I tried using free proxies with the search ranking checker tool Serposcope, but CAPTCHAs appeared frequently and it was completely unusable.

The log file /var/log/serposcope/*.log had a large amount of logs like the following that detected CAPTCHAs:

[google-0] INFO  c.s.s.s.g.s.GoogleScraper - GOT status=[302] exception=[none]
[google-0] DEBUG c.s.s.s.g.s.GoogleScraper - captcha form detected via proxy:http://144.217.158.153:3128/

Since most free proxy servers are probably used in similar ways, those IP addresses might be under suspicion from Google.

That’s all from the Gemba.