optimum value. From the temperature profiles applied to the selected composition, the best result was obtained in the furnace profile with a maximum temperature 1540oC. The studies have shown that if an optimum carbon / sodium sulphate balance is achieved, anthracite has a positive effect on melting and fining, and a high quality glass can be obtained. As the amount of anthracite increased, it was observed that SO2 release started at lower temperatures due to the early start of carbon, silica and sodium sulphate reactions. In case of addition of anthracite to the batch, it was seen that the foam thickness decreased compared to the batch without anthracite. Also it was determined that as the amount of anthracite increased, the bubble counts decreased. On the other hand, it was observed that the melt containing a small amount of anthracite can rapidly cleared from bubbles. The batch containing the highest amount of sodium sulphate demonstrated the highest amount of SO2 emission and height of foam formation. The batch containing the lowest amount of sodium sulphate was cleared  from bubbles more slowly. It was determined that the melting started earlier, if the increase in temperature was slow at the stage of solid-solid state reactions during melting. Especially from elevated temperatures such as 1300-1350oC, it was observed that the increase in temperature was critical for fining, and it should be slow.  In the temperature profiles, it was determined that the waiting time was more important at the critical stages, especially in the sulphate decomposition at 1450 ° C, than the maximum temperature.